feat(core/audio): add square-wave synth and audio recorder

src/Chip8.Core/Audio/AudioRecorder.cs

@@ -0,0 +1,65 @@
+using System;
+
+namespace Chip8.Core.Audio;
+
+public sealed class AudioRecorder
+{
+    public const int defaultSampleRate = 44100;
+    public const double defaultBuzzerFrequency = 440.0;
+    public const double frameSeconds = 1.0 / 60.0;
+
+    readonly int sampleRate;
+    readonly double buzzerFrequency;
+
+    public AudioRecorder(int sampleRate = defaultSampleRate, double buzzerFrequency = defaultBuzzerFrequency)
+    {
+        if (sampleRate <= 0)
+        {
+            throw new ArgumentOutOfRangeException(nameof(sampleRate), "sample rate must be positive");
+        }
+        this.sampleRate = sampleRate;
+        this.buzzerFrequency = buzzerFrequency;
+    }
+
+    public AudioTrack record(Machine machine, int frames, int instructionsPerFrame)
+    {
+        if (frames < 0)
+        {
+            throw new ArgumentOutOfRangeException(nameof(frames), "frame count cannot be negative");
+        }
+
+        var track = new AudioTrack(sampleRate);
+        int samplesPerFrame = track.samplesForSeconds(frameSeconds);
+
+        for (int frame = 0; frame < frames && !machine.exited; frame++)
+        {
+            for (int cycle = 0; cycle < instructionsPerFrame; cycle++)
+            {
+                machine.step();
+            }
+
+            appendFrame(machine, track, samplesPerFrame);
+            machine.tickTimers();
+        }
+
+        return track;
+    }
+
+    void appendFrame(Machine machine, AudioTrack track, int samplesPerFrame)
+    {
+        if (!machine.soundActive)
+        {
+            track.appendSilence(samplesPerFrame);
+            return;
+        }
+
+        if (machine.platform == Platform.XoChip)
+        {
+            track.appendPattern(machine.audioPattern, machine.audioFrequency, samplesPerFrame);
+        }
+        else
+        {
+            track.appendSquare(buzzerFrequency, samplesPerFrame);
+        }
+    }
+}

src/Chip8.Core/Audio/AudioSynth.cs

@@ -0,0 +1,63 @@
+using System;
+
+namespace Chip8.Core.Audio;
+
+public static class AudioSynth
+{
+    public const int patternBits = 128;
+
+    public static short amplitudeToPcm(double amplitude)
+    {
+        double clamped = amplitude < 0 ? 0 : amplitude > 1 ? 1 : amplitude;
+        return (short)(clamped * short.MaxValue);
+    }
+
+    public static bool[] decodePattern(byte[] pattern)
+    {
+        var bits = new bool[patternBits];
+        for (int index = 0; index < patternBits; index++)
+        {
+            int sourceByte = index / 8;
+            int bit = 7 - (index % 8);
+            byte value = sourceByte < pattern.Length ? pattern[sourceByte] : (byte)0;
+            bits[index] = (value & (1 << bit)) != 0;
+        }
+        return bits;
+    }
+
+    public static short[] square(double frequency, int sampleRate, int sampleCount, double amplitude = 0.4, double duty = 0.5)
+    {
+        var samples = new short[sampleCount];
+        short high = amplitudeToPcm(amplitude);
+        short low = (short)-high;
+        double step = frequency / sampleRate;
+        double phase = 0.0;
+
+        for (int index = 0; index < sampleCount; index++)
+        {
+            samples[index] = (phase % 1.0) < duty ? high : low;
+            phase += step;
+        }
+
+        return samples;
+    }
+
+    public static short[] renderPattern(byte[] pattern, double playbackRate, int sampleRate, int sampleCount, double amplitude = 0.4)
+    {
+        bool[] bits = decodePattern(pattern);
+        var samples = new short[sampleCount];
+        short high = amplitudeToPcm(amplitude);
+        short low = (short)-high;
+        double step = playbackRate / sampleRate;
+        double position = 0.0;
+
+        for (int index = 0; index < sampleCount; index++)
+        {
+            int bit = ((int)position) % patternBits;
+            samples[index] = bits[bit] ? high : low;
+            position += step;
+        }
+
+        return samples;
+    }
+}