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Fix MIDI pickup crossing detection never firing #3835

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Fix MIDI pickup crossing detection never firing #3835

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6 changes: 5 additions & 1 deletion src/audiomixerboard.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -95,7 +95,7 @@ static bool midiPickupShouldApply ( int midiValue, int currentValue, int toleran
// Handles the case where rapid movement causes MIDI values to "skip over" the software value
if ( recentMidiValues.size() >= 2 )
{
int prevMidi = recentMidiValues.back();
int prevMidi = recentMidiValues[recentMidiValues.size() - 2];

@ann0see ann0see Jul 28, 2026

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Can this get out of bounds? E.g access -2?

@ignotus666 ignotus666 Jul 28, 2026

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Since if ( recentMidiValues.size() >= 2 ), there shouldn't be any danger of that.

The problem raised is in fact correct and the fix looks like it's correct too. I haven't had time to test it and don't know when I will - hopefully some time next week. This bug probably went unnoticed in testing by me because I modified my controller to output the full stream of CC messages even when moving it fast.

@mcfnord mcfnord Aug 9, 2026

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🤖 AI: ignotus666 got it right — the size() >= 2 guard is what prevents it, and now it's measured.

Swept with the two patched functions extracted verbatim into a driver built -fsanitize=address,undefined -D_GLIBCXX_DEBUG, which bounds-checks std::deque::operator[]: every midiValue x currentValue pair in 0..127, tolerance 0..8, ascending and descending sequences, starting from an empty buffer. 589,824 calls, buffer lengths 0, 1 and 2 all exercised, zero diagnostics. Since the buffer never exceeds MIDI_PICKUP_HISTORY (2), size() - 2 is always index 0.

The guard is load-bearing: at size() == 1, size() - 2 is unsigned and wraps to 18446744073709551615.

// Check if current and previous MIDI values bracket the software value
if ( ( prevMidi <= currentValue && midiValue >= currentValue ) || ( prevMidi >= currentValue && midiValue <= currentValue ) )
return true;
Expand All @@ -117,7 +117,11 @@ static bool midiPickupTryApply ( int midiValue, int currentValue, int tolerance,
tempPickup.push_back ( midiValue );

if ( !midiPickupShouldApply ( midiValue, currentValue, tolerance, tempPickup ) )
{
// keep this value: the next message needs it to detect a crossing
pickupBuffer = tempPickup;
return true; // Still waiting for pickup
}

// Picked up. Stop waiting
waitingForPickup = false;
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