[Bug]: toBitSet() ignores its argument and returns a bitset of every track

[EugineKh]

Detailed steps on how to reproduce the bug

toBitSet (const juce::Array<Track*>&)
(modules/tracktion_engine/model/edit/tracktion_EditUtilities.cpp:250) uses its argument
only to reach the Edit. The loop walks allTracks instead of tracks, so a bit is set for
every track in the Edit and the result never depends on what was asked for:

juce::BigInteger toBitSet (const juce::Array<Track*>& tracks)
{
    juce::BigInteger bitset;

    if (auto first = tracks[0])
    {
        auto allTracks = getAllTracks (first->edit);

        for (auto t : allTracks)          // <-- should be `tracks`
            if (int index = allTracks.indexOf (t); index >= 0)
                bitset.setBit (index);
    }

    return bitset;
}

Steps: create an Edit with three audio tracks, call toBitSet with an array holding only
the first one, and read back the bitset — all three bits are set.

The mask does reach the renderer — Renderer::Parameters::tracksToDo becomes
CreateNodeParams::allowedTracks (Renderer.cpp:38) — so this is not a value that gets
ignored downstream; a caller asking for a subset silently renders or measures the whole Edit.

This is easy to miss in-engine: Renderer::renderToFile (Renderer.cpp:672) passes
getAllTracks (edit), for which "every track" happens to be the right answer, and the one
in-engine call that does pass a subset — tracktion_NodeRendering.test.cpp:203,
toBitSet ({ t }) — is on an Edit with a single audio track. TestUtilities.h:112
(expectPeak → measureStatistics) and host code are where it bites.

The engine's own tests already work around it. Both
tracktion_EditNodeBuilder.test.cpp:34 and tracktion_Plugins.test.cpp:117 carry a private
getTracksMask() built from Track::getIndexInEditTrackList() rather than calling
toBitSet, and each asserts exactly the invariant toBitSet breaks:

static juce::BigInteger getTracksMask (const juce::Array<Track*>& tracks)
{
    juce::BigInteger tracksMask;

    for (auto t : tracks)
        tracksMask.setBit (t->getIndexInEditTrackList());

    jassert (tracksMask.countNumberOfSetBits() == tracks.size());
    return tracksMask;
}

The two index spaces agree, so the fix below is equivalent to that helper:
getIndexInEditTrackList() walks visitAllTracksRecursive, and getAllTracks is
visitAllTracks (f, true), which dispatches to the same visitAllRecursive
(tracktion_TrackUtils.cpp:123).

Observed on develop at 0d55ef0c.

What is the expected behaviour?

The returned bitset holds exactly the tracks passed in, as Renderer.h:54 describes
("An bitset of tracks to render"), so that Renderer::Parameters::tracksToDo and
measureStatistics can address a subset — and so that the two private getTracksMask()
copies in the test files can go back to calling the shared helper.

Unit test to reproduce the error?

class ToBitSetTests  : public juce::UnitTest
{
public:
    ToBitSetTests()
        : juce::UnitTest ("ToBitSet", "tracktion_engine")
    {
    }

    void runTest() override
    {
        using namespace tracktion::engine;
        auto& engine = *Engine::getEngines()[0];
        auto edit = Edit::createSingleTrackEdit (engine);
        edit->ensureNumberOfAudioTracks (3);
        auto audioTracks = getAudioTracks (*edit);

        beginTest ("toBitSet returns only the tracks it was given");
        {
            const auto allTracks = getAllTracks (*edit);
            juce::Array<Track*> justTheFirst { static_cast<Track*> (audioTracks[0]) };

            const auto bits = toBitSet (justTheFirst);
            expectEquals (bits.countNumberOfSetBits(), 1);
            expect (bits[allTracks.indexOf (audioTracks[0])]);
            expect (! bits[allTracks.indexOf (audioTracks[1])]);   // FAILS: every bit is set
        }
    }
};

static ToBitSetTests toBitSetTests;

Suggested fix

diff --git a/modules/tracktion_engine/model/edit/tracktion_EditUtilities.cpp b/modules/tracktion_engine/model/edit/tracktion_EditUtilities.cpp
index 0d15e1c9..fa964b88 100644
--- a/modules/tracktion_engine/model/edit/tracktion_EditUtilities.cpp
+++ b/modules/tracktion_engine/model/edit/tracktion_EditUtilities.cpp
@@ -255,7 +255,7 @@ juce::BigInteger toBitSet (const juce::Array<Track*>& tracks)
     {
         auto allTracks = getAllTracks (first->edit);
 
-        for (auto t : allTracks)
+        for (auto t : tracks)
             if (int index = allTracks.indexOf (t); index >= 0)
                 bitset.setBit (index);
     }

The README says third-party pull requests cannot be taken directly, so this is here as an
issue — happy to send it however suits you.

Operating systems

Linux

What versions of the operating systems?

Debian 13 (trixie), kernel 6.16, GCC 14.2

Architectures

Intel/AMD 64-bit

Testing on the develop branch

The bug is present on the develop branch (0d55ef0c).

Code of Conduct

• I agree to follow the Code of Conduct