S4 Tier 0: the bank plays — VST3 marshals MIDI to the S3 core, reads the live bank + resolves WAV the M4 way, mono downmix, lock-free load handoff, LICE sample-pick

CMakeLists.txt

@@ -621,6 +621,18 @@ target_include_directories(editor_geometry PUBLIC src/vst)
 add_library(bridge_marshal STATIC src/vst/bridge_marshal.cpp)
 target_include_directories(bridge_marshal PUBLIC src/vst)
 
+# sample_map (Phase S4) — PURE mapping logic for the Tier-0 instrument: the live bank
+# blob -> selected sample (via the SHARED bank_book JSON parse, NOT a second parser),
+# interleaved->mono downmix (the Tier-0 channel policy), the Tier-0 chromatic keymap
+# build, and the selected-sample instance-state (de)serialization. Links the three pure
+# modules it composes — bank_book (shared JSON), wav_trim (shared WAV parse), and
+# sampler_core (the Keymap/SampleData it yields) — and NEITHER SDK. The VST3 shell
+# (reasampler_processor.cpp) does the bridge read + file I/O off the audio thread, then
+# calls these; the process callback stays allocation-free.
+add_library(sample_map STATIC src/vst/sample_map.cpp)
+target_include_directories(sample_map PUBLIC src/vst src)
+target_link_libraries(sample_map PUBLIC bank_book wav_trim sampler_core)
+
 add_executable(editor_geometry_tests tests/test_editor_geometry.cpp)
 target_link_libraries(editor_geometry_tests PRIVATE editor_geometry)
 add_test(NAME editor_geometry_tests COMMAND editor_geometry_tests)
@@ -629,6 +641,13 @@ add_executable(bridge_marshal_tests tests/test_bridge_marshal.cpp)
 target_link_libraries(bridge_marshal_tests PRIVATE bridge_marshal)
 add_test(NAME bridge_marshal_tests COMMAND bridge_marshal_tests)
 
+# sample_map: the S4 mapping heart. Links ONLY sample_map (+ its pure deps) — NEITHER
+# the VST3 SDK nor the REAPER SDK — the same structural plain-data-boundary proof the
+# sampler_core test enforces.
+add_executable(sample_map_tests tests/test_sample_map.cpp)
+target_link_libraries(sample_map_tests PRIVATE sample_map)
+add_test(NAME sample_map_tests COMMAND sample_map_tests)
+
 # ---------------------------------------------------------------------------
 # 4) The REAPER extension — a loadable module (dlopen'd by REAPER, not linked).
 # ---------------------------------------------------------------------------
@@ -788,7 +807,14 @@ if(WIN32)
         ${VST3_SDK}/public.sdk/source/main/moduleinit.cpp
         ${LICE_SRC}
     )
-    target_link_libraries(reasampler_vst PRIVATE vst3_sdk editor_geometry bridge_marshal)
+    # editor_geometry + bridge_marshal: the pure spike helpers. sample_map (S4): the pure
+    # bank->keymap mapping + state (de)ser the processor drives off the audio thread;
+    # linking it pulls its pure deps (bank_book, wav_trim, sampler_core, bank_model,
+    # peaks) transitively. capture_paths: the shared M4 path resolution (resolveBankFile /
+    # projectDirOfRpp) the bridge + processor use. Its PUBLIC include dirs (src, src/vst)
+    # give the shell TUs their headers (ext_keys.h, bank_book.h, sampler_core.h, ...).
+    target_link_libraries(reasampler_vst PRIVATE vst3_sdk editor_geometry bridge_marshal
+        sample_map capture_paths)
     # SDK_INC gives reaper_vst3_interfaces.h + reaper_plugin_functions.h for the bridge;
     # WDL_INC gives LICE for the editor. The VST3 SDK headers come from vst3_sdk PUBLIC.
     target_include_directories(reasampler_vst PRIVATE ${SDK_INC} ${WDL_INC})

PLAN.md

@@ -193,15 +193,22 @@ following the active project works; it never captures and never inserts into the
 arrange (read-only over the bank).
 **Depends on:** S1, S2, S3.
 
-- [ ] VST3 `process` marshalling: read MIDI note-on/off/velocity off the event bus,
+- [x] VST3 `process` marshalling: read MIDI note-on/off/velocity off the event bus,
-  drive the S3 core, write per-voice audio to the output bus.
+  drive the S3 core, write per-voice audio to the output bus. (Block-granular event
-- [ ] Live-state seam: read the bank index + selected sample's root note from
+  timing at Tier 0; sample-accurate offset scheduling is a later tier.)
+- [x] Live-state seam: read the bank index + selected sample's root note from
   `"reasampler"` ext-state via the bridge; resolve the WAV audio path the M4
-  project-relative way (shared convention with `persist`, not re-implemented).
+  project-relative way (shared convention with `persist`, not re-implemented — the
-- [ ] Sample selection UI (minimal, in the `IPlugView` LICE editor or a
+  parent-of-.rpp derivation is extracted to `capture_paths::projectDirOfRpp`, which both
-  parameters-only default view): choose which bank sample this instance plays.
+  `persist` and the bridge call). Bank JSON parsed via the shared `bank_book` path (the
-- [ ] Tier-0 playback: chromatic-from-root, basic polyphony, amp envelope,
+  spike string-scan reader retired); ext-state key names shared via pure `ext_keys.h`.
-  velocity→volume — plays in REAPER's routing/record/render path like any VSTi.
+- [x] Sample selection UI (minimal, in the `IPlugView` LICE editor): a clickable list
+  of the bank's samples; the pick is the instance's own VST3 component state
+  (setState/getState), never written back to the bank.
+- [x] Tier-0 playback: chromatic-from-root, basic polyphony (16 voices), amp envelope,
+  velocity→volume — plays in REAPER's routing/record/render path like any VSTi. Sample
+  load / decode / keymap build happen off the audio thread and hand to `process` via a
+  lock-free atomic pointer swap (graveyard-reclaim); `process` never allocates.
 
 ## S5 — Tier 1: "a keymap" (zoned multisamples, per-sample root notes)
 **Goal:** Multiple bank samples zoned across the keyboard (key ranges), each with its

src/capture_paths.cpp

@@ -5,6 +5,7 @@
 #include <cstdint>
 #include <cstdio>
 #include <cstring>  // std::memcmp
+#include <filesystem>
 #include <vector>
 
 namespace reasampler {
@@ -213,6 +214,15 @@ std::string resolveBankFile(const std::string& projectDir,
     return dir + "/" + rel;
 }
 
+std::string projectDirOfRpp(const std::string& rppPath) {
+    // An unsaved project reports an empty .rpp path; keep it empty so downstream
+    // resolution refuses (no default-location fallback). Mirrors persist.cpp's prior
+    // projectDirOf exactly: parent_path of the .rpp, then normalizeSlashes.
+    if (rppPath.empty()) return {};
+    std::string dir = std::filesystem::path(rppPath).parent_path().string();
+    return normalizeSlashes(dir);
+}
+
 BankRelocation deriveRelocationPlan(const std::string& oldProjectDir,
                                     const std::string& newProjectDir) {
     BankRelocation r;

src/capture_paths.h

@@ -121,6 +121,14 @@ std::string bankRelativeForName(const std::string& fileName);
 std::string resolveBankFile(const std::string& projectDir,
                             const std::string& relativePath);
 
+// The project directory that holds a .rpp: its parent directory, forward-slashed,
+// trailing slash stripped. Empty in -> empty out (an unsaved project has an empty
+// .rpp path, which must stay empty so resolveBankFile refuses to resolve — the
+// no-default-location invariant). This is the M4 convention persist uses to place
+// the bank alongside the .rpp; extracted here (pure) so the VST3 instrument resolves
+// audio paths the SAME way persist does rather than re-implementing the derivation.
+std::string projectDirOfRpp(const std::string& rppPath);
+
 // A relocation plan for the physical bank folder on Save-As to a new project
 // location. The index's relative paths do NOT change (they are relative to the
 // project dir, which is what moved with the .rpp), so relocation is purely a

src/ext_keys.h

@@ -0,0 +1,38 @@
+#pragma once
+// ext_keys — the SINGLE SOURCE OF TRUTH for the "reasampler" project ext-state
+// namespace + key names, shared by the extension (writer, via persist.h) and the
+// VST3 instrument (reader, via the bridge). Both sides include this header so the
+// wire contract cannot drift between the two artifacts (the S4 reviewer flagged the
+// spike's duplicated constants as a drift risk).
+//
+// PURE HEADER: NO REAPER types, NO VST3 types, NO SWELL, NO vendor/ includes. Just
+// string constants, so both the REAPER-facing persist shell and the SDK-facing VST
+// bridge can include it without pulling either SDK.
+//
+// FOREVER-STABLE once shipped: these strings key every already-saved project's
+// stored state. Changing any of them orphans that state. See persist.h for the
+// per-key retirement / migration semantics — this header only owns the spellings.
+
+namespace reasampler {
+
+// The ext-state namespace all ReaSampler project state is stored under.
+inline constexpr const char* kProjExtNamespace = "reasampler";
+
+// The multi-bank key: the whole serialized BankBook (pool + named banks). This is
+// the key the VST3 instrument reads to see the live bank (read-only, S4). persist.h
+// documents its authority + the legacy-key migration around it.
+inline constexpr const char* kProjExtBanksKey = "banks";
+
+// The retired legacy single-bank key (read once on load to migrate into the pool).
+inline constexpr const char* kProjExtIndexKey = "bank_index";
+
+// The Design-View model key.
+inline constexpr const char* kProjExtViewKey = "view_state";
+
+// The docked panel's tail-setting key.
+inline constexpr const char* kProjExtTailKey = "tail_setting";
+
+// The per-project minted-GUID identity key.
+inline constexpr const char* kProjExtGuidKey = "project_guid";
+
+}  // namespace reasampler

src/persist.cpp

@@ -128,11 +128,11 @@ void* readActiveProject(std::string& rppPathOut) {
 // Parent directory of the .rpp, forward-slashed, no trailing slash. Empty in ->
 // empty out. Mirrors capture.cpp's derivation so the bank sits alongside the
 // .rpp (NOT GetProjectPathEx, which returns the recording path — see capture.cpp
-// for the full rationale). normalizeSlashes lives in capture_paths (pure).
+// for the full rationale). The derivation itself is projectDirOfRpp in capture_paths
+// (pure) — the SAME convention the VST3 instrument resolves audio paths by, so both
+// artifacts share one implementation rather than duplicating the parent-of-.rpp step.
 std::string projectDirOf(const std::string& rppPath) {
-    if (rppPath.empty()) return {};
+    return projectDirOfRpp(rppPath);
-    std::string dir = fs::path(rppPath).parent_path().string();
-    return normalizeSlashes(dir);
 }
 
 // GetProjExtState needs a caller-supplied buffer; the index JSON can be large

src/persist.h

@@ -22,6 +22,7 @@
 #include "app_version.h"
 #include "bank_book.h"
 #include "bank_model.h"
+#include "ext_keys.h"
 #include "owned_manifest.h"
 #include "prune_reconcile.h"
 #include "tail_control.h"
@@ -29,71 +30,50 @@
 
 namespace reasampler {
 
-// The ext-state namespace every ReaSampler key is stored under. CHANNEL-DERIVED (Phase V,
+// The ext-state namespace + the WIRE-SHARED key names are the contract between this
-// V4): the pure app_version module owns the one channel-qualified string — "reasampler" on
+// extension (writer) and the VST3 instrument (reader), so they live in ext_keys.h
-// stable (byte-identical to the pre-V4 build) or "reasampler_beta" on the isolated beta
+// (pure, REAPER-free) and are included above — not duplicated here. The namespace is
-// build. FOREVER-STABLE per channel once shipped: changing either orphans every already-
+// CHANNEL-DERIVED (Phase V, V4): ext_keys.h's kProjExtNamespace / this projExtNamespace()
-// saved project's state. Beta reads/writes ONLY its own namespace — a project saved by
+// both delegate to app_version's extStateNamespace() — "reasampler" on stable (byte-
-// stable shows empty/default state in beta and vice versa; that isolation is the accepted
+// identical to the pre-V4 build) or "reasampler_beta" on the isolated beta build. Both
-// V4 safety property (no cross-namespace read, migration, or fallback), not a bug.
+// artifacts read the ONE app_version symbol, so the instrument reads exactly the namespace
-// Returns const char* (not a constexpr literal) because the string is channel-derived at
+// the extension writes, per channel. Beta reads/writes ONLY its own namespace — a project
-// build time; the accessor is the single call point for all persist reads/writes below.
+// saved by stable shows empty/default state in beta and vice versa; that isolation is the
+// accepted V4 safety property (no cross-namespace read, migration, or fallback), not a bug.
+// The per-key semantics persist relies on (spellings owned by ext_keys.h):
+//   * kProjExtBanksKey  : the whole serialized BankBook (pool + named banks).
+//     AUTHORITATIVE going forward; the VST reads this key to see the live bank.
+//   * kProjExtIndexKey  : RETIRED legacy single-bank key. No longer WRITTEN (cleared
+//     on save); READ once on load to migrate a legacy project into the pool.
+//   * kProjExtViewKey   : the Design-View ViewModeModel JSON.
+//   * kProjExtTailKey   : the docked panel's TailSetting JSON.
+//   * kProjExtGuidKey   : the per-project minted GUID (content-based identity; poll()
+//     tells a Save-As from a recycled-pointer project switch by it).
+// All are FOREVER-STABLE once shipped: changing any strands every already-saved
+// project's stored state under that key.
+//
+// The accessor form of the namespace: ext_keys.h's kProjExtNamespace is the value; this
+// is the const char* the SetProjExtState/GetProjExtState calls in persist.cpp pass. Kept
+// as an accessor (not a literal) because the string is channel-derived at build time.
 inline const char* projExtNamespace() { return extStateNamespace().c_str(); }
 
-// The RETIRED legacy ext-state key: pre-multi-bank projects stored the whole
+// The two EXTENSION-ONLY keys — NOT part of the VST wire contract (the instrument
-// serialized BankIndex here (single bank). Phase B2 no longer WRITES it — on save
+// reads only banks/view/tail/guid), so they stay here rather than in ext_keys.h:
-// the key is cleared (SetProjExtState with "" deletes it) and the book is written
+//
-// under kProjExtBanksKey instead. It is still READ once, on load of a legacy
+// owned_files — the owned-file manifest JSON (project-relative files the capture path
-// project, to migrate its single index into the pool (BankBook's parse-time
+// itself created; Phase B B-cap seam, consumed by Phase R prune to tell the bank system's
-// promotion). FOREVER-STABLE as a read key for that migration path.
+// own orphans from hand-dropped files). A SIBLING key alongside banks/view/tail — NOT
-inline constexpr const char* kProjExtIndexKey = "bank_index";
+// folded into `banks`, so it stays decoupled from membership. FOREVER-STABLE: changing it
-
+// strands every saved project's ownership record (prune falls back to an empty manifest —
-// The multi-bank ext-state key (Phase B): one key holds the whole serialized
-// BankBook — the pool folded in as bank-zero plus every named bank, each with its
-// own BankIndex, ordinals, and the active-bank id. AUTHORITATIVE going forward;
-// supersedes kProjExtIndexKey. FOREVER-STABLE once shipped: changing it orphans
-// every already-saved project's banks.
-inline constexpr const char* kProjExtBanksKey = "banks";
-
-// The ext-state key the Design-View ViewModeModel JSON is stored under (one key
-// holds the whole serialized model: modes + membership + show-both + snapshots +
-// active mode). Distinct from kProjExtIndexKey — one namespace, two keys.
-// FOREVER-STABLE: changing it orphans every already-saved project's view state.
-inline constexpr const char* kProjExtViewKey = "view_state";
-
-// The ext-state key the docked panel's TailSetting JSON (mode + manualMs) is stored
-// under, so the tail choice travels inside the .rpp and loads per project. Distinct
-// from the index/view keys — one namespace, three keys. FOREVER-STABLE: changing it
-// orphans every already-saved project's tail setting (which then falls back to the
-// default — graceful, but the user's saved choice would be lost).
-inline constexpr const char* kProjExtTailKey = "tail_setting";
-
-// The ext-state key holding the owned-file manifest JSON (the set of project-relative
-// files the capture path itself created — Phase B B-cap seam, consumed by Phase R
-// prune to distinguish the bank system's own orphans from hand-dropped files). A
-// SIBLING key alongside banks/view_state/tail_setting — NOT folded into the `banks`
-// blob, so it stays decoupled from bank membership (removing an index entry is not a
-// manifest removal). One namespace, four content keys. FOREVER-STABLE: changing it
-// strands every already-saved project's ownership record, so Phase R prune could no
-// longer tell the tool's own files apart (it would fall back to an empty manifest —
 // graceful, but the attribution safety net is lost until the next capture rebuilds it).
 inline constexpr const char* kProjExtOwnedKey = "owned_files";
 
-// The ext-state key holding the ReaSampler version that last WROTE this project
+// version — the ReaSampler version that last WROTE this project (Phase V, V1). Written on
-// (Phase V, V1). Written on every save alongside the banks/view/tail keys, so every
+// every save, so every saved .rpp records which build produced its state — the seam a
-// saved .rpp records which build produced its state — the seam a future within-channel
+// future within-channel forward migration keys off. An absent key is the explicit
-// forward migration keys off ("this was written by 0.9.01, I am 0.9.05"). An absent
+// pre-versioning case, read silently, never an error. FOREVER-STABLE key string.
-// key is the explicit pre-versioning case (a project saved before this shipped), read
-// silently, never an error. FOREVER-STABLE key string once shipped.
 inline constexpr const char* kProjExtVersionKey = "version";
 
-// The ext-state key holding a GUID we mint per project to establish CONTENT-BASED
-// project identity (REAPER exposes no stable per-project GUID). poll() uses it to
-// tell a genuine Save-As (same GUID, new .rpp path) apart from a project switch
-// onto a recycled ReaProject* pointer (different GUID). FOREVER-STABLE: changing
-// it strands the identity of every already-saved project. See persist.cpp.
-inline constexpr const char* kProjExtGuidKey = "project_guid";
-
 // Owns the session's BankBook (Phase B: pool + named banks) and drives persistence
 // against the active REAPER project. One instance lives for the extension's
 // lifetime (main.cpp). It tracks

src/vst/bridge_marshal.cpp

@@ -13,66 +13,4 @@ std::optional<std::string> decodeGetProjExtState(int apiReturn,
     return buffer;
 }
 
-std::optional<std::string> extractJsonStringField(const std::string& json,
-                                                  const std::string& key) {
-    // Find the member token: "key" followed (after optional whitespace) by ':' then a
-    // quoted string. Scan for each candidate occurrence of the quoted key so a value
-    // that happens to contain the key text can't produce a false match.
-    const std::string needle = "\"" + key + "\"";
-    size_t searchFrom = 0;
-
-    while (true) {
-        const size_t keyPos = json.find(needle, searchFrom);
-        if (keyPos == std::string::npos) return std::nullopt;
-
-        size_t i = keyPos + needle.size();
-        searchFrom = i;  // next candidate starts after this key token
-
-        // Skip whitespace to the ':'.
-        while (i < json.size() &&
-               (json[i] == ' ' || json[i] == '\t' || json[i] == '\n' ||
-                json[i] == '\r')) {
-            ++i;
-        }
-        if (i >= json.size() || json[i] != ':') continue;  // not a member — keep looking
-        ++i;
-
-        // Skip whitespace to the value.
-        while (i < json.size() &&
-               (json[i] == ' ' || json[i] == '\t' || json[i] == '\n' ||
-                json[i] == '\r')) {
-            ++i;
-        }
-        if (i >= json.size() || json[i] != '"') return std::nullopt;  // value not a string
-        ++i;
-
-        // Read the string body, honoring the common JSON escapes.
-        std::string out;
-        while (i < json.size()) {
-            const char c = json[i];
-            if (c == '\\') {
-                if (i + 1 >= json.size()) return std::nullopt;  // dangling escape
-                const char e = json[i + 1];
-                switch (e) {
-                    case '"': out.push_back('"'); break;
-                    case '\\': out.push_back('\\'); break;
-                    case '/': out.push_back('/'); break;
-                    case 'n': out.push_back('\n'); break;
-                    case 't': out.push_back('\t'); break;
-                    case 'r': out.push_back('\r'); break;
-                    case 'b': out.push_back('\b'); break;
-                    case 'f': out.push_back('\f'); break;
-                    default: out.push_back(e); break;  // pass through unknown escapes
-                }
-                i += 2;
-                continue;
-            }
-            if (c == '"') return out;  // closing quote — done
-            out.push_back(c);
-            ++i;
-        }
-        return std::nullopt;  // unterminated string
-    }
-}
-
 }  // namespace reasampler::vst

src/vst/bridge_marshal.h

@@ -1,18 +1,21 @@
-// bridge_marshal.h — PURE marshalling helpers for the REAPER VST-host bridge read
+// bridge_marshal.h — PURE marshalling helper for the REAPER VST-host bridge read.
-// (Phase S1). NO VST3, NO REAPER types at the boundary.
+// NO VST3, NO REAPER types at the boundary.
 //
 // The bridge shell (reaper_bridge.cpp) resolves REAPER API functions by name over the
-// host callback and invokes them; the fiddly-and-easy-to-get-wrong parts around those
+// host callback and invokes them; the one fiddly-and-easy-to-get-wrong part around
-// calls — interpreting GetProjExtState's int return, walking EnumProjExtState's
+// GetProjExtState — interpreting its int return against the buffer it filled — is pure
-// index-until-false contract into a key set, and extracting a single value out of the
+// and unit-tested here. Mirror of capture_paths / wav_trim splitting the arithmetic out
-// "reasampler" bank JSON — are pure and unit-tested here. Mirror of capture_paths /
+// of a REAPER-facing shell.
-// wav_trim splitting the arithmetic out of a REAPER-facing shell.
+//
+// The S1 spike ALSO carried a string-scan JSON reader (extractJsonStringField) as a
+// stand-in until the instrument could parse the bank properly. S4 retired it: the
+// instrument now parses the "reasampler" bank blob through the SHARED bank_book /
+// bank_model JSON path (sample_map.cpp), so there is no second JSON parser. This module
+// is back to its one honest job — the API-return decode.
 //
 // Verified against vendor/reaper-sdk/sdk/reaper_plugin_functions.h:
 //   int  GetProjExtState (ReaProject*, extname, key, valOutNeedBig, valOutNeedBig_sz);
 //        -- returns the length written (0 when the key is absent).
-//   bool EnumProjExtState(ReaProject*, extname, idx, keyOut, keyOut_sz, valOut, valOut_sz);
-//        -- returns false when idx is past the last entry.
 
 #pragma once
 
@@ -31,17 +34,4 @@ namespace reasampler::vst {
 std::optional<std::string> decodeGetProjExtState(int apiReturn,
                                                   const std::string& buffer);
 
-// Extract the string value for `key` out of a flat one-level JSON object — the shape
-// persist.cpp writes under the "reasampler" ext-state (e.g. the bank blob's top-level
-// fields). This is a deliberately small, dependency-free reader for the SPIKE's
-// "read a known value" proof, NOT a general JSON parser: it finds "key" as an object
-// member and returns its string value, handling the common escapes (\" \\ \n \t \/).
-// Returns nullopt if the key is absent or its value is not a string.
-//
-// The real instrument (S4) will read the bank index through the shared bank_model JSON
-// path, not this helper; this exists only to give the S1 bridge read a testable,
-// REAPER-free decode step.
-std::optional<std::string> extractJsonStringField(const std::string& json,
-                                                  const std::string& key);
-
 }  // namespace reasampler::vst

src/vst/editor_geometry.cpp

@@ -53,4 +53,23 @@ HitTarget hitTest(const EditorLayout& layout, int x, int y) {
     return HitTarget::kNone;
 }
 
+Rect sampleRowRect(const EditorLayout& layout, int index) {
+    if (index < 0) return Rect{};
+    const int top = layout.canvas.top + index * kSampleRowHeight;
+    return Rect{layout.canvas.left, top, layout.canvas.right, top + kSampleRowHeight};
+}
+
+int sampleRowHitTest(const EditorLayout& layout, int rowCount, int x, int y) {
+    if (rowCount <= 0) return -1;
+    // Must be within the canvas horizontally and at/below its top.
+    if (x < layout.canvas.left || x >= layout.canvas.right) return -1;
+    if (y < layout.canvas.top) return -1;
+    const int index = (y - layout.canvas.top) / kSampleRowHeight;
+    if (index < 0 || index >= rowCount) return -1;
+    // Guard the bottom edge: a click below the last row's canvas bottom is outside.
+    const Rect r = sampleRowRect(layout, index);
+    if (y >= r.bottom) return -1;
+    return index;
+}
+
 }  // namespace reasampler::vst

src/vst/editor_geometry.h

@@ -56,4 +56,23 @@ enum class HitTarget {
 // is kNone in the spike.
 HitTarget hitTest(const EditorLayout& layout, int x, int y);
 
+// --- Sample-selection list (S4 Tier-0 UI) -----------------------------------
+//
+// The Tier-0 editor lists the bank's samples as a vertical stack of fixed-height rows
+// below the title bar; clicking a row selects that sample. This is the pure geometry:
+// the row rectangles and the point->row hit-test, unit-tested outside the DAW while the
+// shell draws the names and routes the click into the processor's reloadFromBank.
+
+// The fixed row height (px) for one sample entry. Exposed so the shell and tests agree.
+inline constexpr int kSampleRowHeight = 22;
+
+// The rectangle for row `index` (0-based) of the sample list, laid out top-down inside
+// the layout's canvas. Rows beyond what the canvas can show are still computed (the
+// shell clips at paint time); a negative index yields an empty rect. Pure.
+Rect sampleRowRect(const EditorLayout& layout, int index);
+
+// The row index a click at (x, y) lands on, given `rowCount` rows, or -1 for a click
+// outside the list (above the first row, past the last, or on the title bar). Pure.
+int sampleRowHitTest(const EditorLayout& layout, int rowCount, int x, int y);
+
 }  // namespace reasampler::vst

src/vst/reaper_bridge.cpp

@@ -5,6 +5,8 @@
 #include <vector>
 
 #include "bridge_marshal.h"
+#include "capture_paths.h"  // projectDirOfRpp (shared M4 project-dir derivation)
+#include "ext_keys.h"       // kProjExtNamespace (shared wire contract)
 
 // The VST3 base types must be included before REAPER's VST3 interface header, which
 // uses FUnknown / CStringA / uint32 / DECLARE_CLASS_IID / PLUGIN_API from
@@ -27,21 +29,17 @@ namespace Steinberg {
 // interface (FUnknownPtr uses the iid), so the definition lives with its sole use.
 DEF_CLASS_IID(Steinberg::IReaperHostApplication)
 
-// The "reasampler" ext-state namespace + bank key. Kept in sync with persist.h by
+// The "reasampler" ext-state namespace is the SHARED wire contract between the
-// value (the extension writes them); we only READ here, so we duplicate the two string
+// extension (writer) and this instrument (reader); it lives in ext_keys.h (pure,
-// constants rather than pull the whole REAPER-facing persist.h into the VST artifact.
+// REAPER-free) — reasampler::kProjExtNamespace — so the two artifacts read one symbol
-// If persist.h's kProjExtNamespace / kProjExtBanksKey ever change, these must follow —
+// and cannot drift. The S1 spike duplicated it locally; that duplication is retired.
-// they are the shared wire contract between the extension (writer) and instrument
-// (reader). VERIFY against persist.h.
-namespace {
-constexpr const char* kReasamplerNamespace = "reasampler";
-}
 
 namespace reasampler::vst {
 
 bool ReaperBridge::connect(Steinberg::FUnknown* context) {
     getProjExtState_ = nullptr;
     enumProjExtState_ = nullptr;
+    enumProjects_ = nullptr;
     hostApp_ = nullptr;
     if (!context) return false;
 
@@ -58,6 +56,10 @@ bool ReaperBridge::connect(Steinberg::FUnknown* context) {
         reaper->getReaperApi("GetProjExtState"));
     enumProjExtState_ = reinterpret_cast<EnumProjExtStateFn>(
         reaper->getReaperApi("EnumProjExtState"));
+    // EnumProjects(-1, ...) yields the active project AND its .rpp path — the same call
+    // persist.cpp uses, so the instrument derives the project directory identically.
+    enumProjects_ = reinterpret_cast<EnumProjectsFn>(
+        reaper->getReaperApi("EnumProjects"));
 
     return getProjExtState_ != nullptr;
 }
@@ -74,15 +76,35 @@ std::optional<std::string> ReaperBridge::readReasamplerExtState(const std::strin
     // REAPER treats null as the current project for these calls, so we pass it through
     // rather than bailing — but if the read yields nothing the caller sees nullopt.
 
-    // GetProjExtState writes into a caller buffer; size it generously for a JSON blob
+    // GetProjExtState writes into a caller buffer; the bank blob can be large (many
-    // and let the pure decoder interpret the result. The buffer is NUL-terminated by
+    // samples), so grow the buffer until the value fits rather than risk a silent
-    // REAPER on success.
+    // truncation — mirrors persist.cpp's getProjExtStateString growing strategy. The
-    std::vector<char> buf(64 * 1024, '\0');
+    // return value is the value length; if it fits strictly inside the buffer it is
-    const int rv = getProjExtState_(proj, kReasamplerNamespace, key.c_str(), buf.data(),
+    // complete, else grow and retry up to a 16 MB ceiling.
-                                    static_cast<int>(buf.size()));
+    for (int cap = 1 << 16; cap <= (1 << 24); cap <<= 2) {
+        std::vector<char> buf(static_cast<std::size_t>(cap), '\0');
+        const int rv = getProjExtState_(proj, kProjExtNamespace, key.c_str(),
+                                        buf.data(), cap);
+        if (rv <= 0) return std::nullopt;  // absent / empty key
+        std::string s(buf.data());
+        if (static_cast<int>(s.size()) + 1 < cap) {
+            return decodeGetProjExtState(rv, s);
+        }
+        // else: possibly truncated -> grow and retry.
+    }
+    return std::nullopt;  // pathologically large (>16 MB) — give up rather than loop
+}
 
-    // Marshal the raw result through the pure decoder (handles the absent-key case).
+std::string ReaperBridge::activeProjectDir() {
-    return decodeGetProjExtState(rv, std::string(buf.data()));
+    if (!enumProjects_) return {};
+    // idx=-1 is the current project tab; the out-buffer receives the full .rpp path,
+    // EMPTY for a never-saved project. Same call + convention as persist.cpp; the pure
+    // projectDirOfRpp turns the .rpp path into the project directory (parent, forward-
+    // slashed) and keeps an unsaved project's empty path empty (no default-location
+    // fallback — the tool's invariant).
+    std::vector<char> buf(4096, '\0');
+    enumProjects_(-1, buf.data(), static_cast<int>(buf.size()));
+    return projectDirOfRpp(std::string(buf.data()));
 }
 
 }  // namespace reasampler::vst

src/vst/reaper_bridge.h

@@ -44,8 +44,19 @@ public:
     // Read a "reasampler" ext-state value by key from the host's active project.
     // Returns nullopt when unconnected, when the project can't be resolved, or when the
     // key is absent. This is the S1 read-spike entry point.
+    //
+    // NOT REAL-TIME SAFE (it allocates a read buffer and calls into REAPER): callers on
+    // the audio thread MUST NOT invoke it. The S4 instrument reads on the main/UI thread
+    // and hands a snapshot to the process path (see reasampler_processor.cpp).
     std::optional<std::string> readReasamplerExtState(const std::string& key);
 
+    // The active project's directory (the folder holding its .rpp), forward-slashed,
+    // no trailing slash — the M4 convention persist uses to place the bank alongside
+    // the .rpp. Empty for an unsaved project or when unconnected. The instrument
+    // resolves relative sample paths against this the SAME way persist does
+    // (capture_paths::projectDirOfRpp over EnumProjects(-1)'s .rpp path). Not RT-safe.
+    std::string activeProjectDir();
+
 private:
     // Resolved REAPER API function pointers (by name via getReaperApi). Signatures
     // verified against reaper_plugin_functions.h.
@@ -54,10 +65,15 @@ private:
     using EnumProjExtStateFn = bool (*)(void* proj, const char* extname, int idx,
                                         char* keyOut, int keyOut_sz, char* valOut,
                                         int valOut_sz);
+    // EnumProjects(-1, projfnOut, sz) -> active project + its .rpp path (SDK line
+    // ~1264). The instrument uses idx=-1 (current tab) so it follows the active project,
+    // and reads the .rpp path from the out-buffer exactly as persist.cpp does.
+    using EnumProjectsFn = void* (*)(int idx, char* projfnOut, int projfnOut_sz);
 
     void* hostApp_ = nullptr;  // IReaperHostApplication* (opaque here; used in .cpp)
     GetProjExtStateFn getProjExtState_ = nullptr;
     EnumProjExtStateFn enumProjExtState_ = nullptr;
+    EnumProjectsFn enumProjects_ = nullptr;
 };
 
 }  // namespace reasampler::vst

src/vst/reasampler_editor.cpp

@@ -7,7 +7,10 @@
 #include <string>
 
 #include "editor_geometry.h"
+#include "ext_keys.h"
 #include "reaper_bridge.h"
+#include "reasampler_processor.h"
+#include "sample_map.h"
 
 #ifdef _WIN32
 #include <windowsx.h>  // GET_X_LPARAM / GET_Y_LPARAM
@@ -47,13 +50,25 @@ void drawText(LICE_IBitmap* bmp, const Rect& r, const char* s, COLORREF col) {
 #endif
 }  // namespace
 
-ReaSamplerEditor::ReaSamplerEditor(ReaperBridge* bridge)
+ReaSamplerEditor::ReaSamplerEditor(ReaSamplerProcessor* processor)
-    : CPluginView(nullptr), bridge_(bridge) {
+    : CPluginView(nullptr), processor_(processor) {
     // Default view size; the host may resize (canResize() == true).
     ViewRect r(0, 0, 420, 260);
     setRect(r);
 }
 
+void ReaSamplerEditor::refreshSampleList() {
+    // Main/UI thread only — reads the live bank over the bridge (allocates, calls REAPER).
+    if (!processor_) {
+        samples_.clear();
+        selectedId_.clear();
+        return;
+    }
+    auto banks = processor_->bridge().readReasamplerExtState(reasampler::kProjExtBanksKey);
+    samples_ = banks ? listSamples(*banks) : std::vector<SampleChoice>{};
+    selectedId_ = processor_->selectedSampleId();
+}
+
 ReaSamplerEditor::~ReaSamplerEditor() {
 #ifdef _WIN32
     // Defensive teardown: the host normally calls removed() (which destroys the child)
@@ -102,6 +117,9 @@ void ReaSamplerEditor::attachedToParent() {
         classRegistered = true;
     }
 
+    // Snapshot the live bank so the first paint shows the sample list.
+    refreshSampleList();
+
     const ViewRect& r = getRect();
     childHwnd_ = CreateWindowExW(
         0, kChildClassName, L"", WS_CHILD | WS_VISIBLE, 0, 0, r.getWidth(),
@@ -144,17 +162,13 @@ void ReaSamplerEditor::paint(HDC hdc) {
 
     const EditorLayout layout = layoutEditor(w, h);
 
-    // Title band.
+    // Title band: the plugin name + whether a live bank is linked.
     LICE_FillRect(&bmp, layout.titleBar.left, layout.titleBar.top,
                   layout.titleBar.width(), layout.titleBar.height(), kColTitleBg, 1.0f,
                   0);
-    // Read a known live-state value over the bridge to prove the read spike. Show the
-    // raw ext-state presence (never the full blob) so the title reflects live project
-    // state without dumping JSON into the UI.
     std::string title = "ReaSampler Instrument";
-    if (bridge_ && bridge_->isConnected()) {
+    if (processor_ && processor_->bridge().isConnected()) {
-        auto banks = bridge_->readReasamplerExtState("banks");
+        title += samples_.empty() ? "  [bank: empty]" : "  [pick a sample]";
-        title += banks ? "  [bank: linked]" : "  [bank: none]";
     } else {
         title += "  [host: no bridge]";
     }
@@ -162,27 +176,42 @@ void ReaSamplerEditor::paint(HDC hdc) {
                    layout.titleBar.right - 8, layout.titleBar.bottom};
     drawText(&bmp, titleText, title.c_str(), kRgbText);
 
-    // The clickable button — fill reflects the last hit-test (the routing proof).
+    // Sample list: one row per bank sample, the selected one highlighted. Clip drawing
-    LICE_FillRect(&bmp, layout.button.left, layout.button.top, layout.button.width(),
+    // to rows that fall within the canvas (sampleRowRect computes all; we skip off-screen
-                  layout.button.height(), buttonHit_ ? kColBtnHitBg : kColBtnBg, 1.0f,
+    // ones so a huge bank doesn't waste paint).
-                  0);
+    for (int i = 0; i < static_cast<int>(samples_.size()); ++i) {
-    LICE_DrawRect(&bmp, layout.button.left, layout.button.top, layout.button.width(),
+        const Rect row = sampleRowRect(layout, i);
-                  layout.button.height(), kColBtnBorder, 1.0f, 0);
+        if (row.top >= layout.canvas.bottom) break;  // past the visible area
-    Rect btnText{layout.button.left + 8, layout.button.top, layout.button.right,
+        const bool sel = !selectedId_.empty() && samples_[i].id == selectedId_;
-                 layout.button.bottom};
+        LICE_FillRect(&bmp, row.left, row.top, row.width(), row.height(),
-    drawText(&bmp, btnText, buttonHit_ ? "clicked" : "click me", kRgbText);
+                      sel ? kColBtnHitBg : kColBtnBg, 1.0f, 0);
+        if (sel) {
+            LICE_DrawRect(&bmp, row.left, row.top, row.width() - 1, row.height() - 1,
+                          kColBtnBorder, 1.0f, 0);
+        }
+        Rect textR{row.left + 8, row.top, row.right - 8, row.bottom};
+        const std::string& name = samples_[i].displayName;
+        drawText(&bmp, textR, name.empty() ? samples_[i].id.c_str() : name.c_str(),
+                 kRgbText);
+    }
 
     BitBlt(hdc, 0, 0, w, h, bmp.getDC(), 0, 0, SRCCOPY);
 }
 
 void ReaSamplerEditor::onClick(int x, int y) {
+    if (!processor_) return;
     RECT cr{};
     GetClientRect(childHwnd_, &cr);
     const EditorLayout layout = layoutEditor(cr.right - cr.left, cr.bottom - cr.top);
-    if (hitTest(layout, x, y) == HitTarget::kButton) {
+    const int row = sampleRowHitTest(layout, static_cast<int>(samples_.size()), x, y);
-        buttonHit_ = !buttonHit_;
+    if (row < 0) return;
+
+    // Select this sample and rebuild the instrument OFF the audio thread (this WM_
+    // handler runs on the UI thread). reloadFromBank reads the id we just set.
+    processor_->setSelectedSampleId(samples_[row].id);
+    processor_->reloadFromBank();
+    selectedId_ = samples_[row].id;
     InvalidateRect(childHwnd_, nullptr, FALSE);
-    }
 }
 
 LRESULT CALLBACK ReaSamplerEditor::wndProc(HWND hwnd, UINT msg, WPARAM wParam,

src/vst/reasampler_editor.h

@@ -13,21 +13,27 @@
 
 #pragma once
 
+#include <string>
+#include <vector>
+
 #include "public.sdk/source/common/pluginview.h"
 
+#include "sample_map.h"  // SampleChoice (the list the editor draws)
+
 #ifdef _WIN32
 #include <windows.h>
 #endif
 
 namespace reasampler::vst {
 
-class ReaperBridge;
+class ReaSamplerProcessor;
 
 class ReaSamplerEditor : public Steinberg::CPluginView {
 public:
-    // `bridge` is owned by the processor and outlives the editor; the editor reads
+    // `processor` owns this editor's lifetime domain and outlives it; the editor reads
-    // (never mutates) it to show a live-state readout. May be null (non-REAPER host).
+    // the live bank through it (the sample list) and drives selection + reload when the
-    explicit ReaSamplerEditor(ReaperBridge* bridge);
+    // user clicks a row. May be null (defensive — a real host always supplies one).
+    explicit ReaSamplerEditor(ReaSamplerProcessor* processor);
     ~ReaSamplerEditor() override;
 
     // Accept only the Windows HWND platform type (D5: Windows-only).
@@ -48,7 +54,7 @@ private:
 #ifdef _WIN32
     // Draw the current surface into the child window's DC via a LICE bitmap.
     void paint(HDC hdc);
-    // Route a client-space click through editor_geometry::hitTest.
+    // Route a client-space click: select the sample row under (x, y), if any.
     void onClick(int x, int y);
 
     static LRESULT CALLBACK wndProc(HWND hwnd, UINT msg, WPARAM wParam, LPARAM lParam);
@@ -56,10 +62,16 @@ private:
     HWND childHwnd_ = nullptr;
 #endif
 
-    ReaperBridge* bridge_ = nullptr;
+    // Re-read the bank's sample list from the live bridge into `samples_`. Main/UI
-    // Latched on click so the paint reflects the last hit-test result — the spike's
+    // thread only (reads ext-state); called on attach and after a selection reload.
-    // proof that host->click->draw routing round-trips.
+    void refreshSampleList();
-    bool buttonHit_ = false;
+
+    ReaSamplerProcessor* processor_ = nullptr;
+    // The bank's samples, snapshotted for the current paint. Refreshed off the audio
+    // thread; the paint just draws it.
+    std::vector<SampleChoice> samples_;
+    // The currently-selected sample id, mirrored for the paint's highlight.
+    std::string selectedId_;
 };
 
 }  // namespace reasampler::vst

src/vst/reasampler_processor.cpp

@@ -2,16 +2,64 @@
 
 #include "reasampler_processor.h"
 
+#include <cstdint>
+#include <fstream>
+#include <vector>
+
+#include "pluginterfaces/base/ibstream.h"
 #include "pluginterfaces/vst/ivstaudioprocessor.h"
+#include "pluginterfaces/vst/ivstevents.h"
 #include "pluginterfaces/vst/vstspeaker.h"
 
+#include "capture_paths.h"   // resolveBankFile (shared M4 path resolution)
+#include "ext_keys.h"        // kProjExtBanksKey (shared wire contract)
 #include "reasampler_editor.h"
+#include "sample_map.h"       // selectSample, downmixToMono, buildTier0Keymap, state (de)ser
+#include "wav_trim.h"         // parseWavLayout, extractFloatFrames (shared WAV parse)
 
 using namespace Steinberg;
 using namespace Steinberg::Vst;
 
 namespace reasampler::vst {
 
+namespace {
+
+// Tier-0 fixed instrument shape (Tier 2 makes these editable). A gentle amp envelope so
+// notes neither click on nor cut off abruptly; sustain at unity (velocity does the
+// dynamics), a short release for a natural tail. Times are in seconds, converted to
+// frames against the live sample rate at build time.
+constexpr double kAttackSeconds = 0.003;
+constexpr double kDecaySeconds = 0.0;
+constexpr double kSustainLevel = 1.0;
+constexpr double kReleaseSeconds = 0.060;
+constexpr std::size_t kMaxVoices = 16;
+
+AdsrParams tier0Adsr(double sampleRate) {
+    const double sr = sampleRate > 0.0 ? sampleRate : 44100.0;
+    AdsrParams p;
+    p.attackFrames = static_cast<std::int64_t>(kAttackSeconds * sr);
+    p.decayFrames = static_cast<std::int64_t>(kDecaySeconds * sr);
+    p.sustainLevel = kSustainLevel;
+    p.releaseFrames = static_cast<std::int64_t>(kReleaseSeconds * sr);
+    return p;
+}
+
+// Read a whole file into a byte buffer. Off-thread only (blocking file I/O). Empty on
+// any failure — the caller treats an unreadable WAV as "nothing to play".
+std::vector<std::uint8_t> readFileBytes(const std::string& path) {
+    std::vector<std::uint8_t> bytes;
+    std::ifstream f(path, std::ios::binary | std::ios::ate);
+    if (!f) return bytes;
+    const std::streamoff size = f.tellg();
+    if (size <= 0) return bytes;
+    f.seekg(0, std::ios::beg);
+    bytes.resize(static_cast<std::size_t>(size));
+    if (!f.read(reinterpret_cast<char*>(bytes.data()), size)) bytes.clear();
+    return bytes;
+}
+
+}  // namespace
+
 FUnknown* ReaSamplerProcessor::createInstance(void* /*context*/) {
     // The host owns the returned reference. Cast up to the combined interface the SDK
     // exposes (IAudioProcessor) so the FUnknown refcount is correctly rooted.
@@ -23,7 +71,7 @@ tresult PLUGIN_API ReaSamplerProcessor::initialize(FUnknown* context) {
     if (result != kResultOk) return result;
 
     // Connect the REAPER bridge. Non-fatal if it fails (non-REAPER host): the
-    // instrument still loads, the editor just shows "no bridge".
+    // instrument still loads, it just has no live bank to play.
     bridge_.connect(context);
 
     // Instrument bus topology: one event input (MIDI in, 16 channels), one stereo audio
@@ -35,46 +83,204 @@ tresult PLUGIN_API ReaSamplerProcessor::initialize(FUnknown* context) {
 }
 
 tresult PLUGIN_API ReaSamplerProcessor::terminate() {
+    // process() is not running at terminate. Free the live instrument and drain the
+    // graveyard. Take the pointer out of the atomic first so nothing else races it.
+    std::lock_guard<std::mutex> lock(reloadMutex_);
+    delete live_.exchange(nullptr);
+    graveyard_.clear();
     return SingleComponentEffect::terminate();
 }
 
-tresult PLUGIN_API ReaSamplerProcessor::setActive(TBool /*state*/) {
+tresult PLUGIN_API ReaSamplerProcessor::setActive(TBool state) {
-    // Nothing to allocate/free in the silent skeleton; S4 will size voice buffers here
+    // Activating: build the instrument from the currently-selected sample so the first
-    // against the setupProcessing block size.
+    // block after activation can play. Deactivating: process is now GUARANTEED stopped by
+    // the host, so this is the safe point to reclaim the graveyard (the displaced engines
+    // no reload could free while active). The build/drain are off the audio thread —
+    // setActive is a main/UI-thread call.
+    if (state) {
+        reloadFromBank();
+    } else {
+        std::lock_guard<std::mutex> lock(reloadMutex_);
+        graveyard_.clear();
+    }
     return kResultOk;
 }
 
 tresult PLUGIN_API ReaSamplerProcessor::setupProcessing(ProcessSetup& setup) {
+    sampleRate_ = setup.sampleRate;
+    maxBlockSize_ = setup.maxSamplesPerBlock;
     return SingleComponentEffect::setupProcessing(setup);
 }
 
+tresult PLUGIN_API ReaSamplerProcessor::setState(IBStream* state) {
+    if (!state) return kResultFalse;
+    // Read the whole component-state blob (the selected sample id, versioned). The blob
+    // is small; read in one shot into a growable buffer.
+    std::vector<std::uint8_t> bytes;
+    std::uint8_t chunk[256];
+    int32 got = 0;
+    while (state->read(chunk, sizeof(chunk), &got) == kResultOk && got > 0) {
+        bytes.insert(bytes.end(), chunk, chunk + got);
+        if (got < static_cast<int32>(sizeof(chunk))) break;
+    }
+    setSelectedSampleId(deserializeSelection(bytes));
+    // Rebuild from the restored selection (off-thread — setState is a load-time call).
+    reloadFromBank();
+    return kResultOk;
+}
+
+tresult PLUGIN_API ReaSamplerProcessor::getState(IBStream* state) {
+    if (!state) return kResultFalse;
+    const std::vector<std::uint8_t> bytes = serializeSelection(selectedSampleId());
+    if (!bytes.empty()) {
+        state->write(const_cast<std::uint8_t*>(bytes.data()),
+                     static_cast<int32>(bytes.size()), nullptr);
+    }
+    return kResultOk;
+}
+
+std::string ReaSamplerProcessor::selectedSampleId() {
+    std::lock_guard<std::mutex> lock(selectionMutex_);
+    return selectedSampleId_;
+}
+
+void ReaSamplerProcessor::setSelectedSampleId(const std::string& id) {
+    std::lock_guard<std::mutex> lock(selectionMutex_);
+    selectedSampleId_ = id;
+}
+
+std::string ReaSamplerProcessor::reloadFromBank() {
+    // OFF THE AUDIO THREAD. Serialize concurrent reloads (editor click + setState) so
+    // the retired-slot free is single-writer. This mutex is NEVER taken on the audio
+    // thread — process() only touches the atomic.
+    std::lock_guard<std::mutex> lock(reloadMutex_);
+
+    // 1. Read the live bank + resolve the project dir over the bridge (allocates,
+    //    calls REAPER — fine here, off-thread).
+    std::optional<std::string> banksJson =
+        bridge_.readReasamplerExtState(kProjExtBanksKey);
+    const std::string projectDir = bridge_.activeProjectDir();
+
+    std::string resolvedId;
+    std::unique_ptr<LoadedInstrument> built;
+
+    if (banksJson) {
+        // 2. Pick the sample (shared bank_book JSON parse — NOT a second parser).
+        std::optional<SelectedSample> sel =
+            selectSample(*banksJson, selectedSampleId());
+        if (sel) {
+            // 3. Resolve the project-relative WAV path the M4 way persist does, read +
+            //    decode it (file I/O off-thread), downmix to the core's mono contract.
+            const std::string abs = resolveBankFile(projectDir, sel->relativePath);
+            if (!abs.empty()) {
+                const std::vector<std::uint8_t> bytes = readFileBytes(abs);
+                const WavLayout layout = parseWavLayout(bytes);
+                if (layout.valid) {
+                    const std::size_t frames = layout.frameCount();
+                    std::vector<AudioSample> interleaved =
+                        extractFloatFrames(bytes, layout, 0, frames);
+                    std::vector<AudioSample> mono =
+                        downmixToMono(interleaved, layout.channelCount);
+                    if (!mono.empty()) {
+                        Keymap km = buildTier0Keymap(
+                            std::move(mono),
+                            static_cast<int>(layout.sampleRate), sel->rootNote,
+                            sel->loop);
+                        built = std::make_unique<LoadedInstrument>(
+                            std::move(km), kMaxVoices, tier0Adsr(sampleRate_));
+                        // Record which id actually resolved so a first-sample fallback
+                        // (empty stored id) becomes the concrete selection.
+                        resolvedId = selectedSampleId();
+                    }
+                }
+            }
+        }
+    }
+
+    // 4. Publish. Atomically install the new instrument; the DISPLACED one goes to the
+    //    graveyard (process may still be reading it this block — it is reclaimed only when
+    //    process is stopped, in setActive(false)/terminate). A null `built` (no bank /
+    //    unreadable WAV) installs silence. `built` is heap-owned; release() hands
+    //    ownership to the atomic, and the exchanged pointer is re-owned by the graveyard.
+    LoadedInstrument* prev = live_.exchange(built.release());
+    if (prev) graveyard_.emplace_back(prev);
+    return resolvedId;
+}
+
 tresult PLUGIN_API ReaSamplerProcessor::process(ProcessData& data) {
-    // Silent skeleton: emit silence on the output bus so the instrument runs cleanly in
+    // REAL-TIME: no allocation, no IO, no locks. Load the live instrument once for the
-    // REAPER's render/record path without a null buffer. S4 marshals MIDI->core->audio.
+    // whole block (a single atomic acquire).
-    if (data.numOutputs > 0 && data.outputs && data.numSamples > 0) {
+    LoadedInstrument* inst = live_.load(std::memory_order_acquire);
+
+    // Marshal MIDI note-on/off from the event input into the voice engine. Tier 0 maps
+    // events at block granularity (no per-event sample-offset split) — audible timing is
+    // within one block, adequate for Tier 0; sample-accurate scheduling is a later tier.
+    if (inst && data.inputEvents) {
+        const int32 count = data.inputEvents->getEventCount();
+        for (int32 i = 0; i < count; ++i) {
+            Event e;
+            if (data.inputEvents->getEvent(i, e) != kResultOk) continue;
+            if (e.type == Event::kNoteOnEvent) {
+                // A note-on with velocity 0 is a note-off by MIDI convention.
+                const int vel = static_cast<int>(e.noteOn.velocity * 127.0f + 0.5f);
+                if (vel <= 0) {
+                    inst->engine.noteOff(e.noteOn.pitch);
+                } else {
+                    inst->engine.noteOn(e.noteOn.pitch, vel);
+                }
+            } else if (e.type == Event::kNoteOffEvent) {
+                inst->engine.noteOff(e.noteOff.pitch);
+            }
+        }
+    }
+
+    if (data.numOutputs <= 0 || !data.outputs || data.numSamples <= 0) {
+        return kResultOk;
+    }
     AudioBusBuffers& out = data.outputs[0];
+    const int32 frames = data.numSamples;
+
+    // 64-bit host processing is not supported by the mono float core; emit silence
+    // rather than mis-render. REAPER runs 32-bit float by default.
+    if (data.symbolicSampleSize != kSample32) {
         for (int32 ch = 0; ch < out.numChannels; ++ch) {
-            if (data.symbolicSampleSize == kSample32) {
-                if (float* buf = out.channelBuffers32[ch]) {
-                    for (int32 i = 0; i < data.numSamples; ++i) buf[i] = 0.f;
-                }
-            } else if (data.symbolicSampleSize == kSample64) {
             if (double* buf = out.channelBuffers64[ch]) {
-                    for (int32 i = 0; i < data.numSamples; ++i) buf[i] = 0.0;
+                for (int32 i = 0; i < frames; ++i) buf[i] = 0.0;
             }
         }
-        }
-        // Flag output silence so the host can optimize (nothing plays yet).
         out.silenceFlags = (out.numChannels >= 64)
                                ? ~0ULL
                                : ((1ULL << out.numChannels) - 1);
+        return kResultOk;
     }
+
+    // Render mono into channel 0's buffer, then replicate to the other channels (the
+    // core is mono-per-sample). Clear channel 0 first (render ADDS), then mix.
+    float* ch0 = out.numChannels > 0 ? out.channelBuffers32[0] : nullptr;
+    if (ch0) {
+        for (int32 i = 0; i < frames; ++i) ch0[i] = 0.f;
+        if (inst) {
+            inst->engine.render(ch0, static_cast<std::size_t>(frames));
+        }
+        // Duplicate the mono render across the remaining output channels.
+        for (int32 ch = 1; ch < out.numChannels; ++ch) {
+            if (float* buf = out.channelBuffers32[ch]) {
+                for (int32 i = 0; i < frames; ++i) buf[i] = ch0[i];
+            }
+        }
+    }
+
+    // Report silence only when nothing is loaded (lets the host optimize when idle).
+    // With an instrument loaded we clear the flag so a ringing voice is not skipped.
+    out.silenceFlags = inst ? 0 : ((out.numChannels >= 64)
+                                       ? ~0ULL
+                                       : ((1ULL << out.numChannels) - 1));
     return kResultOk;
 }
 
 IPlugView* PLUGIN_API ReaSamplerProcessor::createView(FIDString name) {
     if (name && FIDStringsEqual(name, ViewType::kEditor)) {
-        return new ReaSamplerEditor(&bridge_);
+        return new ReaSamplerEditor(this);
     }
     return nullptr;
 }

src/vst/reasampler_processor.h

@@ -1,21 +1,53 @@
-// reasampler_processor.h — the VST3 SingleComponentEffect skeleton (Phase S1). THIN
+// reasampler_processor.h — the VST3 SingleComponentEffect (Phase S4, Tier 0). Wires the
-// shell: an instrument that declares an event-input bus + a stereo audio-output bus,
+// pure S3 sampler core into a real VSTi: it declares an event-input bus + a stereo audio
-// sets up processing, and runs an empty (silent) process. Nothing plays yet — S4 wires
+// output bus, marshals host MIDI note-on/off into the VoiceEngine, and renders the
-// the pure sampler core into process(); S1 only proves REAPER hosts it.
+// engine's audio into the output bus — so a chosen bank sample plays chromatically from
+// its root note in REAPER's routing/record/render path.
 //
 // SingleComponentEffect is the SDK's combined processor+controller base — sanctioned
-// for a non-distributable, REAPER-only plugin under D5/D6 (verified: SDK class
+// for a non-distributable, REAPER-only plugin under D5/D6. It gives us
-// reference). It gives us addAudioOutput/addEventInput and the IEditController seat, so
+// addAudioOutput/addEventInput, IComponent setState/getState for the instance's own
-// createView() can hand the host our IPlugView LICE editor.
+// state (the selected sample), and the IEditController seat so createView() can hand the
+// host our IPlugView LICE editor.
+//
+// REAL-TIME DISCIPLINE (S4 hard constraint). The audio thread (process) does NO
+// allocation, NO file I/O, NO bridge calls, NO locks. Sample loading — bridge ext-state
+// read, WAV decode, path resolve, keymap build, VoiceEngine construction — all happens
+// OFF the audio thread (reloadFromBank, driven from the main/UI thread) and is handed to
+// process via a single atomic pointer swap. See the LoadedInstrument handoff below.
 
 #pragma once
 
+#include <atomic>
+#include <cstdint>
+#include <memory>
+#include <mutex>
+#include <string>
+#include <vector>
+
 #include "public.sdk/source/vst/vstsinglecomponenteffect.h"
 
 #include "reaper_bridge.h"
+#include "sampler_core.h"
 
 namespace reasampler::vst {
 
+// One fully-built, ready-to-play instrument snapshot: the decoded keymap and the voice
+// engine that plays it. The engine holds a reference into the keymap, so the two MUST
+// live and die together at a STABLE address — hence this is heap-allocated and neither
+// copyable nor movable. The audio thread only ever reads it through an atomic pointer;
+// it is built and destroyed off the audio thread.
+struct LoadedInstrument {
+    Keymap keymap;
+    VoiceEngine engine;
+
+    LoadedInstrument(Keymap km, std::size_t maxVoices, const AdsrParams& adsr)
+        : keymap(std::move(km)), engine(maxVoices, keymap, adsr) {}
+
+    LoadedInstrument(const LoadedInstrument&) = delete;
+    LoadedInstrument& operator=(const LoadedInstrument&) = delete;
+};
+
 class ReaSamplerProcessor : public Steinberg::Vst::SingleComponentEffect {
 public:
     ReaSamplerProcessor() = default;
@@ -30,10 +62,16 @@ public:
     Steinberg::tresult PLUGIN_API terminate() override;
     Steinberg::tresult PLUGIN_API setActive(Steinberg::TBool state) override;
 
+    // Instance state = the selected bank sample id (D-B: a performance choice the
+    // instrument owns; NEVER written back to the bank). Component-state, so a saved
+    // REAPER project restores which sample each instance plays.
+    Steinberg::tresult PLUGIN_API setState(Steinberg::IBStream* state) override;
+    Steinberg::tresult PLUGIN_API getState(Steinberg::IBStream* state) override;
+
     //--- from IAudioProcessor ----------------------------------------------
     Steinberg::tresult PLUGIN_API setupProcessing(
         Steinberg::Vst::ProcessSetup& setup) override;
-    // Empty in the spike: emits silence (S4 fills it).
+    // Marshals MIDI -> VoiceEngine -> audio output. Real-time safe (no alloc/IO/lock).
     Steinberg::tresult PLUGIN_API process(
         Steinberg::Vst::ProcessData& data) override;
 
@@ -41,8 +79,50 @@ public:
     // Hands the host our LICE IPlugView editor.
     Steinberg::IPlugView* PLUGIN_API createView(Steinberg::FIDString name) override;
 
+    // Called by the editor (main/UI thread) when the user picks a sample, and internally
+    // on load. Reads the live bank over the bridge, resolves+decodes the selected WAV
+    // OFF the audio thread, and publishes the built instrument to process() via an
+    // atomic swap. Safe to call with no bridge / no bank (leaves silence). Returns the
+    // resolved selection id ("" if nothing was loaded) for the editor to reflect.
+    std::string reloadFromBank();
+
+    // The bridge, for the editor's live-state readout + sample list. Owned here; the
+    // editor borrows it (outlives the editor).
+    ReaperBridge& bridge() { return bridge_; }
+    // The current selection id (main/UI thread reads for the editor). Guarded by
+    // selectionMutex_ — never touched on the audio thread.
+    std::string selectedSampleId();
+    void setSelectedSampleId(const std::string& id);
+
 private:
     ReaperBridge bridge_;
+
+    // --- The audio-thread handoff (S4 real-time discipline) -----------------
+    // process() atomically loads `live_` at block start and marshals/renders against it —
+    // a single atomic acquire, no lock, no free on the audio thread.
+    //
+    // reloadFromBank() (off-thread, serialized by reloadMutex_) builds a new
+    // LoadedInstrument and atomically swaps it into `live_`. The DISPLACED instrument is
+    // NOT freed on the reload path: process() may still be mid-block reading it, and two
+    // rapid reloads could otherwise free a pointer process is using. Instead it is parked
+    // in `graveyard_` and reclaimed only when process is GUARANTEED stopped — at
+    // setActive(false) / terminate(), which the host never runs concurrently with
+    // process. The graveyard grows by one engine per reload during a session (bounded by
+    // user sample switches — a few objects), a deliberate leak-until-deactivate trade for
+    // a lock-free, race-free audio thread. Tier 2 can add epoch-based reclaim if needed.
+    std::atomic<LoadedInstrument*> live_{nullptr};
+    std::vector<std::unique_ptr<LoadedInstrument>> graveyard_;  // freed only when stopped
+    std::mutex reloadMutex_;  // serializes off-thread reloads + graveyard access
+
+    // The selected sample id (instance state). Off-thread only; a small mutex guards the
+    // string against a getState/editor race. NOT read on the audio thread.
+    std::mutex selectionMutex_;
+    std::string selectedSampleId_;
+
+    // Latched from setupProcessing so setActive/reload can size against it. Read
+    // off-thread only.
+    double sampleRate_ = 44100.0;
+    Steinberg::int32 maxBlockSize_ = 4096;
 };
 
 }  // namespace reasampler::vst

src/vst/sample_map.cpp

@@ -0,0 +1,129 @@
+// sample_map — pure implementation. See sample_map.h. NO VST3 / REAPER / SWELL /
+// vendor includes; standard library + the pure bank_book / wav_trim / sampler_core.
+
+#include "sample_map.h"
+
+#include <cstring>  // std::memcpy
+
+namespace reasampler {
+
+namespace {
+
+// Translate a bank_model Sample's S2 intrinsics into the core's SampleLoop. The bank
+// stores loop points as an optional LoopPoints (both-or-neither); the core wants a
+// SampleLoop with an explicit hasLoop. Absent -> no loop.
+SampleLoop loopFromSample(const Sample& s) {
+    SampleLoop out;
+    if (s.loop) {
+        out.hasLoop = true;
+        out.start = s.loop->start;
+        out.end = s.loop->end;
+    }
+    return out;
+}
+
+// A distilled SelectedSample from a bank_model Sample. rootNote defaults to middle C
+// (60) when the bank left the intrinsic empty — Tier 0 still plays, just centered on
+// C rather than a captured pitch (surfaced: an un-rooted sample plays unity at C4).
+SelectedSample distill(const Sample& s) {
+    SelectedSample out;
+    out.relativePath = s.relativePath;
+    out.rootNote = s.rootNote ? *s.rootNote : 60;
+    out.loop = loopFromSample(s);
+    return out;
+}
+
+}  // namespace
+
+std::optional<SelectedSample> selectSample(const std::string& banksJson,
+                                           const std::string& sampleId) {
+    if (banksJson.empty()) return std::nullopt;
+    std::optional<BankBook> book = BankBook::deserialize(banksJson);
+    if (!book) return std::nullopt;  // malformed -> nothing to play (never throw)
+
+    // Search every bank (pool first, then named — banks() is ordinal order) for the
+    // stored id. A sample lives in exactly one bank, so first hit wins.
+    if (!sampleId.empty()) {
+        for (const Bank& b : book->banks()) {
+            if (const Sample* s = b.index.query(sampleId)) {
+                return distill(*s);
+            }
+        }
+    }
+
+    // No stored id, or the id no longer resolves (the sample was deleted/moved out):
+    // fall back to the FIRST sample in ordinal order so a fresh instance plays.
+    for (const Bank& b : book->banks()) {
+        if (!b.index.all().empty()) {
+            return distill(b.index.all().front());
+        }
+    }
+    return std::nullopt;  // bank has zero samples anywhere
+}
+
+std::vector<SampleChoice> listSamples(const std::string& banksJson) {
+    std::vector<SampleChoice> out;
+    if (banksJson.empty()) return out;
+    std::optional<BankBook> book = BankBook::deserialize(banksJson);
+    if (!book) return out;
+    for (const Bank& b : book->banks()) {
+        for (const Sample& s : b.index.all()) {
+            out.push_back(SampleChoice{s.id, s.displayName});
+        }
+    }
+    return out;
+}
+
+std::vector<AudioSample> downmixToMono(const std::vector<AudioSample>& interleaved,
+                                       int channelCount) {
+    std::vector<AudioSample> out;
+    if (channelCount <= 0 || interleaved.empty()) return out;
+    const std::size_t stride = static_cast<std::size_t>(channelCount);
+    const std::size_t frames = interleaved.size() / stride;
+    out.resize(frames);
+    const double inv = 1.0 / static_cast<double>(channelCount);
+    for (std::size_t f = 0; f < frames; ++f) {
+        double acc = 0.0;
+        const std::size_t base = f * stride;
+        for (std::size_t c = 0; c < stride; ++c) {
+            acc += static_cast<double>(interleaved[base + c]);
+        }
+        out[f] = static_cast<AudioSample>(acc * inv);
+    }
+    return out;
+}
+
+Keymap buildTier0Keymap(std::vector<AudioSample> monoFrames, int sampleRate,
+                        int rootNote, const SampleLoop& loop) {
+    SampleData data;
+    data.frames = std::move(monoFrames);
+    data.sampleRate = sampleRate > 0 ? sampleRate : 44100;
+    data.rootNote = rootNote;
+    data.loop = loop;
+    return Keymap::singleSampleChromatic(std::move(data));
+}
+
+std::vector<std::uint8_t> serializeSelection(const std::string& sampleId) {
+    std::vector<std::uint8_t> out;
+    out.resize(4 + sampleId.size());
+    const std::uint32_t v = kSelectionStateVersion;
+    out[0] = static_cast<std::uint8_t>(v & 0xFF);
+    out[1] = static_cast<std::uint8_t>((v >> 8) & 0xFF);
+    out[2] = static_cast<std::uint8_t>((v >> 16) & 0xFF);
+    out[3] = static_cast<std::uint8_t>((v >> 24) & 0xFF);
+    std::memcpy(out.data() + 4, sampleId.data(), sampleId.size());
+    return out;
+}
+
+std::string deserializeSelection(const std::vector<std::uint8_t>& bytes) {
+    if (bytes.size() < 4) return {};  // no version tag -> no selection
+    const std::uint32_t v = static_cast<std::uint32_t>(bytes[0]) |
+                            (static_cast<std::uint32_t>(bytes[1]) << 8) |
+                            (static_cast<std::uint32_t>(bytes[2]) << 16) |
+                            (static_cast<std::uint32_t>(bytes[3]) << 24);
+    if (v != kSelectionStateVersion) return {};  // unknown version -> ignore
+    return std::string(reinterpret_cast<const char*>(bytes.data() + 4),
+                       bytes.size() - 4);
+}
+
+}  // namespace reasampler

src/vst/sample_map.h

@@ -0,0 +1,110 @@
+#pragma once
+// sample_map — PURE mapping logic for the S4 Tier-0 instrument: turn the live
+// "reasampler" bank ext-state + a decoded WAV into the plain data the sampler core
+// plays, and (de)serialize the instance's selected-sample choice for VST3 component
+// state. NO VST3, NO REAPER, NO SWELL, NO vendor/ includes at the boundary — the
+// mirror of capture_paths / wav_trim / bridge_marshal splitting the fiddly, testable
+// arithmetic out of a host-facing shell.
+//
+// WHY IT EXISTS (S4 seams). The instrument reads the bank over the live-state seam
+// (the "banks" ext-state blob) and the audio over the file seam (the on-disk WAV).
+// Both of those raw inputs cross the bridge/file boundary in the shell; everything
+// after — parse the bank with the SHARED bank_model/bank_book JSON path (NOT a second
+// parser; the S1 spike's string-scan reader is retired), pick the selected sample,
+// downmix its decoded PCM to the core's mono contract, and build the Tier-0 chromatic
+// Keymap — is pure and unit-tested here.
+//
+// It links bank_book (the shared BankBook::deserialize) and wav_trim (the shared
+// 32-bit-float WAV parse — no third WAV reader) and sampler_core (the Keymap /
+// SampleData it produces). All three are pure; this stays pure.
+
+#include <cstdint>
+#include <optional>
+#include <string>
+#include <vector>
+
+#include "bank_book.h"       // BankBook::deserialize (shared bank JSON parse)
+#include "sampler_core.h"    // Keymap, SampleData, SampleLoop
+#include "wav_trim.h"        // parseWavLayout, extractFloatFrames (shared WAV parse)
+
+namespace reasampler {
+
+// The bank sample this instance is bound to, distilled from the live "banks" blob:
+// the project-relative WAV path the file seam must resolve+decode, plus the S2 bank
+// intrinsics the core repitches / loops by. A pure value — no host, no PCM yet.
+struct SelectedSample {
+    std::string relativePath;   // project-relative; the shell resolves it (M4 convention)
+    int rootNote = 60;          // S2 intrinsic; defaults to middle C when the bank left it empty
+    SampleLoop loop;            // S2 intrinsic; hasLoop=false when the bank left it empty
+};
+
+// Resolve the bound sample from the live bank blob. `banksJson` is the raw "banks"
+// ext-state value the bridge read (may be empty / malformed — an unsaved or pre-bank
+// project). `sampleId` is this instance's stored selection.
+//
+// Precedence, all pure:
+//   * empty / malformed banksJson            -> nullopt (nothing to play)
+//   * sampleId names a sample in ANY bank    -> that sample (searched pool + named)
+//   * sampleId empty or not found, bank has   -> the FIRST sample in ordinal order
+//     >= 1 sample                                (a sensible default so a fresh
+//                                                 instance plays SOMETHING; the UI can
+//                                                 then pick a specific one)
+//   * bank has zero samples                  -> nullopt
+//
+// The "first sample" fallback is deliberate: Tier 0 is "the bank plays", and a brand-
+// new instance with no stored selection should map the bank's first sample rather than
+// stay silent until the user opens the editor.
+std::optional<SelectedSample> selectSample(const std::string& banksJson,
+                                           const std::string& sampleId);
+
+// All (id, displayName) pairs across every bank in ordinal order (pool first), for the
+// selection UI to list. Empty for an empty / malformed blob. Pure projection over the
+// shared parse — the UI never parses JSON itself.
+struct SampleChoice {
+    std::string id;
+    std::string displayName;
+};
+std::vector<SampleChoice> listSamples(const std::string& banksJson);
+
+// Downmix interleaved float frames (the shape wav_trim::extractFloatFrames yields:
+// [f0c0,f0c1,...,f1c0,...]) to the core's MONO contract by AVERAGING channels per
+// frame. `channelCount` is the interleave stride (>= 1). CHANNEL POLICY (Tier 0,
+// documented + surfaced): the S3 core is mono-per-sample by design; bank WAVs preserve
+// their source channel count, so a stereo (or N-channel) capture is folded to a single
+// mono stream here by an equal-weight average. Averaging (not "take L", not summing) is
+// the least-surprising, no-clip default — a centered mono source stays unity, and a
+// hard-panned source is attenuated rather than silenced or doubled. Empty / zero-stride
+// in -> empty out. Pure.
+std::vector<AudioSample> downmixToMono(const std::vector<AudioSample>& interleaved,
+                                       int channelCount);
+
+// Build the Tier-0 chromatic keymap for one decoded, mono sample: one zone spanning
+// the whole keyboard, repitched from `rootNote`, looped per `loop`. This is the
+// single-sample degenerate case (Keymap::singleSampleChromatic) with the S2 intrinsics
+// threaded in. `monoFrames` is the downmixed PCM; `sampleRate` is the WAV's rate.
+Keymap buildTier0Keymap(std::vector<AudioSample> monoFrames, int sampleRate,
+                        int rootNote, const SampleLoop& loop);
+
+// --- Instance state (VST3 setState/getState) --------------------------------
+//
+// The instrument's OWN state is which bank sample it plays (D-B: the selection is a
+// performance choice, held by the instrument, never written back to the bank). It is a
+// single string id. serialize/deserialize keep the on-the-wire form explicit and
+// versioned so a future Tier can extend it without breaking already-saved instances.
+//
+// Format (v1): a 4-byte little-endian version tag (== 1) followed by the id bytes. No
+// length prefix is needed — the id runs to the end of the stream (the host tells us the
+// byte count). deserializeSelection tolerates a truncated / wrong-version / empty blob
+// by returning "" (no selection — the instrument falls back to the bank's first sample),
+// never throwing across the host boundary.
+
+inline constexpr std::uint32_t kSelectionStateVersion = 1;
+
+// The selected-sample id serialized to bytes for IBStream (getState).
+std::vector<std::uint8_t> serializeSelection(const std::string& sampleId);
+
+// The selected-sample id parsed back from IBStream bytes (setState). Unknown version,
+// too-short, or empty -> "" (graceful no-selection).
+std::string deserializeSelection(const std::vector<std::uint8_t>& bytes);
+
+}  // namespace reasampler

src/vst/sampler_core.cpp

@@ -289,18 +289,29 @@ void VoiceEngine::noteOff(int note) {
     if (target != kNoVoice) voices_[target].release();
 }
 
-void VoiceEngine::render(std::vector<AudioSample>& out, std::size_t frameCount) {
+void VoiceEngine::render(AudioSample* out, std::size_t frameCount) {
-    const std::size_t base = out.size();
+    // Real-time safe: no allocation, no resize — mix straight into the caller's buffer.
-    out.resize(base + frameCount, 0.0f);  // S4: caller must pre-reserve — no allocation allowed under the VST3 process callback.
+    // The VST3 process callback hands us the host's output channel buffer here, so the
+    // audio thread never touches the heap (S4 real-time discipline).
+    if (out == nullptr || frameCount == 0) return;
     for (Voice& voice : voices_) {
         if (!voice.active()) continue;
         for (std::size_t f = 0; f < frameCount; ++f) {
             if (!voice.active()) break;
-            out[base + f] += voice.renderFrame();
+            out[f] += voice.renderFrame();
         }
     }
 }
 
+void VoiceEngine::render(std::vector<AudioSample>& out, std::size_t frameCount) {
+    // Off-thread / test path: grow the buffer (this allocates — never call under
+    // process), zero-fill the appended span, then delegate to the RT mix loop so both
+    // overloads share exactly one summation path.
+    const std::size_t base = out.size();
+    out.resize(base + frameCount, 0.0f);
+    render(out.data() + base, frameCount);
+}
+
 std::size_t VoiceEngine::activeVoiceCount() const {
     std::size_t n = 0;
     for (const Voice& v : voices_) {

src/vst/sampler_core.h

@@ -236,9 +236,20 @@ public:
     // the older tail to ring — matches hardware behavior). No-op if none match.
     void noteOff(int note);
 
-    // Renders `frameCount` mono output frames, summing all active voices, appending to
+    // REAL-TIME render (S4): sums all active voices into the caller-provided buffer
-    // `out` (does not clear it — the caller owns mixing/clearing). Voices that finish
+    // `out[0..frameCount)`, ADDING to whatever is there (the caller clears or mixes —
-    // mid-block go idle and stop contributing.
+    // this never touches memory it does not own and NEVER allocates). This is the
+    // audio-thread entry point: the VST3 process callback passes the host's own output
+    // channel buffer, so no allocation, resize, or heap traffic happens under process.
+    // Voices that finish mid-block go idle and stop contributing. `out` must point at
+    // at least `frameCount` writable samples; a null `out` or zero count is a no-op.
+    void render(AudioSample* out, std::size_t frameCount);
+
+    // TEST / off-thread convenience: appends `frameCount` summed frames to `out`
+    // (grows it — DO NOT call on the audio thread; it allocates). Delegates to the
+    // real-time overload after sizing the buffer, so both paths share one mix loop.
+    // Does not clear existing contents — appends, matching the pre-S4 contract the
+    // unit tests rely on.
     void render(std::vector<AudioSample>& out, std::size_t frameCount);
 
     // Count of currently active voices (for tests / diagnostics).

tests/test_bridge_marshal.cpp

@@ -1,11 +1,12 @@
 // Standalone tests for reasampler::vst::bridge_marshal — no VST3, no REAPER, no test
 // framework. Same fast assert loop as the sibling pure tests: assert the REAPER
-// bridge-read marshalling (GetProjExtState result decode + a small JSON string-field
+// bridge-read marshalling (GetProjExtState result decode) directly, so the DAW-facing
-// reader) directly, so the DAW-facing shell only has to invoke the API.
+// shell only has to invoke the API.
 //
 // Covers: decodeGetProjExtState hit/absent/zero-return/empty-buffer (the stale-buffer
-// guard); extractJsonStringField present/absent/escapes/whitespace/value-vs-key
+// guard). The S1 spike's extractJsonStringField string-scan reader was retired in S4
-// disambiguation/non-string-value/malformed.
+// (the instrument now parses the bank through the shared bank_book JSON path), so its
+// cases are gone with it.
 
 #include "../src/vst/bridge_marshal.h"
 
@@ -44,72 +45,11 @@ static void testDecodeEmptyBuffer() {
     CHECK(!v.has_value());
 }
 
-// --- extractJsonStringField ---------------------------------------------------
-
-static void testExtractPresent() {
-    const std::string json = R"({"guid":"ABC-123","name":"kick"})";
-    auto g = extractJsonStringField(json, "guid");
-    CHECK(g && *g == "ABC-123");
-    auto n = extractJsonStringField(json, "name");
-    CHECK(n && *n == "kick");
-}
-
-static void testExtractAbsent() {
-    const std::string json = R"({"guid":"ABC-123"})";
-    CHECK(!extractJsonStringField(json, "missing").has_value());
-}
-
-static void testExtractWhitespaceTolerant() {
-    const std::string json = "{ \"guid\" :   \"X\" , \"n\":\"y\" }";
-    auto g = extractJsonStringField(json, "guid");
-    CHECK(g && *g == "X");
-}
-
-static void testExtractEscapes() {
-    // \" \\ \/ \n \t all decode.
-    const std::string json = R"({"path":"a\\b\/c\"d\ne"})";
-    auto p = extractJsonStringField(json, "path");
-    CHECK(p && *p == "a\\b/c\"d\ne");
-}
-
-static void testExtractValueContainingKeyText() {
-    // A VALUE that contains the key text must not be mistaken for the member. Here the
-    // first "guid" occurrence is inside another value; the real member comes later.
-    const std::string json = R"({"note":"the guid is here","guid":"REAL"})";
-    auto g = extractJsonStringField(json, "guid");
-    CHECK(g && *g == "REAL");
-}
-
-static void testExtractNonStringValue() {
-    // A numeric/object value is not a string — return nullopt rather than garbage.
-    const std::string json = R"({"count":42,"name":"ok"})";
-    CHECK(!extractJsonStringField(json, "count").has_value());
-    // The sibling string field still reads.
-    auto n = extractJsonStringField(json, "name");
-    CHECK(n && *n == "ok");
-}
-
-static void testExtractMalformed() {
-    CHECK(!extractJsonStringField(R"({"guid":"unterminated)", "guid").has_value());
-    CHECK(!extractJsonStringField(R"({"guid":)", "guid").has_value());
-    CHECK(!extractJsonStringField(R"({"guid")", "guid").has_value());
-    CHECK(!extractJsonStringField("", "guid").has_value());
-    // Dangling escape at end of string.
-    CHECK(!extractJsonStringField(R"({"guid":"abc\)", "guid").has_value());
-}
-
 int main() {
     testDecodeHit();
     testDecodeAbsentKey();
     testDecodeNegativeReturn();
     testDecodeEmptyBuffer();
-    testExtractPresent();
-    testExtractAbsent();
-    testExtractWhitespaceTolerant();
-    testExtractEscapes();
-    testExtractValueContainingKeyText();
-    testExtractNonStringValue();
-    testExtractMalformed();
 
     if (g_fail == 0) std::printf("bridge_marshal: all tests passed\n");
     return g_fail != 0;

tests/test_editor_geometry.cpp

@@ -131,6 +131,63 @@ static void testHitTestMatchesDrawnButton() {
     }
 }
 
+// --- sample list (S4) ---------------------------------------------------------
+
+static void testSampleRowRectStacks() {
+    const EditorLayout L = layoutEditor(400, 260);
+    const Rect r0 = sampleRowRect(L, 0);
+    const Rect r1 = sampleRowRect(L, 1);
+    // Row 0 starts at the canvas top and spans its full width.
+    CHECK(r0.top == L.canvas.top);
+    CHECK(r0.left == L.canvas.left && r0.right == L.canvas.right);
+    CHECK(r0.height() == kSampleRowHeight);
+    // Row 1 sits directly below row 0 (no gap, no overlap).
+    CHECK(r1.top == r0.bottom);
+    CHECK(r1.height() == kSampleRowHeight);
+    // A negative index is an empty rect.
+    CHECK(sampleRowRect(L, -1).width() == 0 && sampleRowRect(L, -1).height() == 0);
+}
+
+static void testSampleRowHitTestMapsClickToRow() {
+    const EditorLayout L = layoutEditor(400, 260);
+    const int rows = 5;
+    // A click in the vertical middle of row 2 resolves to index 2.
+    const Rect r2 = sampleRowRect(L, 2);
+    const int midY = (r2.top + r2.bottom) / 2;
+    CHECK(sampleRowHitTest(L, rows, 200, midY) == 2);
+    // Row 0's top-left corner hits row 0.
+    const Rect r0 = sampleRowRect(L, 0);
+    CHECK(sampleRowHitTest(L, rows, r0.left, r0.top) == 0);
+}
+
+static void testSampleRowHitTestMisses() {
+    const EditorLayout L = layoutEditor(400, 260);
+    const int rows = 3;
+    // Above the first row (in the title bar) -> no row.
+    CHECK(sampleRowHitTest(L, rows, 200, L.titleBar.top) == -1);
+    // Below the last row -> no row.
+    const Rect last = sampleRowRect(L, rows - 1);
+    CHECK(sampleRowHitTest(L, rows, 200, last.bottom + 1) == -1);
+    // Left of the canvas -> no row.
+    CHECK(sampleRowHitTest(L, rows, L.canvas.left - 1, last.top) == -1);
+    // Zero rows -> always -1.
+    CHECK(sampleRowHitTest(L, 0, 200, L.canvas.top + 1) == -1);
+}
+
+// The drawn-row <-> hit-test agreement: every pixel inside a row rect must resolve to
+// that row's index (the same load-bearing invariant as the button).
+static void testSampleRowHitTestMatchesDrawnRows() {
+    const EditorLayout L = layoutEditor(320, 200);
+    const int rows = 4;
+    for (int i = 0; i < rows; ++i) {
+        const Rect r = sampleRowRect(L, i);
+        if (r.top >= L.canvas.bottom) break;  // clipped rows aren't clickable targets
+        const int y = (r.top + r.bottom) / 2;
+        if (y >= L.canvas.bottom) continue;
+        CHECK(sampleRowHitTest(L, rows, r.left + 1, y) == i);
+    }
+}
+
 int main() {
     testContainsHalfOpen();
     testContainsDegenerate();
@@ -141,6 +198,10 @@ int main() {
     testHitTestMissesNonButton();
     testHitTestButtonBoundary();
     testHitTestMatchesDrawnButton();
+    testSampleRowRectStacks();
+    testSampleRowHitTestMapsClickToRow();
+    testSampleRowHitTestMisses();
+    testSampleRowHitTestMatchesDrawnRows();
 
     if (g_fail == 0) std::printf("editor_geometry: all tests passed\n");
     return g_fail != 0;

tests/test_sample_map.cpp

@@ -0,0 +1,275 @@
+// Standalone tests for reasampler::sample_map — no VST3, no REAPER, no test framework.
+// Same fast assert loop as the sibling pure tests. This module is the S4 mapping heart:
+// bank blob -> selected sample (through the SHARED bank_book JSON parse), interleaved ->
+// mono downmix (the Tier-0 channel policy), the Tier-0 chromatic keymap build, and the
+// selected-sample instance-state (de)serialization.
+//
+// Every assertion is written to FAIL if the mapping were wrong: the bank blobs are built
+// by serializing a real BankBook (so we exercise the shared parse, not a fixture string),
+// and the selection / downmix / keymap / state values are checked against independently
+// computed expectations.
+//
+// Covers: selectSample by-id hit (across pool + named banks), first-sample fallback for
+// an empty / unknown id, empty & malformed blob -> nullopt, zero-samples -> nullopt,
+// rootNote/loop intrinsic threading incl. the middle-C default; listSamples ordinal
+// order + empty/malformed; downmixToMono mono passthrough / stereo average / 3-ch
+// average / zero-stride / empty; buildTier0Keymap single full-keyboard zone with the
+// root + loop + rate threaded and rate defaulting; selection state round-trip + empty id
+// + wrong-version / truncated -> "".
+
+#include "../src/vst/sample_map.h"
+
+#include <cmath>
+#include <cstdio>
+#include <string>
+
+#include "../src/bank_book.h"
+#include "../src/bank_model.h"
+
+using namespace reasampler;
+
+static int g_fail = 0;
+#define CHECK(cond) do { if(!(cond)) { \
+    std::printf("FAIL line %d: %s\n", __LINE__, #cond); ++g_fail; } } while(0)
+
+// Build a Sample with the fields sample_map reads. Relative path is required by
+// BankIndex::add (relative-only invariant); a content hash is set so dedup does not
+// collapse distinct entries.
+static Sample makeSample(const std::string& id, const std::string& name,
+                         const std::string& rel, std::optional<int> root) {
+    Sample s;
+    s.id = id;
+    s.displayName = name;
+    s.relativePath = rel;
+    s.contentHash = "hash-" + id;
+    s.rootNote = root;
+    return s;
+}
+
+// A serialized BankBook: the pool carries `poolSamples`, and one named bank "Drums"
+// carries `drumSamples`. Returns the JSON the instrument would read from ext-state.
+static std::string bookJson(const std::vector<Sample>& poolSamples,
+                            const std::vector<Sample>& drumSamples) {
+    BankBook book;
+    for (const Sample& s : poolSamples) book.pool().index.add(s);
+    if (!drumSamples.empty()) {
+        book.createBank("drums-id", "Drums");
+        BankIndex* di = book.index("drums-id");
+        for (const Sample& s : drumSamples) di->add(s);
+    }
+    return book.serialize();
+}
+
+// --- selectSample -------------------------------------------------------------
+
+static void testSelectByIdHit() {
+    const std::string json = bookJson(
+        {makeSample("a", "Kick", "reasampler_bank/a.wav", 36)},
+        {makeSample("b", "Snare", "reasampler_bank/b.wav", 38)});
+    // A sample in the NAMED bank resolves by id (search spans every bank).
+    auto sel = selectSample(json, "b");
+    CHECK(sel.has_value());
+    CHECK(sel && sel->relativePath == "reasampler_bank/b.wav");
+    CHECK(sel && sel->rootNote == 38);
+}
+
+static void testSelectFirstSampleFallbackOnEmptyId() {
+    const std::string json = bookJson(
+        {makeSample("a", "Kick", "reasampler_bank/a.wav", 36)},
+        {makeSample("b", "Snare", "reasampler_bank/b.wav", 38)});
+    // No stored selection -> the FIRST sample in ordinal order (pool first).
+    auto sel = selectSample(json, "");
+    CHECK(sel.has_value());
+    CHECK(sel && sel->relativePath == "reasampler_bank/a.wav");
+    CHECK(sel && sel->rootNote == 36);
+}
+
+static void testSelectFirstSampleFallbackOnUnknownId() {
+    const std::string json = bookJson(
+        {makeSample("a", "Kick", "reasampler_bank/a.wav", 36)}, {});
+    // A stored id that no longer resolves falls back to the first sample, not silence.
+    auto sel = selectSample(json, "deleted-id");
+    CHECK(sel.has_value());
+    CHECK(sel && sel->relativePath == "reasampler_bank/a.wav");
+}
+
+static void testSelectRootNoteDefault() {
+    const std::string json = bookJson(
+        {makeSample("a", "Loop", "reasampler_bank/a.wav", std::nullopt)}, {});
+    // A sample with no root-note intrinsic defaults to middle C (60).
+    auto sel = selectSample(json, "a");
+    CHECK(sel.has_value());
+    CHECK(sel && sel->rootNote == 60);
+}
+
+static void testSelectLoopThreaded() {
+    Sample s = makeSample("a", "Pad", "reasampler_bank/a.wav", 60);
+    s.loop = LoopPoints{100, 500};
+    const std::string json = bookJson({s}, {});
+    auto sel = selectSample(json, "a");
+    CHECK(sel.has_value());
+    CHECK(sel && sel->loop.hasLoop);
+    CHECK(sel && sel->loop.start == 100 && sel->loop.end == 500);
+}
+
+static void testSelectNoLoopIsAbsent() {
+    const std::string json = bookJson(
+        {makeSample("a", "OneShot", "reasampler_bank/a.wav", 60)}, {});
+    auto sel = selectSample(json, "a");
+    CHECK(sel.has_value());
+    CHECK(sel && !sel->loop.hasLoop);  // absent loop -> hasLoop false (not a zero loop)
+}
+
+static void testSelectEmptyBlob() {
+    CHECK(!selectSample("", "a").has_value());
+}
+
+static void testSelectMalformedBlob() {
+    CHECK(!selectSample("{not valid json", "a").has_value());
+}
+
+static void testSelectZeroSamples() {
+    // A valid book with NO samples anywhere -> nothing to play.
+    const std::string json = bookJson({}, {});
+    CHECK(!selectSample(json, "").has_value());
+    CHECK(!selectSample(json, "anything").has_value());
+}
+
+// --- listSamples --------------------------------------------------------------
+
+static void testListSamplesOrdinalOrder() {
+    const std::string json = bookJson(
+        {makeSample("a", "Kick", "reasampler_bank/a.wav", 36),
+         makeSample("c", "Hat", "reasampler_bank/c.wav", 42)},
+        {makeSample("b", "Snare", "reasampler_bank/b.wav", 38)});
+    const std::vector<SampleChoice> list = listSamples(json);
+    // Pool samples (insertion order) come before the named bank's.
+    CHECK(list.size() == 3);
+    CHECK(list.size() == 3 && list[0].id == "a" && list[0].displayName == "Kick");
+    CHECK(list.size() == 3 && list[1].id == "c");
+    CHECK(list.size() == 3 && list[2].id == "b" && list[2].displayName == "Snare");
+}
+
+static void testListSamplesEmptyAndMalformed() {
+    CHECK(listSamples("").empty());
+    CHECK(listSamples("{garbage").empty());
+    CHECK(listSamples(bookJson({}, {})).empty());
+}
+
+// --- downmixToMono ------------------------------------------------------------
+
+static bool approx(double a, double b) { return std::fabs(a - b) < 1e-6; }
+
+static void testDownmixMonoPassthrough() {
+    const std::vector<AudioSample> in{0.1f, -0.2f, 0.3f};
+    const std::vector<AudioSample> out = downmixToMono(in, 1);
+    CHECK(out.size() == 3);
+    CHECK(out.size() == 3 && approx(out[0], 0.1) && approx(out[1], -0.2) &&
+          approx(out[2], 0.3));
+}
+
+static void testDownmixStereoAverages() {
+    // Two frames, stereo interleaved: frame0 = (1.0, 0.0) -> 0.5; frame1 = (0.4, 0.6) -> 0.5.
+    const std::vector<AudioSample> in{1.0f, 0.0f, 0.4f, 0.6f};
+    const std::vector<AudioSample> out = downmixToMono(in, 2);
+    CHECK(out.size() == 2);
+    CHECK(out.size() == 2 && approx(out[0], 0.5) && approx(out[1], 0.5));
+}
+
+static void testDownmixThreeChannelAverages() {
+    // One 3-channel frame (0.3, 0.3, 0.6) -> 0.4.
+    const std::vector<AudioSample> in{0.3f, 0.3f, 0.6f};
+    const std::vector<AudioSample> out = downmixToMono(in, 3);
+    CHECK(out.size() == 1);
+    CHECK(out.size() == 1 && approx(out[0], 0.4));
+}
+
+static void testDownmixDegenerate() {
+    CHECK(downmixToMono({}, 2).empty());          // empty input
+    CHECK(downmixToMono({0.1f, 0.2f}, 0).empty()); // zero stride
+    CHECK(downmixToMono({0.1f, 0.2f}, -1).empty()); // negative stride
+}
+
+// --- buildTier0Keymap ---------------------------------------------------------
+
+static void testBuildKeymapSingleFullZone() {
+    SampleLoop loop;
+    loop.hasLoop = true;
+    loop.start = 10;
+    loop.end = 90;
+    const Keymap km = buildTier0Keymap({0.1f, 0.2f, 0.3f}, 48000, 40, loop);
+    // One sample, one zone spanning the whole keyboard, rooted at 40.
+    CHECK(km.samples.size() == 1);
+    CHECK(km.zones.size() == 1);
+    CHECK(km.zones.size() == 1 && km.zones[0].lowNote == 0 && km.zones[0].highNote == 127);
+    CHECK(km.zones.size() == 1 && km.zones[0].rootNote == 40);
+    CHECK(km.samples.size() == 1 && km.samples[0].sampleRate == 48000);
+    CHECK(km.samples.size() == 1 && km.samples[0].rootNote == 40);
+    CHECK(km.samples.size() == 1 && km.samples[0].frames.size() == 3);
+    CHECK(km.samples.size() == 1 && km.samples[0].loop.hasLoop &&
+          km.samples[0].loop.start == 10 && km.samples[0].loop.end == 90);
+    // Resolution: any note lands in the single zone.
+    CHECK(km.resolve(0, 100).matched);
+    CHECK(km.resolve(127, 100).matched);
+}
+
+static void testBuildKeymapRateDefault() {
+    // A zero/invalid rate defaults to 44100 rather than producing a divide-by-zero-shaped
+    // sample rate downstream.
+    const Keymap km = buildTier0Keymap({0.1f}, 0, 60, SampleLoop{});
+    CHECK(km.samples.size() == 1 && km.samples[0].sampleRate == 44100);
+}
+
+// --- selection state (setState/getState) --------------------------------------
+
+static void testSelectionStateRoundTrip() {
+    const std::string id = "sample-guid-123";
+    const std::vector<std::uint8_t> bytes = serializeSelection(id);
+    // Versioned: 4-byte tag + the id bytes.
+    CHECK(bytes.size() == 4 + id.size());
+    CHECK(deserializeSelection(bytes) == id);
+}
+
+static void testSelectionStateEmptyId() {
+    const std::vector<std::uint8_t> bytes = serializeSelection("");
+    CHECK(bytes.size() == 4);            // just the version tag
+    CHECK(deserializeSelection(bytes) == "");
+}
+
+static void testSelectionStateWrongVersion() {
+    std::vector<std::uint8_t> bytes = serializeSelection("id");
+    bytes[0] = 0xEE;  // corrupt the version tag
+    CHECK(deserializeSelection(bytes) == "");  // unknown version -> no selection
+}
+
+static void testSelectionStateTruncated() {
+    CHECK(deserializeSelection({}) == "");             // empty
+    CHECK(deserializeSelection({1, 0, 0}) == "");      // fewer than 4 bytes (no tag)
+}
+
+int main() {
+    testSelectByIdHit();
+    testSelectFirstSampleFallbackOnEmptyId();
+    testSelectFirstSampleFallbackOnUnknownId();
+    testSelectRootNoteDefault();
+    testSelectLoopThreaded();
+    testSelectNoLoopIsAbsent();
+    testSelectEmptyBlob();
+    testSelectMalformedBlob();
+    testSelectZeroSamples();
+    testListSamplesOrdinalOrder();
+    testListSamplesEmptyAndMalformed();
+    testDownmixMonoPassthrough();
+    testDownmixStereoAverages();
+    testDownmixThreeChannelAverages();
+    testDownmixDegenerate();
+    testBuildKeymapSingleFullZone();
+    testBuildKeymapRateDefault();
+    testSelectionStateRoundTrip();
+    testSelectionStateEmptyId();
+    testSelectionStateWrongVersion();
+    testSelectionStateTruncated();
+
+    if (g_fail == 0) std::printf("sample_map: all tests passed\n");
+    return g_fail != 0;
+}