heldNote.ts

import type { LyricsWord } from "./tauri/lyrics";

/** A letter: a character with the combining marks that follow it. */
const LETTER = /\P{M}\p{M}*/gu;

/** Shortest hold that reads as a held note, whatever the word. */
const HELD_MIN_MS = 1000;
/** A long word sung at an ordinary pace is not held: this much per letter. */
const HELD_MS_PER_LETTER = 220;
/** Longer words are sung, not held — and a wave through them reads as noise. */
const HELD_MAX_LETTERS = 8;

/**
 * Whether `word` is a held note: sung long enough for its length. A
 * two-letter word held for a second is one; a seven-letter word sung over
 * the same second is ordinary pace.
 *
 * Only real word timing counts. An estimated word (it carries `fillEndMs`)
 * has a length made up from the line's, and would light up at random.
 */
export function isHeldNote(word: LyricsWord | undefined): boolean {
  if (!word || word.fillEndMs !== undefined || word.endMs < 0) return false;
  // Every letter counts, a shaped script's included: it moves as one
  // block, but the rule is about how long each letter is held.
  const letters = letterCount(word.text.trim());
  if (letters === 0 || letters > HELD_MAX_LETTERS) return false;
  const heldMs = word.endMs - word.timeMs;
  return heldMs >= HELD_MIN_MS && heldMs >= letters * HELD_MS_PER_LETTER;
}

/**
 * Scripts whose letters change shape with their neighbours (Arabic's
 * joined forms, the Indic conjuncts, Thai's stacked vowels…). Cut into
 * separate boxes they fall apart, so a held word in one of them moves as
 * a whole.
 */
const SHAPED_SCRIPT =
  /[\p{Script=Arabic}\p{Script=Syriac}\p{Script=Nko}\p{Script=Mongolian}\p{Script=Hebrew}\p{Script=Devanagari}\p{Script=Bengali}\p{Script=Gurmukhi}\p{Script=Gujarati}\p{Script=Oriya}\p{Script=Tamil}\p{Script=Telugu}\p{Script=Kannada}\p{Script=Malayalam}\p{Script=Sinhala}\p{Script=Thai}\p{Script=Lao}\p{Script=Tibetan}\p{Script=Myanmar}\p{Script=Khmer}]/u;

const EMOJI_SEQUENCE =
  /\p{Extended_Pictographic}|\p{Regional_Indicator}|\u200D/u;

/**
 * The letters a held word ripples through: each character with the
 * combining marks that follow it (an accent typed as a separate mark
 * stays on its letter), or the whole word in a script that shapes its
 * letters together, or when it holds an emoji sequence.
 */
export function heldNoteLetters(text: string): string[] {
  // An emoji sequence (skin tone, joiner, flag) is one picture made of
  // several code points; cut apart it falls to pieces too.
  if (SHAPED_SCRIPT.test(text) || EMOJI_SEQUENCE.test(text)) {
    return text ? [text] : [];
  }
  return text.match(LETTER) ?? [];
}

/**
 * How many letters `text` has, as a reader counts them: a shaped script's
 * letters one by one, and an emoji sequence — joined figures, a skin tone,
 * a flag — as one.
 */
function letterCount(text: string): number {
  const folded = text
    // A joiner and the figure it attaches, a skin tone, a presentation
    // selector: parts of the picture before them.
    .replace(/‍\p{Extended_Pictographic}/gu, "")
    .replace(/[\u{1F3FB}-\u{1F3FF}]/gu, "")
    .replace(/️/gu, "")
    // A flag is two regional indicators.
    .replace(/\p{Regional_Indicator}{2}/gu, "F");
  return (folded.match(LETTER) ?? []).filter(isLetterSegment).length;
}

/**
 * Whether a segment of `heldNoteLetters` is sung — a letter, a digit, an
 * emoji — rather than punctuation, which keeps its place in the word but
 * neither counts toward the hold nor moves.
 */
export function isLetterSegment(segment: string): boolean {
  return SUNG.test(segment);
}

const SUNG = /[\p{L}\p{N}\p{Extended_Pictographic}\p{Regional_Indicator}]/u;