/**
 * Defines an audio file's waveform, the bars the player seeks on: how many
 * peaks it has, and how they are taken from the samples. The server makes an
 * uploaded file's from its decoded audio (`api/lib/ffmpeg.ts`), and the
 * dump form a local file's in the browser (`src/utils/player/waveform.ts`),
 * both with {@link peaksOf}, so the two draw alike.
 *
 * @module
 */

/** How many peaks, and so bars, a waveform has. */
export const WAVEFORM_PEAKS = 150;

/**
 * Returned by `GET /api/dumps/:dumpId/waveform` for an audio file dump, to the
 * player's waveform.
 */
export interface DumpWaveform {
  /** {@link WAVEFORM_PEAKS} heights from 0 to 1, as {@link peaksOf} gives. */
  peaks: number[];
}

/**
 * Returns `n` peaks of `samples`, each the loudest of its slice, scaled so the
 * loudest of all is 1 and rounded to three places: a quiet recording fills the
 * waveform as a loud one does. `samples` may be signed, as audio is, or
 * already peaks of smaller slices. Silence gives zeros.
 */
export function peaksOf(
  samples: ArrayLike<number>,
  n: number = WAVEFORM_PEAKS,
): number[] {
  const peaks: number[] = [];
  for (let i = 0; i < n; i++) {
    // Slices cover every sample; with fewer samples than peaks, some share.
    const start = Math.floor((i * samples.length) / n);
    const end = Math.max(start + 1, Math.floor(((i + 1) * samples.length) / n));
    let peak = 0;
    for (let j = start; j < Math.min(end, samples.length); j++) {
      peak = Math.max(peak, Math.abs(samples[j]));
    }
    peaks.push(peak);
  }
  const loudest = Math.max(...peaks);
  return peaks.map((p) =>
    loudest > 0 ? Math.round((p / loudest) * 1000) / 1000 : 0
  );
}