halkyon/apps/todo/tool/completion_sound.dart

// Writes Todo's chime of a task done (decided by the developer, 10 Nov
// 2026), played on desktops: two soft bell tones a fifth apart, A5 then E6,
// each fading away, quiet, a quarter of a second in all. Synthesized here, so
// it is ours and needs no licence of anyone's; run from apps/todo:
//
//   dart run tool/completion_sound.dart
//
// It writes halkyon_todo_flutter/assets/sounds/task_done.wav, a mono 16-bit
// WAV at 44.1 kHz; the same every run.
import 'dart:io';
import 'dart:math' as math;
import 'dart:typed_data';

const _rate = 44100;

/// The loudest the chime gets, of full scale.
const _peak = 0.3;

/// Each tone: when it starts and how long it rings, in seconds.
const _tones = [
  (hertz: 880.0, start: 0.0, length: 0.16),
  (hertz: 1318.51, start: 0.07, length: 0.18),
];

void main() {
  final length = _tones
      .map((tone) => tone.start + tone.length)
      .reduce(math.max);
  final count = (length * _rate).ceil();
  final samples = Float64List(count);
  for (final tone in _tones) {
    final first = (tone.start * _rate).round();
    final last = math.min(count, first + (tone.length * _rate).round());
    for (var index = first; index < last; index++) {
      final t = (index - first) / _rate;
      // A few milliseconds rising, so it doesn't click, then fading as a
      // bell does; a quiet octave above for its shimmer.
      final attack = math.min(1, t / 0.004);
      final fade = math.exp(-t * 28);
      final end = math.min(1, (last - index) / (_rate * 0.01));
      final wave =
          math.sin(2 * math.pi * tone.hertz * t) +
          0.2 * math.sin(4 * math.pi * tone.hertz * t);
      samples[index] += wave * attack * fade * end;
    }
  }
  final loudest = samples.map((sample) => sample.abs()).reduce(math.max);
  final pcm = ByteData(count * 2);
  for (var index = 0; index < count; index++) {
    pcm.setInt16(
      index * 2,
      (samples[index] / loudest * _peak * 32767).round(),
      Endian.little,
    );
  }
  final file = File('halkyon_todo_flutter/assets/sounds/task_done.wav')
    ..createSync(recursive: true)
    ..writeAsBytesSync(_wav(pcm.buffer.asUint8List()));
  stdout.writeln('Wrote ${file.path} (${file.lengthSync()} bytes).');
}

/// [pcm] in a WAV file's RIFF header: one channel, 16 bits at [_rate].
Uint8List _wav(Uint8List pcm) {
  final header = ByteData(44);
  void text(int offset, String value) {
    for (var index = 0; index < value.length; index++) {
      header.setUint8(offset + index, value.codeUnitAt(index));
    }
  }

  text(0, 'RIFF');
  header.setUint32(4, 36 + pcm.length, Endian.little);
  text(8, 'WAVE');
  text(12, 'fmt ');
  header
    ..setUint32(16, 16, Endian.little)
    ..setUint16(20, 1, Endian.little) // PCM
    ..setUint16(22, 1, Endian.little) // one channel
    ..setUint32(24, _rate, Endian.little)
    ..setUint32(28, _rate * 2, Endian.little) // bytes a second
    ..setUint16(32, 2, Endian.little) // bytes a frame
    ..setUint16(34, 16, Endian.little); // bits a sample
  text(36, 'data');
  header.setUint32(40, pcm.length, Endian.little);
  return Uint8List.fromList([...header.buffer.asUint8List(), ...pcm]);
}