char-slop/chordpad.vibed.software

git clone https://git.t4t.associates/char-slop/chordpad.vibed.software

Charlotte Sominitial commit3c7bc6e2026-09-30

import type { Chord } from "./chord.ts";
import { mod, pcOf } from "./pitch.ts";

const UPPER_CENTER = 65;
const BASS_CENTER = 41;

/** distance between two voicings, pairing every note with its nearest neighbour on the other side */
function motion(from: readonly number[], to: readonly number[]): number {
  const nearest = (x: number, ys: readonly number[]) =>
    Math.min(...ys.map(y => Math.abs(x - y)));
  return (
    (to.reduce((s, x) => s + nearest(x, from), 0) +
      from.reduce((s, y) => s + nearest(y, to), 0)) /
    2
  );
}

const mean = (xs: readonly number[]) => xs.reduce((a, b) => a + b, 0) / xs.length;

/** stack the pitch classes in close position, starting from `low` */
function closeStack(low: number, pcs: readonly number[]): number[] {
  return pcs.map(pc => low + mod(pc - low, 12)).sort((a, b) => a - b);
}

/**
 * a piano voicing: one bass note, plus the chord's upper structure led smoothly from `prev`
 * (the previous chord's voicing, bass first)
 */
export function voiceChord(chord: Chord, prev?: readonly number[]): number[] {
  const root = pcOf(chord.root);
  const bassPc = chord.bass ? pcOf(chord.bass) : root;

  let upper = chord.tones.map(t => mod(root + t, 12));
  // thin out dense chords: the fifth goes first, then the root when the bass already has it
  if (upper.length > 4 && chord.tones.includes(7))
    upper = upper.filter(p => p !== mod(root + 7, 12));
  if (upper.length > 4 && bassPc === root) upper = upper.filter(p => p !== root);

  const prevBass = prev?.[0];
  const prevUpper = prev?.slice(1);

  let bass = 0;
  let bassCost = Infinity;
  for (let m = 33; m <= 52; m++) {
    if (mod(m, 12) !== bassPc) continue;
    const cost = Math.abs(m - (prevBass ?? BASS_CENTER)) + 0.5 * Math.abs(m - BASS_CENTER);
    if (cost < bassCost) [bass, bassCost] = [m, cost];
  }

  let best: number[] = [];
  let bestCost = Infinity;
  for (let low = 50; low <= 70; low++) {
    if (!upper.includes(mod(low, 12))) continue;
    const close = closeStack(low, upper);
    const drop2 =
      close.length >= 4
        ? [...close.slice(0, -2), close.at(-2)! - 12, close.at(-1)!].sort((a, b) => a - b)
        : undefined;
    for (const [candidate, penalty] of [
      [close, 0],
      [drop2, 1],
    ] as const) {
      if (!candidate || candidate[0] - bass < 5) continue;
      const cost =
        (prevUpper?.length ? motion(prevUpper, candidate) : 0) +
        0.25 * Math.abs(mean(candidate) - UPPER_CENTER) +
        penalty;
      if (cost < bestCost) [best, bestCost] = [candidate, cost];
    }
  }

  return [bass, ...best];
}

/** move the upper structure to its next inversion up (+1) or down (-1), keeping the bass */
export function invertVoicing(notes: readonly number[], direction: 1 | -1): number[] {
  const [bass, ...upper] = [...notes].sort((a, b) => a - b);
  if (!upper.length) return [bass + 12 * direction];
  if (direction > 0) upper.push(upper.shift()! + 12);
  else if (upper.at(-1)! - 12 > bass) upper.unshift(upper.pop()! - 12);
  return [bass, ...upper].sort((a, b) => a - b);
}