Arpeggiator

A MIDI effect that turns held chords into rhythmic note sequences.
Rewriting a MidiBuffer in processBlock
Timing notes from the sample rate
One AudioParameterFloat for speed
Source: examples/Plugins/ArpeggiatorPluginDemo.h on GitHub.
ArpeggiatorPluginDemo.h
cpp
/*******************************************************************************
 The block below describes the properties of this PIP. A PIP is a short snippet
 of code that can be read by the Projucer and used to generate a JUCE project.

 BEGIN_JUCE_PIP_METADATA

 name:                  ArpeggiatorPlugin
 version:               1.0.0
 vendor:                JUCE
 website:               http://juce.com
 description:           Arpeggiator audio plugin.

 dependencies:          juce_audio_basics, juce_audio_devices, juce_audio_formats,
                        juce_audio_plugin_client, juce_audio_processors,
                        juce_audio_utils, juce_core, juce_data_structures,
                        juce_events, juce_graphics, juce_gui_basics, juce_gui_extra,
                        juce_audio_processors_headless
 exporters:             xcode_mac, vs2022, vs2026

 moduleFlags:           JUCE_STRICT_REFCOUNTEDPOINTER=1

 type:                  AudioProcessor
 mainClass:             Arpeggiator

 useLocalCopy:          1

 pluginCharacteristics: pluginWantsMidiIn, pluginProducesMidiOut, pluginIsMidiEffectPlugin

 END_JUCE_PIP_METADATA

*******************************************************************************/

#pragma once


//==============================================================================
class Arpeggiator final : public AudioProcessor
{
public:

    //==============================================================================
    Arpeggiator()
        : AudioProcessor (BusesProperties()) // add no audio buses at all
    {
        addParameter (speed = new AudioParameterFloat ({ "speed", 1 }, "Arpeggiator Speed", 0.0, 1.0, 0.5));
    }

    //==============================================================================
    void prepareToPlay (double sampleRate, int samplesPerBlock) override
    {
        ignoreUnused (samplesPerBlock);

        notes.clear();
        currentNote = 0;
        lastNoteValue = -1;
        time = 0;
        rate = static_cast<float> (sampleRate);
    }

    void releaseResources() override {}

    void processBlock (AudioBuffer<float>& buffer, MidiBuffer& midi) override
    {
        // A pure MIDI plugin shouldn't be provided any audio data
        jassert (buffer.getNumChannels() == 0);

        // however we use the buffer to get timing information
        auto numSamples = buffer.getNumSamples();

        // get note duration
        auto noteDuration = static_cast<int> (std::ceil (rate * 0.25f * (0.1f + (1.0f - (*speed)))));

        for (const auto metadata : midi)
        {
            const auto msg = metadata.getMessage();
            if      (msg.isNoteOn())  notes.add (msg.getNoteNumber());
            else if (msg.isNoteOff()) notes.removeValue (msg.getNoteNumber());
        }

        midi.clear();

        if ((time + numSamples) >= noteDuration)
        {
            auto offset = jmax (0, jmin ((int) (noteDuration - time), numSamples - 1));

            if (lastNoteValue > 0)
            {
                midi.addEvent (MidiMessage::noteOff (1, lastNoteValue), offset);
                lastNoteValue = -1;
            }

            if (notes.size() > 0)
            {
                currentNote = (currentNote + 1) % notes.size();
                lastNoteValue = notes[currentNote];
                midi.addEvent (MidiMessage::noteOn  (1, lastNoteValue, (uint8) 127), offset);
            }

        }

        time = (time + numSamples) % noteDuration;
    }

    using AudioProcessor::processBlock;

    //==============================================================================
    bool isMidiEffect() const override                     { return true; }

    //==============================================================================
    AudioProcessorEditor* createEditor() override          { return new GenericAudioProcessorEditor (*this); }
    bool hasEditor() const override                        { return true; }

    //==============================================================================
    const String getName() const override                  { return "Arpeggiator"; }

    bool acceptsMidi() const override                      { return true; }
    bool producesMidi() const override                     { return true; }
    double getTailLengthSeconds() const override           { return 0; }

    //==============================================================================
    int getNumPrograms() override                          { return 1; }
    int getCurrentProgram() override                       { return 0; }
    void setCurrentProgram (int) override                  {}
    const String getProgramName (int) override             { return "None"; }
    void changeProgramName (int, const String&) override   {}

    //==============================================================================
    void getStateInformation (MemoryBlock& destData) override
    {
        MemoryOutputStream (destData, true).writeFloat (*speed);
    }

    void setStateInformation (const void* data, int sizeInBytes) override
    {
        speed->setValueNotifyingHost (MemoryInputStream (data, static_cast<size_t> (sizeInBytes), false).readFloat());
    }

private:
    //==============================================================================
    AudioParameterFloat* speed;
    int currentNote, lastNoteValue;
    int time;
    float rate;
    SortedSet<int> notes;

    //==============================================================================
    JUCE_DECLARE_NON_COPYABLE_WITH_LEAK_DETECTOR (Arpeggiator)
};
Arpeggiator ​

Arpeggiator
