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* What a chord name means: its notes, and the name the guitar fingering
* database (chords-db) files it under. "C#m7b5/G" → root C#, the notes
* C# E G B, over G.
*/
const LETTER: Record<string, number> = {
C: 0,
D: 2,
E: 4,
F: 5,
G: 7,
A: 9,
B: 11,
}
const SHARPS = ['C', 'C#', 'D', 'D#', 'E', 'F', 'F#', 'G', 'G#', 'A', 'A#', 'B']
const FLATS = ['C', 'Db', 'D', 'Eb', 'E', 'F', 'Gb', 'G', 'Ab', 'A', 'Bb', 'B']
/** chords-db's own name for each root (its file keys). */
const DB_ROOT = [
'C',
'Csharp',
'D',
'Eb',
'E',
'F',
'Fsharp',
'G',
'Ab',
'A',
'Bb',
'B',
]
/**
* Each chord type: its intervals in semitones above the root, and the
* suffix chords-db uses. Written forms that mean the same thing share an
* entry ("m", "min", "-"; "maj7", "M7", "Δ7").
*/
const TYPES: {names: string[]; intervals: number[]; db: string}[] = [
{names: ['', 'maj', 'M'], intervals: [0, 4, 7], db: 'major'},
{names: ['m', 'min', '-', 'mi'], intervals: [0, 3, 7], db: 'minor'},
{names: ['dim', '°', 'o'], intervals: [0, 3, 6], db: 'dim'},
{names: ['dim7', '°7', 'o7'], intervals: [0, 3, 6, 9], db: 'dim7'},
{names: ['aug', '+', '+5', '#5'], intervals: [0, 4, 8], db: 'aug'},
{names: ['sus2'], intervals: [0, 2, 7], db: 'sus2'},
{names: ['sus4', 'sus'], intervals: [0, 5, 7], db: 'sus4'},
{names: ['7sus4', '7sus'], intervals: [0, 5, 7, 10], db: '7sus4'},
{names: ['9sus4', '9sus'], intervals: [0, 5, 7, 10, 14], db: '7sus4'},
{names: ['6'], intervals: [0, 4, 7, 9], db: '6'},
{names: ['69', '6/9'], intervals: [0, 4, 7, 9, 14], db: '69'},
{names: ['m6', 'min6', '-6'], intervals: [0, 3, 7, 9], db: 'm6'},
{names: ['m69', 'm6/9'], intervals: [0, 3, 7, 9, 14], db: 'm69'},
{names: ['7'], intervals: [0, 4, 7, 10], db: '7'},
{names: ['7b5', '7-5'], intervals: [0, 4, 6, 10], db: '7b5'},
{
names: ['7#5', '7+5', 'aug7', '+7', '7+'],
intervals: [0, 4, 8, 10],
db: 'aug7',
},
{names: ['7b9', '7-9'], intervals: [0, 4, 7, 10, 13], db: '7b9'},
{names: ['7#9', '7+9'], intervals: [0, 4, 7, 10, 15], db: '7#9'},
{names: ['9'], intervals: [0, 4, 7, 10, 14], db: '9'},
{names: ['11'], intervals: [0, 4, 7, 10, 14, 17], db: '11'},
{names: ['13'], intervals: [0, 4, 7, 10, 14, 21], db: '13'},
{
names: ['maj7', 'M7', 'Δ7', 'Δ', 'ma7'],
intervals: [0, 4, 7, 11],
db: 'maj7',
},
{names: ['maj9', 'M9', 'Δ9'], intervals: [0, 4, 7, 11, 14], db: 'maj9'},
{names: ['maj13', 'M13'], intervals: [0, 4, 7, 11, 14, 21], db: 'maj13'},
{names: ['m7', 'min7', '-7', 'mi7'], intervals: [0, 3, 7, 10], db: 'm7'},
{
names: ['m7b5', 'ø', 'ø7', 'm7-5', '-7b5'],
intervals: [0, 3, 6, 10],
db: 'm7b5',
},
{names: ['m9', 'min9', '-9'], intervals: [0, 3, 7, 10, 14], db: 'm9'},
{names: ['m11', 'min11', '-11'], intervals: [0, 3, 7, 10, 14, 17], db: 'm11'},
{
names: ['mmaj7', 'mM7', 'mΔ7', 'm(maj7)'],
intervals: [0, 3, 7, 11],
db: 'mmaj7',
},
{names: ['add9', 'add2'], intervals: [0, 4, 7, 14], db: 'add9'},
{names: ['madd9', 'm(add9)'], intervals: [0, 3, 7, 14], db: 'madd9'},
{names: ['5'], intervals: [0, 7], db: 'major'},
]
export type ChordInfo = {
/** As written, tidied: "C#m7" */
name: string
root: number
/** Pitch classes, root first */
tones: number[]
/** Note names, spelled to suit the root: "C# E G# B" */
notes: string[]
/** The bass note under a slash ("/G"), if any */
bass: number | null
bassName: string | null
/** For the fingering lookup */
dbKey: string
dbSuffix: string
/** The chord without its bass note, if the slash chord isn't filed */
dbFallback: string
}
const pc = (letter: string, acc: string) =>
(LETTER[letter] + (acc === '#' ? 1 : acc === 'b' ? -1 : 0) + 12) % 12
/** Letter steps above the root for each interval: a third is an E-something over C. */
const STEP: Record<number, number> = {
0: 0,
1: 1,
2: 1,
3: 2,
4: 2,
5: 3,
6: 4,
7: 4,
8: 4,
9: 5,
10: 6,
11: 6,
13: 1,
14: 1,
15: 1,
17: 3,
21: 5,
}
const LETTERS = 'CDEFGAB'
/** A note named the way the chord spells it: Cm7's third is Eb, not D#. */
function spellInterval(rootLetter: string, rootPc: number, interval: number) {
const letter =
LETTERS[(LETTERS.indexOf(rootLetter) + (STEP[interval] ?? 0)) % 7]
const target = (rootPc + interval) % 12
let diff = (target - LETTER[letter] + 12) % 12
if (diff > 6) diff -= 12
return letter + (diff > 0 ? '#'.repeat(diff) : 'b'.repeat(-diff))
}
/** A bass note: flats in flat keys, sharps otherwise. */
function speller(rootName: string) {
return /b/.test(rootName) || rootName === 'F' ? FLATS : SHARPS
}
export function chordInfo(raw: string): ChordInfo | null {
const name = raw.trim().replace(/[.,]+$/, '')
const m = name.match(/^([A-G])([#b]?)(.*?)(?:\/([A-G])([#b]?))?$/)
if (!m) return null
const [, letter, acc, rest, bassLetter, bassAcc] = m
const type =
TYPES.find((t) => t.names.includes(rest)) ??
TYPES.find((t) => t.names.includes(rest.replace(/[()]/g, '')))
if (!type) return null
const root = pc(letter, acc)
const spell = speller(letter + acc)
const tones = [...new Set(type.intervals.map((i) => (root + i) % 12))]
const bass = bassLetter ? pc(bassLetter, bassAcc) : null
// chords-db files slash chords only for major triads ("C/G"), naming
// the bass with sharps except Bb
const slashDb =
bass !== null && type.db === 'major'
? `/${SHARPS[bass].replace('A#', 'Bb')}`
: null
return {
name,
root,
tones,
notes: [...new Set(type.intervals.map((i) => i % 24))]
.filter((i, k, all) => all.findIndex((j) => j % 12 === i % 12) === k)
.map((i) => spellInterval(letter, root, i)),
bass,
bassName: bass !== null ? spell[bass] : null,
dbKey: DB_ROOT[root],
dbSuffix: slashDb ?? type.db,
dbFallback: type.db,
}
}
export type GuitarPosition = {
frets: number[]
fingers: number[]
baseFret: number
barres: number[]
}
type GuitarDb = {
chords: Record<string, {suffix: string; positions: GuitarPosition[]}[]>
}
let db: Promise<GuitarDb> | null = null
/** Guitar fingerings for a chord, easiest first; loaded on first use. */
export async function guitarPositions(
info: ChordInfo,
): Promise<GuitarPosition[]> {
db ??= import('@tombatossals/chords-db/lib/guitar.json').then(
(m) => (m.default ?? m) as unknown as GuitarDb,
)
const all = (await db).chords[info.dbKey] ?? []
const exact = all.find((c) => c.suffix === info.dbSuffix)
if (exact) return exact.positions
// A slash chord chords-db doesn't have: the chord itself
return all.find((c) => c.suffix === info.dbFallback)?.positions ?? []
}
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