char-slop/chordpad.vibed.software

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

Charlotte Somexperiment: chord connectionsa427fe32026-10-01

import { type Chord, chordFrom } from "./chord.ts";
import { type Key, MODES } from "./key.ts";
import { mod, pcOf, spellDegree } from "./pitch.ts";

// all in root position: where the bass goes is left to voicing

const has = (c: Chord, ...tones: number[]) => tones.every(t => c.tones.includes(t));
const without = (c: Chord, ...tones: number[]) => c.tones.filter(t => !tones.includes(t));
const inKey = (key: Key, pc: number) =>
  MODES[key.mode].steps.includes(mod(pc - pcOf(key.tonic), 12));

const isMajor = (c: Chord) => has(c, 4) && !has(c, 3);
const isMinor = (c: Chord) => has(c, 3) && !has(c, 4);
export const isDominant = (c: Chord): boolean => has(c, 4, 10) && !has(c, 3);

/** C ↔ Cm, keeping everything but the third */
export function parallel(c: Chord): Chord | undefined {
  if (has(c, 6) && !has(c, 7)) return undefined; // diminished: no major counterpart
  if (isMajor(c)) return chordFrom(c.root, [...without(c, 4), 3]);
  if (isMinor(c)) return chordFrom(c.root, [...without(c, 3), 4]);
  return undefined;
}

/** C ↔ Am, Cmaj7 ↔ Am7: the chord a minor third away sharing most of its notes */
export function relative(c: Chord): Chord | undefined {
  if (!has(c, 7)) return undefined;
  if (isMajor(c) && !has(c, 10)) {
    return chordFrom(spellDegree(c.root, 5, 9), has(c, 11) ? [0, 3, 7, 10] : [0, 3, 7]);
  }
  if (isMinor(c) && !has(c, 11)) {
    return chordFrom(spellDegree(c.root, 2, 3), has(c, 10) ? [0, 4, 7, 11] : [0, 4, 7]);
  }
  return undefined;
}

/** G7 → D♭7: the dominant a tritone away, with the same tritone inside it */
export function tritoneSub(c: Chord): Chord | undefined {
  return isDominant(c) ? chordFrom(spellDegree(c.root, 4, 6), c.tones) : undefined;
}

/** add a 7th, picking whichever fits the key (C → Cmaj7, G → G7 in C), or take it away */
export function toggleSeventh(c: Chord, key: Key): Chord | undefined {
  const dim = has(c, 3, 6) && !has(c, 7);
  if (has(c, 10) || has(c, 11) || (dim && has(c, 9)))
    return chordFrom(c.root, without(c, 9, 10, 11));
  const root = pcOf(c.root);
  const [preferred, fallback] = dim ? [10, 9] : isMajor(c) ? [11, 10] : [10, 11];
  const seventh = inKey(key, root + preferred)
    ? preferred
    : inKey(key, root + fallback)
      ? fallback
      : dim
        ? 9
        : 10;
  return chordFrom(c.root, [...c.tones, seventh]);
}

/** replace the third with a 2nd or 4th; asked again, put the key's third back */
export function suspend(c: Chord, key: Key, tone: 2 | 5): Chord | undefined {
  if (has(c, tone) && !has(c, 3) && !has(c, 4)) {
    const third = inKey(key, pcOf(c.root) + 4) ? 4 : 3;
    return chordFrom(c.root, [...without(c, tone), third]);
  }
  return chordFrom(c.root, [...without(c, 3, 4, 2, 5), tone]);
}