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