reasampler/CONTEXT.md

ReaSampler — implementation briefing

A native C++ REAPER extension that captures any arbitrary audio source into a per-project sample bank (cached files + a docked grid), decoupled from the arrange view, with keyboard/MIDI-bindable capture and placement. Built as a precision tool: deterministic, non-destructive, no clutter.

This document is the spec. Read the existing scaffold first (src/main.cpp is the REAPER<->extension contract; the pure/testable-core split in src/mpe_model.* is the pattern to preserve — the MPE model is being replaced, the discipline is not). Verify every REAPER API name and signature against vendor/reaper-sdk/sdk/reaper_plugin_functions.h before use — the API names in this brief are correct-by-intent but treat them as hints, not gospel, and check argument order/types.

The load-bearing principle

Capture and placement are separate acts. Capturing audio writes a file to the bank and adds an index entry — it NEVER puts an item in the arrange. Placement is a distinct, on-demand action (insert / drag). Any code path that auto-inserts a capture into the timeline violates the entire point of the tool and must be rejected in review. This single rule is why the tool exists.

Settled decisions

• Capture modes: offline render (deterministic, the default) AND realtime record (for hardware / performed FX). Both sit behind one capture interface and produce identical bank entries.
• Bank scope: per-project, travels with the .rpp. Files live in a project-relative subfolder; the index persists in project ext state. No absolute paths anywhere in the index.
• Material: must handle full-mix/stem bounces, chops/one-shots, and single-cycle/wavetable grabs equally. That means exact sample-accurate bounds, explicit tail control, channel-count preservation, and loop/zero-crossing handling all matter from day one.

Module architecture

Preserve the scaffold's split: pure, REAPER-free logic in one set of files (unit-tested outside the DAW via the existing tests/ + CTest harness), REAPER- facing shells in another.

Pure (no REAPER types, fully unit-tested):

• bank_model — the Sample metadata struct and the BankIndex (add / remove / query / tier moves / dedup-by-hash) plus JSON serialize/deserialize to a std::string. This is the heart; test it hard.
• peaks — compute waveform min/max bins from raw PCM. Feed it a known signal (sine, ramp) and assert the envelope. We compute our own thumbnails from the captured file rather than depending on REAPER's peak API — we own the file format, so this is simpler, testable, and dependency-free.

REAPER-facing:

• capture — the ICaptureBackend interface plus OfflineRenderBackend and RealtimeRecordBackend. Input: a CaptureRequest (capture scope — item or track, time range, tail, SR/bit-depth/channels, output path). Output: a finished file + a populated Sample handed to bank_model. Capture is always wet; the FX scope (not a wet/dry dial) is the control — the pure render_settings module maps a scope to its render-source bits and its FX-bypass plan (see §Capture FX scope below).
• insert — placement via InsertMedia; conform-to-project-tempo vs literal, as an explicit flag (never silent stretching).
• bank_panel — the docked grid: LICE-drawn thumbnails, audition, multi-select, keyboard navigation. Reuses the docking setup already in mpe_view.cpp.
• persist — project ext state <-> bank_model JSON; project-relative path resolution (resolve bank folder from the current project path).
• actions — registers the capture/placement/slot action family and routes each to the modules above (the command_id + gaccel + hookcommand pattern from main.cpp).

Data model (sketch — refine in code)

Sample: id, display name, relative file path, source mode, source range (start/ end in project time + PPQ), track GUID(s) if applicable, wet/dry, channel count, sample rate, length (seconds + musical/beats), capture tempo, optional key, peak/RMS/LUFS, clip flag, tier (scratch | archive), content hash, provenance (parent sample id + FX-chain snapshot string, when resampled from another sample), created timestamp.

BankIndex: ordered collection of Sample, keyed by id; hash lookup for dedup; tier filtering; JSON round-trip. Scratch tier is auto-prunable; archive is kept.

REAPER API surface (verify all signatures)

Offline render (the crux — prototype this first):

• Drive render settings with GetSetProjectInfo (RENDER_BOUNDSFLAG, RENDER_STARTPOS, RENDER_ENDPOS, RENDER_TAILFLAG, RENDER_TAILMS, RENDER_SRATE, RENDER_CHANNELS, RENDER_SETTINGS) and GetSetProjectInfo_String (RENDER_FILE, RENDER_PATTERN, RENDER_FORMAT). The RENDER_SETTINGS bit choice is driven by capture scope: item scope renders selected media items (single-file), track scope renders selected tracks via master. There is no master-mix scope (to capture the master, render a track). Render is always wet — REAPER has no true pre-FX "dry" render bit; FX scoping is done by the FX-bypass-around-render mechanism, not a render bit (see §Capture FX scope).
• Trigger a no-dialog render via the appropriate render action / RENDER_SETTINGS bit. Confirm the exact command id and the "render without opening dialog" flag against current REAPER — do not assume; test that it runs headless.
• Determinism is a hard requirement: two identical requests must produce bit-identical files (enables the null test below).

Realtime record (track scope — item realtime is deferred):

• Taps the selected track, not the master. Recipe: create a hidden temp track, route a post-fader send from each selected source track into it (CreateTrackSend, default post-fader/full-stereo — captures each track's own output before the parent/master sums it, so the tap is chain-independent by construction and needs no FX bypass), set the temp's record mode to record-output (latency-compensated, B_MAINSEND=0 so it does not sum back), arm (I_RECARM), CSurf_OnRecord, run for the range, OnStopButtonEx, then move the recorded file into the bank and delete the temp track (which drops the sends — no source track is left mutated). Verify I_RECMODE values for output-recording.
• Master→track sends are refused by REAPER as feedback loops — this is why the tap is the selected track's post-fader output, not the master.

Sources & metadata:

• Time selection: GetSet_LoopTimeRange. Razor edits: GetSetMediaTrackInfo_String(track, "P_RAZOREDITS", ...). Tempo: Master_GetTempo / TimeMap2_timeToBeats / GetProjectTimeSignature2. Selected items/tracks: CountSelectedMediaItems / GetSelectedTrack.

Placement:

• InsertMedia(path, mode) at edit cursor / new track / replace selection (verify mode bits). SetEditCurPos. Wrap edits in Undo_BeginBlock2 / Undo_EndBlock2.

Persistence & paths:

• SetProjExtState / GetProjExtState (namespace e.g. "reasampler") for the index JSON. Resolve project folder via EnumProjects / GetProjectPathEx; store the bank under a project-relative subfolder; keep only relative paths in the index.

Build order (each milestone independently testable)

1. bank_model + JSON round-trip + unit tests. (pure — no REAPER)
2. peaks + unit tests. (pure)
3. Offline capture of the time-selection master mix to a wav in the project bank folder; add a Sample; log it to the console. (the render-driving spike)
4. persist: write the index to proj ext state, reload on project open; confirm it survives Save / Save As. (bank travels with the .rpp)
5. bank_panel: docked grid with thumbnails, audition, selection.
6. insert: "insert selected sample at edit cursor" action via InsertMedia.
7. Capture action family: two FX scopes — item (item/take FX only) and track (item FX + the selected track's own track FX), each chain-independent (the out-of-scope chain is neutralized to unity for the render). No master scope — to capture the master, render a track. Each scope captures over a time-or-razor range (razor is a range source, not a mode: razor-when-present, else time selection). Capture is always wet; the scope is the control. All registered as bindable actions (tail off; a tail-on variant is a later opt-in).
8. RealtimeRecordBackend behind the same interface.
9. Slots: "capture to slot N" / "insert slot N", MIDI-bindable (MPC-style).
10. Provenance + "re-capture from source"; null-test verify action.
11. Polish: batch capture (per selected item / per razor area), resample-and-mute- source, conform-on-insert, drag-out to OS.

Precision invariants (enforce, test)

• Null test: a dry offline capture of a range, re-inserted at its source position, nulls to silence against the source. Ship this as a verification action; it is the tool's trust anchor.
• Bit-identical repeats: identical offline requests produce identical files.
• Non-destructive: capture never mutates source items or tracks (realtime's temp track is created and removed cleanly; source routing is restored).
• Exact bounds: no rounding of the requested range; no added silence unless a tail is explicitly requested; channel count preserved (no silent stereo fold).
• Relative paths only in the persisted index.

Non-goals / guardrails

• No auto-insertion of captures into the arrange (see the load-bearing principle).
• Native OS drag-out is deferred to the final milestone — InsertMedia-driven placement is the primary path and must work first.
• Do not depend on REAPER's peak API for thumbnails; compute from the captured file.
• Do not silently time-stretch on insert; conform is opt-in.
• Do not trust this brief's API names blindly — verify against the SDK header.

Open questions to resolve during build

• Exact no-dialog render command/flag on the current REAPER build.
• Realtime record routing that captures wet master output without altering the user's monitoring. Resolved: realtime taps the selected track (track scope), not the master — a post-fader track→temp send is chain-independent by construction, so there is no wet-master-routing problem and monitoring is untouched (see the Realtime record API surface above).
• Audio format for the bank (wav bit depth default; allow float for wavetable fidelity).
• Thumbnail cache: recompute vs store peak bins alongside the index.

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Design View — additive phase spec

Additive section. This is a standalone phase parallel to — not part of — the M0–M11 capture roadmap above. Nothing above changes. Product framing (workflow narrative, screenset differentiation, N-mode reasoning, design-direction calls) lives in docs/product/design-view.md; this section is the authoritative technical spec, matching the house style of the capture spec. Same standing discipline applies: verify every REAPER API name/signature against vendor/reaper-sdk/sdk/reaper_plugin_functions.h before use.

What it is

A track-visibility-plus-processing "mode" system, toggled from the ReaSampler window. Tracks used purely for sound design (scratch oscillators, FX mangling, resampling sources) are tagged into Design mode; the arrangement's real tracks are Arrange mode (the default). Toggling to a mode hides and disables the tracks that don't belong to it. Workflow value: mental separation + clutter elimination — be in Design view, resample into the bank, flip to Arrange, place it.

Design View is a mode projection over REAPER's single arrange timeline — reaching both tracks (parked per mode) and items (lane-split per mode on shared tracks). It approximates two canvases without a second surface: same project, same timeline, but each stance sees its own tracks and its own items. It never duplicates the project or opens a second window.

Two-canvas reach — settled (2026-07-23). The original single-canvas framing here was derived from a REAPER constraint, not chosen as a product stance. Daniel reopened it — "as close as possible to two separate design and arrange canvases, same project, different items and leaves" — and has now signed off on the mechanism and resolved all forks. The "different leaves" half is delivered by track-parking (D1). The "different items" half is delivered by per-item mode membership via REAPER fixed lanes (track-side I_FREEMODE=2, I_NUMFIXEDLANES, C_LANEPLAYS:N; item-side I_FIXEDLANE, C_LANEPLAYS, B_FIXEDLANE_HIDDEN): each mode owns a lane on a shared track, and toggling shows/plays only the active mode's lane. This is an item-visibility projection over the one timeline — the exact analog of today's track-visibility projection — with still no literal second surface, window, or duplicate project. It is an additive sub-phase (Phase D2 / Phase E) on top of D1, spec'd in §Two-canvas sub-phase below. Full framing and the rejected alternatives (timebase-offset, subproject) live in docs/product/design-view.md §Two-canvas direction.

It is the visibility/processing analog of the capture pillar's load-bearing rule: designing and arranging are separate stances on one timeline, and the tool enforces the separation without ever destroying the user's real state.

Settled decisions

• Membership. Default = Arrange; every untagged leaf belongs to it. Leaves opt in to Design (or any mode). No track appears in two modes at once except (a) via an explicit show-both toggle, or (b) parent/folder derivation.
• Parents are derived, never tagged. A parent/folder track is visible in mode M if either (a) any descendant leaf is visible in M (descendant-derived), or (b) the parent belongs to M by its own membership — and an untagged parent is an Arrange member by default. So an untagged folder carrying its own FX/media above all-Design leaves shows in both Arrange (its own default) and Design (derived from its children). A parent is never parked in any mode it is visible in. Rule of thumb: tag leaves; parents follow — the common case, since a content-bearing folder still surfaces wherever its own membership places it. Master track is always visible and never touched.
• N-mode model, two-mode UI. The data model carries arbitrarily many modes; the UI ships Arrange + Design. A mode is (stable id, display name, ordinal). Arrange is special only as the default home for untagged leaves.
• Parking a track (inactive-mode leaf): B_SHOWINTCP=0, B_SHOWINMIXER=0 (hide both panels), B_MAINSEND=0 (out of mix), I_FXEN=0 (FX bypassed), and TrackFX_SetOffline(track, fx, true) for each FX (reclaim CPU). Full CPU-park is the deliberate choice over mix-removal-only.
• Never touches B_MUTE / I_SOLO. The tool owns only visibility, B_MAINSEND, I_FXEN, and per-FX offline — on every managed leaf, tagged or untagged. User mute/solo survives every toggle untouched.
• Persistence. Membership index + last-active mode + per-track flag snapshots ride in the existing "reasampler" project ext-state namespace and travel with the .rpp. On project open, reapply the active mode's visibility + processing.

Precision invariants (enforce, test)

• Non-destructive restore. For every flag the tool drives, snapshot the prior value before parking; on toggle-back restore from the snapshot, never to a hardcoded "on." Round-trip (snapshot → park → restore) returns every driven flag to its captured value. This is the phase's trust anchor — the analog of the capture null test — and is enforced in the pure layer.
• Mute/solo untouched. No toggle ever reads or writes B_MUTE / I_SOLO.
• Master untouched. The tool never drives the master track's visibility flags (the SDK forbids B_SHOWINTCP/B_SHOWINMIXER on master; the invariant agrees).
• GUID-keyed, reorder-safe. Membership keys on track GUID (GetTrackGUID), never track index; tolerates unknown/stale GUIDs (prune on reconcile).
• Relative/portable state only in the persisted view section (GUID strings, mode ids — no absolute paths, no index positions).

Documented caveat

Offlined FX re-instantiate when a track returns to the active mode. Stateful plugins (convolution, loaded samplers, tail-holding effects) re-initialize on return — possible load hitch, un-persisted internal state lost. Accepted cost of the CPU reclaim; surface it at the toggle affordance (tooltip).

Module architecture (preserve the pure/shell split)

Pure (no REAPER types, unit-tested — the mirror of bank_model):

• view_mode_model — mode registry (id/name/ordinal; Arrange + Design seeded); membership index (track GUID → { mode ids } + per-track show-both flag; add / remove / retag / query); folder-tree-aware visibility derivation (given the current parent↔child tree supplied by the shell + the active mode, compute the visible set); the parking/restore planner (given active mode + snapshot record, emit the exact (track, flag, value) operation lists for park and restore — where the restore invariant is enforced); JSON round-trip of modes + membership + show-both + snapshots + active mode.

REAPER-facing:

• view shell — reads I_FOLDERDEPTH across the track list to build the parent↔child tree and feeds it to view_mode_model; applies the planner's operations via SetMediaTrackInfo_Value (B_SHOWINTCP / B_SHOWINMIXER / B_MAINSEND / I_FXEN) and TrackFX_GetCount + per-FX TrackFX_SetOffline; snapshots prior flag values before parking; resolves GUIDs via GetTrackGUID / guidToString / stringToGuid. Never touches master visibility, never touches B_MUTE / I_SOLO.
• persist (slice) — serialize/deserialize the view section into the "reasampler" namespace alongside the bank; on project open, rebuild the tree and reapply the active mode.
• actions (entries) — toggle active mode; activate mode: Arrange / Design; tag/untag selected tracks → mode; show-both for selected tracks. Registered with the command_id / gaccel / hookcommand pattern; toggle + mode-jumps MIDI-bindable.
• UI (in the ReaSampler / bank_panel window) — a segmented mode switch ([ Arrange | Design ]) in the window header, active segment lit; small per-mode membership count; the offlined-FX caveat as a tooltip. Tag/untag acts on the current REAPER track selection, not a per-track widget.

Show-both semantics

A per-track "pin visible across modes" flag that re-enables processing whenever shown. A show-both leaf appears in every mode's visible set and is never parked — its driven flags stay at snapshot/restored values, FX online, in the mix. ("Show but keep parked" is not offered — a visible-but-silent-and-offline track is clutter with a thumbnail.) Stored on the membership record; persists; togglable per selection.

REAPER API surface (verify all signatures)

• Visibility/routing/FX flags via GetMediaTrackInfo_Value (snapshot) / SetMediaTrackInfo_Value (apply): B_SHOWINTCP, B_SHOWINMIXER, B_MAINSEND, I_FXEN. (Note: brief cited B_SHOWINMCP; verified SDK name is B_SHOWINMIXER.)
• Per-FX offline: TrackFX_GetCount + TrackFX_SetOffline(track, fx, offline).
• Folder tree: read I_FOLDERDEPTH per track to derive parent↔child structure.
• GUID keying: GetTrackGUID, guidToString, stringToGuid.
• Persistence: SetProjExtState / GetProjExtState under "reasampler" (shared with the bank index — one blob, two logical sections).
• Wrap flag mutations in Undo_BeginBlock2 / Undo_EndBlock2 as appropriate.

Non-goals / guardrails

• No literal second canvas. A literal second arrange surface, a second window, or a duplicated project stays rejected — reject any such path in review. This guardrail was narrowed (2026-07-23), not lifted: per-item mode separation via REAPER fixed lanes on shared tracks (an item-visibility projection over the one timeline) is now allowed and specified in §Two-canvas sub-phase below. Paths that remain rejected: subproject / second-project-file approaches, and overloading item D_POSITION with mode semantics (timebase-offset regions) — the latter collides with the capture null test. See docs/product/design-view.md §Two-canvas direction for why those were rejected.
• Never touch mute/solo. Any code path reading/writing B_MUTE / I_SOLO is a bug.
• Never drive a manual lane (Phase D2). A mode toggle touches only managed lanes (minted by the mode system, keyed in the lane-ownership index). Any code path that shows, hides, silences, or re-lanes a manual lane — a user-created comp/take lane outside the mode system — is a bug. The tool drives only lanes it created.
• Every leaf is managed. The mode system owns all leaf tracks: an untagged leaf is an Arrange member, and when the active mode is not Arrange it is fully parked and snapshot-restored exactly like a tagged leaf out of its mode. show-both is the only way to opt a leaf out of parking. (Parents stay visibility-only, master is never touched — see below.)
• Restore from snapshot, never to a default. No hardcoded "on" restores.
• Verify API names against the SDK header before use.

Open questions to resolve during build

• Reconcile residuals (bulk behavior shipped — ViewModeModel::reconcile(liveGuids) prunes orphaned snapshots on every toggle/load; folder restructure is self-healing because the tree is rebuilt each toggle; membership is intentionally kept so undo-delete preserves the tag). Two sub-items remain deferred:
  • FX-GUID keying for restoreFxOffline: currently restores by slot index; a reshuffled FX chain while parked will restore to the wrong slot. Fix requires FX-GUID keying + snapshot-schema migration — deferred.
  • Dormant membership entries: truly-deleted tracks accumulate stale entries in persisted view_state (harmless and bounded); natural home is a future user-initiated "compact" action, not automatic pruning (which would reintroduce undo-delete tag-loss).
• Interaction with the user having a screenset active (Design View drives the same flags a screenset recall would; last writer wins — confirm no surprising fight).

Two-canvas sub-phase (Phase D2 / Phase E) — additive to D1

Additive sub-phase, settled 2026-07-23. Extends D1's track-level mode projection to item level so each stance owns its own items as well as its own tracks. Nothing in D1 changes; this wraps it. Runtime floor rises to REAPER 7 for this sub-phase (fixed lanes shipped in v7). Product framing in docs/product/design-view.md §Two-canvas direction.

What it adds

On a track present in both stances (a show-both track, or a folder carrying its own media), each mode owns a fixed lane: the active mode's lane shows and plays; the inactive mode's lane is hidden and silent. A Design take and an Arrange take can then live on the same track, same time position, without colliding on the view. Track-only-in-one-mode content is still handled by D1 track-parking, unchanged.

Settled decisions

• Mechanism: fixed item lanes. Map mode → lane; toggle drives per-lane play/show so only the active mode's lane is present. Items keep their real position and real track — nothing is moved in time or deleted. SDK surface (verified present in vendor/reaper-sdk): track-side I_FREEMODE = 2, I_NUMFIXEDLANES, C_LANEPLAYS:N; item-side I_FIXEDLANE, C_LANEPLAYS, B_FIXEDLANE_HIDDEN. I_FREEMODE changes require UpdateTimeline() to take visible effect.
• Membership: auto-tag by active mode at creation. New content — both new tracks and new items — is tagged to whatever mode is active when it is created. Pre-existing content defaults to Arrange. Membership is exclusive: Design-created content never appears in Arrange and vice versa, except via the existing show-both escape hatch. (This is the same tag-to-active-mode rule as D1 track membership, now reaching items on shared tracks.)
• Inactive-mode content is hidden AND silenced. The off-mode lane is set C_LANEPLAYS = 0 — neither shown nor played — consistent with exclusive membership and with D1's "flipping modes is a real change, not cosmetic." Show-both is the deliberate opt-out for a lane that must stay audible across modes.
• Managed vs. manual lanes — indexed and distinct. Fixed lanes are also REAPER's native comping surface: a user may keep their own manual lanes (comp takes, alternate reads) on a track alongside the mode system's lanes. The tool maintains a lane-ownership index — per (track GUID, lane): managed (which mode owns it) vs manual (user-minted, outside the mode system). Mode operations touch only managed lanes; manual lanes are never shown, hidden, silenced, or re-laned by a toggle, and their C_LANEPLAYS stays exactly as the user set it. Items a user adds to a manual lane are not auto-tagged (auto-tag governs normal timeline content, not hand-managed lanes). The ownership index rides in "reasampler" view_state alongside the membership index, GUID-keyed and portable.
• Capture placement is mode-aware. An explicit placement while in Design mode — including capture-and-place — lands the item in the Design lane; the same rule governs manual insertion. The capture load-bearing principle is untouched: capture still writes a file + index entry and never auto-inserts; this governs only where an explicit placement lands.
• REAPER floor: v7. No version-gate branch — below v7 this sub-phase is simply unavailable.

Precision invariants (unaffected — called out explicitly)

The capture precision invariants — null test, bit-identical repeats, non-destructive, exact bounds, relative-paths-only — are entirely unaffected by this sub-phase. No capture path changes; lanes are a placement/view concern downstream of the written file. Lane assignment and C_LANEPLAYS are reversible flags: the item is never relocated in time or deleted, so the sub-phase extends the D1 non-destructive snapshot/restore contract to a new (item-lane) flag family rather than introducing any destructive operation.

New invariant — mode operations touch only managed lanes. A mode toggle drives only lanes the mode system minted (managed lanes, keyed in the lane-ownership index). Manual lanes — user-created comp/take lanes outside the mode system — are never shown, hidden, silenced, or re-laned by a toggle; their C_LANEPLAYS is left exactly as the user set it. This is the fixed-lane analog of never touch B_MUTE/I_SOLO and never touch master: the tool drives only what it created. Enforceable and testable in the pure layer — the "which lanes may this toggle touch" decision is a pure query over the ownership index; only reading REAPER's live lane state is shell.

Module architecture (preserve the pure/shell split)

• Pure (view_mode_model extension). Lane math — which lane maps to which mode, which C_LANEPLAYS value per mode, the item-lane op family alongside the existing track-flag op family — is REAPER-free and unit-tested, mirroring the D1 planner. The lane-ownership index (per (track GUID, lane): managed-which-mode vs manual) and the "which lanes may this toggle touch" query (managed only) are pure and unit-tested — the planner emits lane ops for managed lanes only and never for manual lanes. The auto-tag decision is pure too: given a set of new track/item GUIDs + the active mode — and, for items, whether the item landed in a manual lane (exempt) — produce the membership writes.
• Shell. Two shell responsibilities. (1) Apply the planner's item-lane ops (I_FREEMODE/I_FIXEDLANE/C_LANEPLAYS/B_FIXEDLANE_HIDDEN via the media-item info setters, UpdateTimeline() after I_FREEMODE changes) — for managed lanes only. (2) Detect new content and read live lane state — see below.
• Persistence. The tool persists which lane maps to which mode and the managed/manual ownership index (a small addition to the "reasampler" view section); REAPER stores fixed lanes and lane-plays in the .rpp natively.

New-content detection (implementation design point)

REAPER exposes no clean "item added" / "track added" event callback. Auto-tagging therefore requires the shell to diff project state on the panel's existing timer — the bank_panel already polls and fingerprints the bank; this extends that machinery to the timeline's tracks and items.

• Each poll, compare the live track/item GUID set against the previous poll's set; any GUID new since the last poll is tagged to the then-active mode.
• Correctness the implementation must handle: the first poll after project open must not mass-tag pre-existing content (pre-existing defaults to Arrange, per the membership rule).
• Manual-lane exemption (design point). An item added to a manual lane is not auto-tagged — auto-tag governs normal timeline content, not lanes the user hand-manages. Distinguishing the two may need a heuristic at detection (e.g., an item whose I_FIXEDLANE is marked manual in the ownership index is exempt; content outside any managed lane on a mode-managed track follows the active-mode rule). The precise rule is an open implementation design point; the settled boundary is that manual-lane content is off-limits to auto-tag.
• Lane-identity fragility (design point). I_FIXEDLANE is the lane's identity and is how the ownership index keys to a lane. Whether the index survives lane reorder/renumber/deletion without going stale is an implementation design point (same class as GUID-keyed reorder-safety for tracks) — flag, don't solve here.
• Pure/shell seam: the detection (diffing REAPER's live set each tick, reading live lane ownership) is shell; the tagging decision and the managed/manual lane query (new GUIDs + active mode + manual-lane exemption ⇒ membership + lane writes) are pure and unit-tested.

REAPER API surface (verify all signatures)

• Fixed lanes — track: I_FREEMODE (=2), I_NUMFIXEDLANES, C_LANEPLAYS:N via GetMediaTrackInfo_Value/SetMediaTrackInfo_Value; item: I_FIXEDLANE, C_LANEPLAYS, B_FIXEDLANE_HIDDEN via GetMediaItemInfo_Value / SetMediaItemInfo_Value. Call UpdateTimeline() after I_FREEMODE changes.
• Detection reuses the bank_panel timer + GUID fingerprinting already in place; item GUIDs via the item's GUID (GetSetMediaItemInfo_String "GUID"), track GUIDs via GetTrackGUID as in D1.
• Verify every name/signature against the SDK header before use — the surface is verified present, but confirm argument order, types, and flag values.

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Multi-bank — additive phase spec

Additive section. This is a standalone phase parallel to — not part of — the M0–M11 capture roadmap and Phase D above. Nothing above changes. Product framing (workflow narrative, pool-privilege reasoning, movement semantics, UI-direction calls) lives in docs/product/multi-bank.md; this section is the authoritative technical spec, matching the house style of the capture and Design View specs. Same standing discipline applies: verify every REAPER API name/signature against vendor/reaper-sdk/sdk/reaper_plugin_functions.h before use.

What it is

The single per-project bank (bank_model / BankIndex) is generalized into a multi-bank system. The existing bank becomes the pool — a default, always-present bank that every capture lands in unless another bank is the active target. On top of the pool the user creates named banks ("Drums", "1-Shots", "Synth Hits") that group samples for a purpose. Samples move freely between any banks, including to and from the pool. Exactly one bank is the active bank — the capture target — the pool by default.

This is the container generalization of the capture pillar's bank. The pool is to banks what Arrange is to modes: structurally one member of an N-collection, but privileged as the default home. The capture pillar's load-bearing rule is untouched — capture still writes a file + an index entry and never inserts into the arrange; the only change is which index the entry lands in.

Settled decisions

• The pool is privileged, not special-cased. Structurally the pool is one BankIndex among many in the container (mirror of "Arrange is just another mode"). Semantically it is privileged: it always exists, is un-deletable, and is un-renamable (fixed id + fixed display name "Pool"). New projects and migrated single-bank projects start with the pool and zero named banks. This keeps the data model uniform (no pool-shaped special type) while the rules layer enforces the three privileges.
• Container in the pure core; a BankIndex per bank. A new pure module bank_book owns an ordered registry of banks, each bank = { stable bank id, display name, ordinal, BankIndex }. BankIndex is untouched — the multi-bank layer wraps it, it does not modify it (additive; no bank-id field on Sample). Bank id is the stable key (GUID-style, minted on bank create); display name and ordinal are mutable (rename / reorder). Display names are unique, enforced in the pure model on create and rename: createBank / renameBank reject a name that duplicates an existing bank's (renaming a bank to its own current name is a no-op success). The comparison is trimmed + case-insensitive (ASCII), so "Drums", "drums", and " Drums " cannot coexist; the pool's reserved name "Pool" is protected by the same check. Uniqueness makes by-name resolution in the action shell unambiguous by construction. The pool is the first, seeded, fixed-id member. bank_book is the mirror of bank_model and view_mode_model: pure, no REAPER types, unit-tested outside the DAW, JSON round-trip.
• Active bank lives in the model, routes through the capture path. bank_book carries the active bank id (defaults to the pool). The capture action family resolves "which bank does this capture land in?" by asking the session for the active bank's BankIndex, then adds exactly as today. No capture backend changes; only the add-target is selected upstream. Activating a bank is a model mutation + a persist write; it never touches the timeline.
• Movement moves the index entry, not the file. Moving a sample from bank A to bank B is an index-only operation: remove the Sample from A's BankIndex, add it to B's. The underlying file stays in the project bank folder — banks are logical groupings over one shared file pool, not separate folders on disk. This keeps movement cheap, non-destructive, and immune to path-rewrite bugs. (Per-bank subfolders on disk are an explicit non-goal — see guardrails.)
• Dedup-by-hash is per-bank. Each BankIndex dedups within itself, unchanged. Moving a sample whose hash already exists in the destination bank collapses onto the existing entry there (the move is a no-op add on the destination side, and the source entry is still removed) — the same collapse semantics BankIndex::add already has, now observed across a move. Cross-bank dedup is not enforced: the same hash may exist in the pool and in a named bank simultaneously (that is the point — copy lets a sample be grouped into "Drums" while still living in the pool).
• Move vs. copy are distinct acts; move is the default. Move removes from source, adds to destination (one logical sample, regrouped) — it is the primary, low-friction gesture, so a sample lives in exactly one bank at a time. Copy adds to destination and leaves the source entry intact (same file, two index entries, two banks) — the deliberate secondary act for the "in two places at once" case. Both are index-only; both share the destination-collapse rule. Under move-as-default the pool is the default home and staging ground, not a permanent superset: moving a sample into a named bank takes it out of the pool. (See product notes for the mental-model reconciliation.)
• Delete drops members; evacuate returns them. Deleting a named bank drops its member index entries (files are not deleted — file lifecycle stays owned by the capture/prune path). A separate evacuate operation moves all of a bank's members back to the pool (index-only, same destination-collapse-by-hash as move), leaving the bank empty. Intended workflow: evacuate then delete to keep the samples, plain delete to drop the grouping and its members. A plain delete of a non-empty bank orphans those members out of every index — their files persist on disk until prune, referenced by no bank — so the UI confirms on non-empty delete and offers evacuate as the alternative. Evacuate cannot be applied to the pool.
• Persistence: a new ext-state key; the pool folds in and the legacy key is retired. The multi-bank state serializes to a new key banks in the existing "reasampler" namespace, alongside view_state and project_guid. The pool's index rides inside the banks blob as bank-zero — persisted identically to any named bank (one blob, one section, one JSON shape). Migration: on load, if a banks key is absent but a legacy bank_index key is present, the legacy index is promoted into the pool inside a freshly-minted banks blob and the book is { pool } with zero named banks — a one-way, lossless promotion. After migration the banks blob is authoritative; the legacy bank_index key is retired (not written or read back going forward). The one-way retirement trades pre-multi-bank backward-read compatibility for the clean single-blob shape — an accepted, forward-only migration consistent with how M4 project state already moves forward.
• Vertical-split UI, pool on top. The bank window splits vertically: pool on top, the named-banks region below (a tab-page strip, one tab per named bank, empty when none exist). Two full-height toggles collapse the split: pool full-height (hide the named-banks region) and banks full-height (hide the pool). The Design View segmented mode switch already in the window header is orthogonal and stays — it governs timeline visibility, not bank grouping; the two coexist in the header/body without interaction.

Precision / invariant implications

• Relative-paths-only survives unchanged. Every BankIndex in the book keeps the relative-path invariant at its add boundary — the book adds no new path handling, because movement is index-only and files never relocate. The invariant is enforced N times (once per bank) by the exact code that enforces it today.
• Non-destructive. Bank create / rename / delete / activate / evacuate and sample move / copy mutate only index + ext-state; no file is written, moved, or deleted, and no timeline item is touched. Deleting a named bank drops its member index entries but does not delete their files; file lifecycle stays owned by the capture/prune path, not the bank container. A file referenced only by the deleted bank becomes an orphan on disk — present but indexed by no bank — until the capture/prune path reclaims it. That orphaned-until-prune window is designed, not accidental; the evacuate verb and the confirm-on-non-empty-delete guardrail exist to keep the user out of it unintentionally.
• Travels-with-the-.rpp preserved. The banks blob rides the same ext-state namespace and the same GUID-primary project-identity / Save-As-relocation machinery as the bank index does today (M4). One shared physical bank folder, one ext-state namespace, now three logical sections (banks + view + identity).
• Determinism / null-test / bit-identical are untouched — they are properties of the capture path and the file, and the multi-bank layer sits above the file entirely.

Module architecture (preserve the pure/shell split)

Pure (no REAPER types, unit-tested — the mirror of bank_model / view_mode_model):

• bank_book — ordered bank registry ({ bank id, display name, ordinal, BankIndex }); pool seeded with fixed id + name; create / rename / reorder / delete named banks (pool-privilege rules enforced here: reject delete/rename of pool; delete drops member index entries; display names unique — create/rename reject a name that duplicates another bank's, trimmed + case-insensitive, "Pool" protected); evacuate a bank (move every member to the pool, index-only, destination-collapse observed; pool cannot be evacuated); active-bank id (get/set, defaults to pool); move and copy a sample between banks (index-only, destination-collapse observed); query a bank's index; JSON round-trip of the whole book (pool-as-bank-zero inside the blob + named banks + per-bank indices + active id + ordinals) and legacy-bank_index→pool migration on parse (one-way; blob authoritative thereafter).

REAPER-facing:

• persist (slice) — serialize/deserialize the book under the banks key in "reasampler" (pool-as-bank-zero inside the blob; no separate bank_index key going forward); migrate a legacy bank_index key into the pool on first load (one-way; blob authoritative thereafter, legacy key retired); reload-on-open and Save-As survival ride the existing M4 machinery. The session exposes the book the way it exposes the bank today; the active bank's BankIndex is what the capture layer adds to.
• bank_panel (extension) — the vertical split: pool grid on top, named-banks tab-page region below; two full-height toggles; the active-bank indicator; the create / rename / delete / activate affordances. The named-banks tab strip is LICE-drawn to match the M5 grid and the Design View segmented switch (not a SWELL-native tab control), with an overflow/scroll affordance so it scales past the ~8–12-tab point. Sample move ships both ways: a "move to bank" menu on the current selection (the bindable front-end for the B3 move action) and drag-between-regions (the direct-manipulation accelerator); copy is the deliberate secondary act, offered on the menu. Drag carries clear drop-target highlighting on the destination region/tab, and a mis-drop is recoverable by design (move is index-only and reversible — the user moves the sample back). Reuses the existing LICE grid render loop per bank region.
• actions (entries) — create bank / rename bank / delete bank (confirm on non-empty delete); evacuate bank → pool; activate bank (direct + cycle); move selected samples → bank; copy selected samples → bank; pool/banks full-height toggles. Registered with the command_id / gaccel / hookcommand pattern; bank-activate + move/copy + evacuate MIDI-bindable to suit the capture-heavy workflow.

REAPER API surface (verify all signatures)

No new REAPER API is invented at spec stage — the multi-bank layer is pure model + persistence + panel UI over machinery M0–M6 already established. Shells will need to verify against the SDK header where they extend existing surfaces:

• Persistence: SetProjExtState / GetProjExtState under "reasampler", new key banks (shared blob machinery from M4 — no new API, new key only).
• Panel UI: the docked-window + LICE-grid surface from M5 (bank_panel), extended to two grid regions + a LICE-drawn tab strip (with overflow/scroll) + toggles. The tab strip, the toggle affordances, and the drag hit-testing are custom-drawn on the M5 LICE surface (not SWELL-native tabs); the "move to bank" menu uses a SWELL popup-menu surface. Verify LICE drawing and any SWELL menu/drag hit-test usage against the M5 reference / SWELL headers, and confirm the drag hit-test does not collide with the M5 grid's multi-select drag. No new REAPER audio API involved.
• Actions: the command_id / gaccel / hookcommand contract from main.cpp (unchanged), new command-id strings under the sampler family prefix.

Non-goals / guardrails

• No per-bank folders on disk. Banks are logical groupings over one shared project bank folder. Do not create a subfolder per bank or move files on bank-move — reject any such path in review (it reintroduces the path-rewrite bug class M4 closed).
• No cross-bank dedup enforcement. The same hash may exist in multiple banks (that is what copy is for). Do not add a global dedup that collapses across banks.
• Pool privileges are inviolable. No action path may delete or rename the pool, leave a project with zero banks, or evacuate the pool (the pool is evacuation's destination, not a source). Enforce in the pure rules layer, not just the UI.
• Delete drops members; files are never deleted by a bank op. Deleting a named bank removes its member index entries only. No bank operation writes, moves, or deletes a file — file lifecycle stays with capture/prune. The UI confirms on non-empty delete and offers evacuate; do not silently orphan members.
• Capture still never inserts into the arrange. The load-bearing principle is unchanged; multi-bank only redirects which index the capture lands in.
• Additive only. Do not alter BankIndex, the M0–M11 capture roadmap, or Phase D semantics. bank_book wraps; it does not modify.
• Verify API names against the SDK header before use.

Open questions to resolve during build

Forks 1–5 are all settled (see product notes → Settled forks and Fork 5 — settled, and the settled-decision prose above). One panel-polish detail remains open.

• Fork 5 — tab rendering + move affordance (B4). Settled (2026-07-23). (5a) The named-banks region is LICE-drawn to match the M5 grid and the Design View segmented switch — not a SWELL-native tab control — with an overflow/scroll affordance in scope from the start so the strip scales past the ~8–12-tab breakdown. (5b) Move ships as both a "move to bank" menu (the precise, MIDI-bindable front-end for the B3 move action) and drag-between-regions (the direct-manipulation accelerator); copy stays the deliberate secondary act via the menu. Drag mis-drop is mitigated by drop-target highlighting and is recoverable by design (move is index-only and reversible). Folded into the bank_panel prose and the API surface below. Verify LICE tab drawing and any SWELL menu/drag hit-test surface against the M5 reference / SWELL headers before use (confirm no collision with the M5 grid's multi-select drag). Analysis in product notes → Fork 5.
• Active-bank indicator placement (B4 polish) — per-region headers vs. a single header readout vs. lit-tab treatment. The "visually unmistakable" requirement is settled (fork 4); only the placement is open. Panel-polish detail.