feat(core/audio): add WAV writer and audio track buffer

src/Chip8.Core/Audio/AudioTrack.cs

@@ -0,0 +1,68 @@
+using System;
+using System.Collections.Generic;
+
+namespace Chip8.Core.Audio;
+
+public sealed class AudioTrack
+{
+    readonly List<short> samples = new();
+    readonly int sampleRate;
+
+    double squarePhase;
+    double patternPosition;
+
+    public AudioTrack(int sampleRate)
+    {
+        if (sampleRate <= 0)
+        {
+            throw new ArgumentOutOfRangeException(nameof(sampleRate), "sample rate must be positive");
+        }
+        this.sampleRate = sampleRate;
+    }
+
+    public int sampleCount => samples.Count;
+
+    public double durationSeconds => (double)samples.Count / sampleRate;
+
+    public int samplesForSeconds(double seconds) => (int)Math.Round(seconds * sampleRate);
+
+    public void appendSilence(int count)
+    {
+        for (int index = 0; index < count; index++)
+        {
+            samples.Add(0);
+        }
+    }
+
+    public void appendSquare(double frequency, int count, double amplitude = 0.4, double duty = 0.5)
+    {
+        short high = AudioSynth.amplitudeToPcm(amplitude);
+        short low = (short)-high;
+        double step = frequency / sampleRate;
+
+        for (int index = 0; index < count; index++)
+        {
+            samples.Add((squarePhase % 1.0) < duty ? high : low);
+            squarePhase += step;
+        }
+    }
+
+    public void appendPattern(byte[] pattern, double playbackRate, int count, double amplitude = 0.4)
+    {
+        bool[] bits = AudioSynth.decodePattern(pattern);
+        short high = AudioSynth.amplitudeToPcm(amplitude);
+        short low = (short)-high;
+        double step = playbackRate / sampleRate;
+
+        for (int index = 0; index < count; index++)
+        {
+            int bit = ((int)patternPosition) % AudioSynth.patternBits;
+            samples.Add(bits[bit] ? high : low);
+            patternPosition += step;
+        }
+    }
+
+    public short[] toSamples() => samples.ToArray();
+
+    public byte[] toWav() => WavFile.write(samples.ToArray(), sampleRate);
+}

src/Chip8.Core/Audio/WavFile.cs

@@ -0,0 +1,52 @@
+using System;
+using System.IO;
+using System.Text;
+
+namespace Chip8.Core.Audio;
+
+public static class WavFile
+{
+    public const int bitsPerSample = 16;
+    public const int channels = 1;
+
+    public static byte[] write(short[] samples, int sampleRate)
+    {
+        if (sampleRate <= 0)
+        {
+            throw new ArgumentOutOfRangeException(nameof(sampleRate), "sample rate must be positive");
+        }
+
+        int dataBytes = samples.Length * sizeof(short);
+        int blockAlign = channels * (bitsPerSample / 8);
+        int byteRate = sampleRate * blockAlign;
+
+        using var stream = new MemoryStream(44 + dataBytes);
+        using var writer = new BinaryWriter(stream, Encoding.ASCII);
+
+        writer.Write(Encoding.ASCII.GetBytes("RIFF"));
+        writer.Write(36 + dataBytes);
+        writer.Write(Encoding.ASCII.GetBytes("WAVE"));
+
+        writer.Write(Encoding.ASCII.GetBytes("fmt "));
+        writer.Write(16);
+        writer.Write((short)1);
+        writer.Write((short)channels);
+        writer.Write(sampleRate);
+        writer.Write(byteRate);
+        writer.Write((short)blockAlign);
+        writer.Write((short)bitsPerSample);
+
+        writer.Write(Encoding.ASCII.GetBytes("data"));
+        writer.Write(dataBytes);
+        foreach (short sample in samples)
+        {
+            writer.Write(sample);
+        }
+
+        writer.Flush();
+        return stream.ToArray();
+    }
+
+    public static void writeFile(string path, short[] samples, int sampleRate) =>
+        File.WriteAllBytes(path, write(samples, sampleRate));
+}