// One latest target in a core shared row. Versions are snapshot tokens, not a
// sample count. The producer never touches the consumer's acknowledgement.
export class Mailbox {
  constructor({ buffer, descriptor }) {
    this.words = new Int32Array(buffer, descriptor.offset, 16);
    this.scratch = new ArrayBuffer(4);
    this.bits = new Int32Array(this.scratch);
    this.float = new Float32Array(this.scratch);
  }
  encode(value) {
    this.float[0] = value;
    return this.bits[0];
  }
  decode(value) {
    this.bits[0] = value;
    return this.float[0];
  }
  write(stroke, sample, seed) {
    const version = (Atomics.load(this.words, 0) + 2) | 0;
    Atomics.store(this.words, 0, version - 1);
    Atomics.store(this.words, 2, stroke);
    Atomics.store(this.words, 3, this.encode(sample.x));
    Atomics.store(this.words, 4, this.encode(sample.y));
    Atomics.store(this.words, 5, this.encode(sample.pressure));
    Atomics.store(this.words, 6, this.encode(seed ?? 0));
    Atomics.store(this.words, 7, seed === undefined ? 0 : 1);
    Atomics.store(this.words, 0, version);
    return version;
  }
  read(stroke) {
    const a = Atomics.load(this.words, 0);
    if (a & 1 || a === Atomics.load(this.words, 1)) return null;
    const id = Atomics.load(this.words, 2);
    const x = this.decode(Atomics.load(this.words, 3));
    const y = this.decode(Atomics.load(this.words, 4));
    const pressure = this.decode(Atomics.load(this.words, 5));
    const seed = this.decode(Atomics.load(this.words, 6));
    const hasSeed = Atomics.load(this.words, 7);
    if (a !== Atomics.load(this.words, 0) || id !== stroke) return null;
    return { version: a, x, y, pressure, seed: hasSeed ? seed : undefined };
  }
  acknowledge(version) {
    Atomics.store(this.words, 1, version);
  }
  get processed() {
    return Atomics.load(this.words, 1);
  }
}