fix(waveform): 4x-oversampled min/max envelope, columnMinMax homed in peaks, panel routed through shared drawWaveform — one gap-free algorithm on all surfaces

src/peaks.cpp

@@ -3,6 +3,7 @@
 #include <algorithm>
 #include <climits>
 #include <cmath>
+#include <cstdint>
 
 // peaks implementation.
 //
@@ -64,6 +65,37 @@ Envelope computeEnvelope(const std::vector<AudioSample>& interleaved,
     return envelope;
 }
 
+MinMax columnMinMax(const ChannelEnvelope& bins, int columnCount, int col) {
+    const int nbins = static_cast<int>(bins.size());
+    if (columnCount <= 0 || nbins == 0) return MinMax{};
+
+    // Clamp col to [0, columnCount-1].
+    if (col < 0) col = 0;
+    if (col >= columnCount) col = columnCount - 1;
+
+    // Half-open bin range for this column, mirroring computeEnvelope's exact partition.
+    // 64-bit products: col*nbins can exceed int range for a large oversampled envelope
+    // (same overflow discipline as computeEnvelope's frame-span arithmetic above).
+    const std::int64_t begin64 = (static_cast<std::int64_t>(col) * nbins) / columnCount;
+    const std::int64_t end64 =
+        (static_cast<std::int64_t>(col) + 1) * nbins / columnCount;
+    // col <= columnCount-1 guarantees begin64 <= (columnCount-1)*nbins/columnCount < nbins.
+    const int colBinBegin = static_cast<int>(begin64);
+
+    // When the column spans no full bin (more columns than bins), use the enclosing bin
+    // so no column is left empty.
+    const int scanEnd = (end64 > begin64) ? static_cast<int>(end64) : colBinBegin + 1;
+    const int clampedEnd = (scanEnd <= nbins) ? scanEnd : nbins;
+
+    MinMax result = bins[static_cast<std::size_t>(colBinBegin)];
+    for (int b = colBinBegin + 1; b < clampedEnd; ++b) {
+        const MinMax& mm = bins[static_cast<std::size_t>(b)];
+        if (mm.min < result.min) result.min = mm.min;
+        if (mm.max > result.max) result.max = mm.max;
+    }
+    return result;
+}
+
 std::size_t lastFrameAboveThreshold(const std::vector<AudioSample>& interleaved,
                                     std::size_t channelCount,
                                     std::size_t frameCount,