Ψ-W2-T2: collapse a capture whose channels are bit-identical to one lossless mono channel, index value measured off the landed file

src/core/capture/CLAUDE.md

@@ -45,7 +45,7 @@ Detail specific to these pure modules:
 
 ## Modules
 
-- `wav_codec` — chunk walker + layout parse + float32 build + size-field patch + content hashes; the single pure RIFF/WAV owner (`wav_trim` is retired; `wav_codec` is the sole owner).
+- `wav_codec` — chunk walker + layout parse + float32 build + size-field patch + the lossless mono collapse + content hashes; the single pure RIFF/WAV owner (`wav_trim` is retired; `wav_codec` is the sole owner).
 - `capture_realtime` (`core/capture`, **renamed from `realtime_record` in Q-W3** — the Q-9 naming rider: pure module takes the stem, the shell takes the suffix, matching `drag_out`/`drag_out_win`) — the M8 realtime-record pure logic: capture scope + FX-tap point → `I_RECMODE`/`I_RECMODE_FLAGS` values, wet/dry → tap point, the recorded-file → `Sample` mapping, and the async record-phase state machine. Depends on `bank_model` for the plain `Sample`/`SourceMode` types. The transport/temp-track/send recipe lives in the shell (`shell/capture/capture_realtime_shell.cpp` + `capture_realtime_finalize.cpp`).
 - `batch_capture` — pure batch-capture planner: maps source ranges to capture units and aggregates results.
 - `capture_paths` — the REAPER-free path arithmetic behind offline capture: bank-subfolder + unique-filename derivation (`deriveBankPaths`, forward-slash form, no filesystem touch), the absolute-render-dir vs. project-relative-index-path split (`BankPaths`), the persist-side inverse (`resolveBankFile`, `projectDirOfRpp`), the Save-As bank-relocation plan (`deriveRelocationPlan`), and the GUID-primary project-identity classifier (`classifyProjectTransition` → `NoOp`/`Load`/`SaveAsRelocate`) the persist-poll timer drives.
@@ -81,6 +81,14 @@ Detail specific to these pure modules:
 - `kRenderPreFaderStems` (&8192) is deliberately **not** used — REAPER offline
   render has no true pre-FX "dry" bit; FX scoping is done entirely by the
   FX-bypass-around-render mechanism, never by a render bit.
+- **The mono collapse changes a capture's content identity, by design.**
+  `hashWavContent` covers the `fmt ` body plus the `data` payload, and the collapse
+  rewrites both — so a collapsed capture does NOT hash-dedup against a stereo twin of
+  the same audio already in the bank. Accepted: the predicate is deterministic over
+  deterministic bytes, so repeats of the same request still dedup against each other,
+  which is what the bit-identical-repeats invariant actually asks for. Do not "fix"
+  this by hashing pre-collapse — that would make two entries with different audio
+  layouts share one identity.
 - `tail_control`'s `kDefaultManualTailMs`/`kManualStepMs` and
   `render_settings`'s `kMaxTailMs`/`kAutoTrimThresholdDb` are separate constants
   in separate files by design (panel-facing default/step vs. runaway-guard cap)

src/core/capture/wav_codec.cpp

@@ -257,6 +257,44 @@ std::vector<std::uint8_t> buildFloat32Wav(int nch, std::uint32_t rate,
     return out;
 }
 
+MonoCollapse collapseToMono(const std::vector<std::uint8_t>& bytes) {
+    MonoCollapse out;
+
+    const WavLayout layout = parseWavLayout(bytes);
+    if (!layout.valid || layout.channelCount < 2) return out;
+
+    const std::size_t frames = layout.frameCount();
+    if (frames == 0) return out;
+
+    const std::size_t stride = layout.channelCount;
+    const std::vector<AudioSample> pcm = extractFloatFrames(bytes, layout, 0, frames);
+    if (pcm.size() != frames * stride) return out;  // short read -> decline, never guess
+
+    // Bit patterns, not values: see the header. memcpy is the only defined float->bits
+    // read, and it compiles to a register move.
+    auto bitsOf = [](AudioSample s) {
+        std::uint32_t bits = 0;
+        std::memcpy(&bits, &s, 4u);
+        return bits;
+    };
+    for (std::size_t f = 0; f < frames; ++f) {
+        const std::uint32_t first = bitsOf(pcm[f * stride]);
+        for (std::size_t c = 1; c < stride; ++c) {
+            if (bitsOf(pcm[f * stride + c]) != first) return out;
+        }
+    }
+
+    // float -> double -> float round-trips exactly (double represents every float),
+    // so channel 0 reaches the rebuilt file unaltered.
+    std::vector<double> mono(frames);
+    for (std::size_t f = 0; f < frames; ++f)
+        mono[f] = static_cast<double>(pcm[f * stride]);
+
+    out.collapsed = true;
+    out.bytes = buildFloat32Wav(1, layout.sampleRate, frames, mono);
+    return out;
+}
+
 std::string hashBytes(const std::uint8_t* data, std::size_t len) {
     // FNV-1a 64-bit: deterministic, no dependencies, adequate for dedup identity.
     std::uint64_t h = kFnvOffsetBasis;

src/core/capture/wav_codec.h

@@ -90,6 +90,32 @@ std::vector<std::uint8_t> buildFloat32Wav(int nch, std::uint32_t rate,
                                           std::size_t frameCount,
                                           const std::vector<double>& interleaved);
 
+// --- Lossless mono collapse ---------------------------------------------------
+
+// The outcome of the bit-identical mono collapse. `collapsed == false` means the
+// caller must leave the source file exactly as it is — it writes nothing.
+struct MonoCollapse {
+    bool collapsed = false;
+    std::vector<std::uint8_t> bytes;  // the rebuilt 1-channel WAV; empty unless collapsed
+};
+
+// Collapses a multi-channel float32 WAV to one channel when EVERY channel of EVERY
+// frame carries the identical float BIT PATTERN. Bit equality, never an epsilon and
+// never `==` on floats: +0.0/-0.0 and two NaNs with differing payloads are NOT
+// identical and are never folded. Frame count, sample rate and bit depth are
+// preserved — only the interleave stride changes — so the collapse cannot lose
+// information, and a lossy downmix (summing differing channels) is not something
+// this can express.
+//
+// Declines for: bytes that do not parse; a file already at one channel; a zero-frame
+// file (no frame of evidence to act on); any differing channel pair.
+//
+// The rebuild is a canonical minimal WAV, so non-audio chunks (a renderer's `bext`
+// timestamp, iXML, LIST) do not survive it. That much hashWavContent already skips —
+// but the collapse rewrites the `fmt ` body and the `data` payload too, which moves
+// the file's content identity; see this directory's CLAUDE.md for what that costs.
+MonoCollapse collapseToMono(const std::vector<std::uint8_t>& bytes);
+
 // --- Content identity (dedup hashes) -----------------------------------------
 
 // Deterministic FNV-1a 64-bit content hash over `len` bytes, as 16-char lowercase

src/core/model/bank_model.h

@@ -88,6 +88,9 @@ struct Sample {
 
     double wetDry = 1.0;            // 1.0 = fully wet, 0.0 = fully dry
 
+    // Channels in the file this entry names — equal to its `fmt ` count by
+    // construction, which is what makes the instrument's mono/stereo read-out and
+    // its waveform lane count agree with the audio. 0 = unknown (pre-field entry).
     int    channelCount = 0;
     int    sampleRate   = 0;
 

src/shell/capture/CLAUDE.md

@@ -51,7 +51,7 @@ detail not covered there:
 
 ## Modules
 
-- `capture` — two CONCRETE backends with deliberately different lifecycles (no shared interface — the former `ICaptureBackend` was deleted in Q-W3, T4-26: one deriver, zero polymorphic call sites): `OfflineRenderBackend` (deterministic default, synchronous) and `RealtimeRecordBackend` (async begin/tick/abort). Input: `CaptureRequest`. Output: finished file + populated `Sample` handed to `bank_model`.
+- `capture` — two CONCRETE backends with deliberately different lifecycles (no shared interface — the former `ICaptureBackend` was deleted in Q-W3, T4-26: one deriver, zero polymorphic call sites): `OfflineRenderBackend` (deterministic default, synchronous) and `RealtimeRecordBackend` (async begin/tick/abort). Input: `CaptureRequest`. Output: finished file + populated `Sample` handed to `bank_model`. It also owns the two file-side steps both backends share: `collapseCapturedFileToMono` (the lossless mono collapse, applied to the landed file) and `stampCaptureSample`, which measures the channel count off that same file so the entry and the audio cannot disagree.
 - `scope_resolve` (`shell/capture`) — scope/source resolution shared by every capture entry point (Q-W3 hoist out of `main.cpp`): razor-else-time range inference, selected-track/selected-item-owning-track collection with canonical GUIDs, and the M10 provenance-assembly inputs (read BEFORE the FX-bypass guard neutralizes the in-scope chain).
 - `render_selection` (`shell/capture`) — the transient track selection a selected-tracks render (`&128`) requires, as a stack RAII guard: REAPER prints whatever tracks are selected, so `renderOffline` makes the request's own tracks BE the selection for the render's duration and restores the user's set on every exit path. Engaged ONLY for that source mode, which leaves a stated residual: a `&32` selected-items render still prints whatever ITEMS the user has selected. Live captures are unaffected (that selection is the source), but a recipe replay of a `SelectedItems` capture renders against whatever happens to be selected then — the recipe stores tracks and a range, never item GUIDs, so this guard cannot close it. Filed in `docs/TODO.md`.
 - `render_isolation` (`shell/capture`) — the transient upstream silencing a ranged ITEM render needs, as a stack RAII guard alongside the two above: the selected-tracks source prints everything flowing INTO the track, so each direct folder child's `B_MAINSEND` and each of the track's receives' `B_MUTE` are cut for the render and restored on every exit path. Direct children only — a grandchild reaches the track through the child that owns it. The child-set walk is pure (`core/capture/track_topology`).

src/shell/capture/capture.cpp

@@ -201,11 +201,27 @@ std::string makeUniqueTag(const std::string& prefix) {
            std::to_string(++counter);
 }
 
+void collapseCapturedFileToMono(const std::string& absolutePath) {
+    const std::vector<std::uint8_t> bytes = util::readFileBytes(absolutePath);
+    if (bytes.empty()) return;
+
+    const MonoCollapse collapse = collapseToMono(bytes);
+    if (!collapse.collapsed) return;
+
+    // One truncating write — the same shape, and the same accepted mid-write residual,
+    // as the realtime Auto-tail trim (capture_realtime_finalize.cpp).
+    std::ofstream out(absolutePath, std::ios::binary | std::ios::trunc);
+    if (!out) return;
+    out.write(reinterpret_cast<const char*>(collapse.bytes.data()),
+              static_cast<std::streamsize>(collapse.bytes.size()));
+}
+
 void stampCaptureSample(Sample& s, const CaptureRequest& req,
                         ReaProject* rateProj, ReaProject* timeSigProj,
                         const std::string& absolutePath) {
-    // Track GUIDs + channel count: echoed from the request (the caller resolved
-    // the selection; the backends stay source-agnostic).
+    // Track GUIDs echoed from the request (the caller resolved the selection; the
+    // backends stay source-agnostic). channelCount starts at the request value only
+    // as the fallback for an unparseable file — the produced FILE overrides it below.
     s.trackGuids   = req.trackGuids;
     s.channelCount = req.channelCount;
 
@@ -238,6 +254,10 @@ void stampCaptureSample(Sample& s, const CaptureRequest& req,
         const std::vector<std::uint8_t> fileBytes = util::readFileBytes(absolutePath);
         if (!fileBytes.empty()) {
             s.contentHash = hashWavContent(fileBytes);
+            // The one authority for the entry's channel count is the file's own `fmt`
+            // — never the render request, which asks for 2 on every capture path.
+            const WavLayout layout = parseWavLayout(fileBytes);
+            if (layout.valid) s.channelCount = static_cast<int>(layout.channelCount);
         }
     }
 
@@ -447,6 +467,14 @@ CaptureResult OfflineRenderBackend::capture(const CaptureRequest& request) {
         }
     }
 
+    // Lossless mono collapse, deliberately AFTER the bounds gate: the gate measures
+    // REAPER's own render against the requested window, so nothing of ours may sit
+    // between the render and that measurement, and a refusal must delete the
+    // renderer's file rather than one this step had already rewritten. The collapse
+    // preserves the frame count, so the two are order-independent in outcome — only
+    // in what each is measuring.
+    collapseCapturedFileToMono(expectedPath);
+
     // Record the request's own bounds (exact) rather than re-measuring the file.
     Sample s;
     // Same uniqueTag that named the file — calling makeUniqueTag() again could

src/shell/capture/capture.h

@@ -53,6 +53,9 @@ struct CaptureRequest {
 
     // 0 sampleRate => follow project rate.
     int         sampleRate   = 0;
+    // What the RENDER is asked for (RENDER_CHANNELS / the realtime record mode), not
+    // what the capture lands as: a dual-mono render is collapsed to 1 channel after
+    // the fact, and the Sample's count comes from the produced file.
     int         channelCount = 2;
     WavBitDepth bitDepth      = WavBitDepth::Float32;
 
@@ -100,8 +103,16 @@ public:
 // backend's family marker ("" offline, "rt-" realtime).
 std::string makeUniqueTag(const std::string& prefix);
 
-// Stamps the metadata shared by both backends onto `s`: trackGuids + channelCount
-// (echoed from the request), resolved sampleRate (request rate, else PROJECT_SRATE
+// Rewrites a just-captured WAV in place as a 1-channel file when its channels are
+// bit-identical (the pure `collapseToMono` decides). Every other file is left
+// untouched, byte for byte, so the not-collapsed path is exactly what the backend
+// produced. Must run BEFORE stampCaptureSample, which measures the landed file.
+void collapseCapturedFileToMono(const std::string& absolutePath);
+
+// Stamps the metadata shared by both backends onto `s`: trackGuids (echoed from the
+// request) + channelCount (measured from the produced file's `fmt`; the request
+// value only as the fallback for a file that cannot be parsed), resolved
+// sampleRate (request rate, else PROJECT_SRATE
 // from `rateProj`), captureTempo, the capture-start time signature
 // (TimeMap_GetTimeSigAtTime against `timeSigProj` — offline passes nullptr for the
 // active project, realtime pins the record's own project), the WAV-aware

src/shell/capture/capture_realtime_finalize.cpp

@@ -184,6 +184,10 @@ CaptureResult finalizeRecording(ReaProject* proj, MediaTrack* temp,
                                                 request.endSeconds);
     }
 
+    // Channel-domain rewrite, after the frame-domain trim so it acts on the final
+    // frame set; it preserves the frame count, so the trimmed length above still holds.
+    collapseCapturedFileToMono(destPath);
+
     // Pure recorded-capture -> Sample mapping (identity, bounds echo, tier).
     RecordedCapture cap;
     cap.relativePath = paths.relativePath;
@@ -194,7 +198,9 @@ CaptureResult finalizeRecording(ReaProject* proj, MediaTrack* temp,
     cap.wetDry       = request.wetDry;
     cap.displayName  = request.baseName;
     cap.trackGuids   = request.trackGuids;
-    cap.channelCount = request.channelCount;
+    // channelCount deliberately left unset here: stampCaptureSample measures it from
+    // the file below. Echoing the request was this path's own defect — it parsed the
+    // recorded layout for the trim and still reported the requested 2.
 
     result.status  = CaptureStatus::Ok;
     result.sample  = sampleFromRecordedCapture(cap);

src/shell/instrument/processor_reload.cpp

@@ -148,8 +148,9 @@ std::string ReaSamplerProcessor::reloadInstrument() {
     //    no-play — no crash, no retry loop.
     if (const SelectedSample* sel = findRef(refs, selId)) {
         // Auto-default: channelModeFor computes the mode from the loaded capture's channel
-        // count (always 2 for extension captures; mono only for ingest-imported mono files).
-        // An unknown count (0) or explicit user choice keeps the mode.
+        // count — 1 for an ingested mono file or a capture whose channels came out
+        // bit-identical and collapsed, 2 otherwise. An unknown count (0) or an explicit
+        // user choice keeps the mode.
         {
             std::lock_guard<std::mutex> cm(channelModeMutex_);
             channelMode_ = channelModeFor(sel->channelCount, channelMode_,