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/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;