Blame for src/lib/abc/parse.ts

import type { AbcMark, AbcNote, AbcParseResult, AbcScore, AbcVoice } from "@/lib/abc/model";

/**
 * Reads the ABC dialect YuE2 and SheetSage2 actually emit.
 *
 * This is deliberately not a general ABC parser. It covers notes, rests,
 * chords, ties, accidentals, octaves, bar lines and repeats, and it *refuses*
 * anything else. Refusing matters more than coverage here: the grid writes the
 * score back out, so a construct it silently drops is a construct the operator
 * loses. When this returns a failure the panel keeps the text editor and says
 * which construct stopped it, which is recoverable. Quietly deleting a tuplet
 * is not.
 */

/** Semitone offset of each natural within an octave. */
const STEP: Record<string, number> = { C: 0, D: 2, E: 4, F: 5, G: 7, A: 9, B: 11 };

/** Sharps and flats a key signature applies, by letter. */
const KEY_SIGNATURES: Record<string, string[]> = {
  C: [], G: ["F"], D: ["F", "C"], A: ["F", "C", "G"], E: ["F", "C", "G", "D"],
  B: ["F", "C", "G", "D", "A"], "F#": ["F", "C", "G", "D", "A", "E"],
  F: ["B"], Bb: ["B", "E"], Eb: ["B", "E", "A"], Ab: ["B", "E", "A", "D"],
  Db: ["B", "E", "A", "D", "G"], Gb: ["B", "E", "A", "D", "G", "C"],
};

const UNSUPPORTED: { pattern: RegExp; name: string }[] = [
  { pattern: /\(\d/, name: "a tuplet" },
  { pattern: /\{/, name: "a grace note" },
  { pattern: /![^!\n]*!/, name: "a decoration" },
  { pattern: /\+[^+\n]*\+/, name: "a decoration" },
];

interface Cursor {
  text: string;
  at: number;
}

export function parseAbc(source: string): AbcParseResult {
  const lines = source.replace(/\r\n?/g, "\n").split("\n");

  const header: string[] = [];
  const voices: AbcVoice[] = [];
  const bodyByVoice = new Map<number, string[]>();

  let meter: [number, number] = [4, 4];
  let unit: [number, number] | null = null;
  let key = "C";
  let tempo = 120;
  let current = -1;
  let sawVoiceLine = false;

  for (const raw of lines) {
    const line = raw.replace(/%.*$/, "").trimEnd();
    if (!line.trim()) continue;

    const field = /^([A-Za-z]):\s*(.*)$/.exec(line);
    if (field) {
      const [, tag, value] = field;

      if (tag === "V") {
        sawVoiceLine = true;
        const [id, ...rest] = value.trim().split(/\s+/);
        const existing = voices.findIndex((v) => v.id === id);
        if (existing >= 0) {
          current = existing;
        } else {
          voices.push({ id: id || `V${voices.length + 1}`, rest: rest.join(" ") });
          current = voices.length - 1;
          bodyByVoice.set(current, []);
        }
        continue;
      }

      if (tag === "M") {
        const m = /^(\d+)\s*\/\s*(\d+)$/.exec(value.trim());
        if (m) meter = [Number(m[1]), Number(m[2])];
        else if (value.trim() === "C") meter = [4, 4];
        else if (value.trim() === "C|") meter = [2, 2];
      }
      if (tag === "L") {
        const m = /^(\d+)\s*\/\s*(\d+)$/.exec(value.trim());
        if (m) unit = [Number(m[1]), Number(m[2])];
      }
      if (tag === "K") key = value.trim() || "C";
      if (tag === "Q") tempo = readTempo(value.trim()) ?? tempo;

      // Header fields stay verbatim so a rewrite cannot reword the tune.
      if (!sawVoiceLine) header.push(line);
      continue;
    }

    if (current < 0) {
      // Music before any V: line is a single implicit voice.
      voices.push({ id: "1", rest: "" });
      current = 0;
      bodyByVoice.set(0, []);
    }
    bodyByVoice.get(current)!.push(line);
  }

  if (!voices.length) {
    return { ok: false, reason: "There is no music in this score yet.", offending: null };
  }

  // L: is optional in ABC. Its default depends on the meter: short meters get
  // a sixteenth, everything else an eighth.
  if (!unit) unit = meter[0] / meter[1] < 0.75 ? [1, 16] : [1, 8];

  const notes: AbcNote[] = [];
  const marks: AbcMark[] = [];
  let repeatsExpanded = false;
  let seq = 0;
  let markSeq = 0;

  for (const [voiceIndex, body] of bodyByVoice) {
    const text = body.join(" ");

    for (const { pattern, name } of UNSUPPORTED) {
      const hit = pattern.exec(text);
      if (hit) {
        return {
          ok: false,
          reason: `This score contains ${name}, which the grid cannot draw without losing it.`,
          offending: hit[0],
        };
      }
    }

    const read = readVoice(text, key);
    if (!read.ok) return read;
    if (read.repeated) repeatsExpanded = true;

    for (const event of read.events) {
      notes.push({
        id: `n${seq++}`,
        voice: voiceIndex,
        start: event.start,
        duration: event.duration,
        midi: event.midi,
        tied: event.tied,
      });
    }

    for (const mark of read.marks) {
      marks.push({ id: `m${markSeq++}`, voice: voiceIndex, ...mark });
    }
  }

  notes.sort((a, b) => a.start - b.start || a.midi - b.midi);
  marks.sort((a, b) => a.start - b.start);

  const score: AbcScore = { header, voices, notes, marks, meter, unit, key, repeatsExpanded, tempo };
  return { ok: true, score };
}

interface Event {
  start: number;
  duration: number;
  midi: number;
  tied: boolean;
}

interface RawMark {
  start: number;
  text: string;
  annotation: boolean;
}

interface VoiceOk {
  ok: true;
  events: Event[];
  marks: RawMark[];
  repeated: boolean;
}

/** Q:1/4=120, Q:120, or Q:"Allegro" 1/4=120 all mean the same thing here. */
function readTempo(value: string): number | null {
  const withUnit = /(\d+)\s*\/\s*(\d+)\s*=\s*(\d+(?:\.\d+)?)/.exec(value);
  if (withUnit) {
    const beat = Number(withUnit[1]) / Number(withUnit[2]);
    const perMinute = Number(withUnit[3]);
    // Normalise to quarter notes, which is what the player counts in.
    return (beat / 0.25) * perMinute;
  }
  const plain = /^(\d+(?:\.\d+)?)$/.exec(value);
  return plain ? Number(plain[1]) : null;
}

function readVoice(text: string, key: string): VoiceOk | Extract<AbcParseResult, { ok: false }> {
  const cursor: Cursor = { text, at: 0 };
  const events: Event[] = [];
  const marks: RawMark[] = [];

  // A repeated section has to be written out, because the grid shows time as
  // one straight line and cannot draw "go back and play that again".
  let sectionStart = 0;
  let sectionEventStart = 0;
  let repeated = false;

  let time = 0;
  // Accidentals last to the end of the bar in ABC, exactly as in notation.
  let barAccidentals: Record<string, number> = {};
  const signature = accidentalsFor(key);

  const pushTiedOrNew = (event: Event) => {
    const previous = events[events.length - 1];
    if (previous && previous.tied && previous.midi === event.midi) {
      previous.duration += event.duration;
      previous.tied = event.tied;
      return;
    }
    events.push(event);
  };

  while (cursor.at < cursor.text.length) {
    const ch = cursor.text[cursor.at];

    if (ch === " " || ch === "\t") {
      cursor.at++;
      continue;
    }

    // Bar lines, in the order that matters: two-character forms first.
    const two = cursor.text.slice(cursor.at, cursor.at + 2);
    if (two === ":|" || two === "::") {
      const from = events.length;
      // Replay everything since the section opened, marks included: a chord
      // symbol that only appeared on the first pass would leave the repeat
      // looking unharmonised.
      const span = time - sectionStart;
      for (let i = sectionEventStart; i < from; i++) {
        const source = events[i];
        events.push({ ...source, start: source.start + span });
      }
      for (const mark of marks.filter((m) => m.start >= sectionStart && m.start < time)) {
        marks.push({ ...mark, start: mark.start + span });
      }
      if (from > sectionEventStart) repeated = true;
      time += span;
      sectionStart = time;
      sectionEventStart = events.length;
      cursor.at += 2;
      barAccidentals = {};
      continue;
    }
    if (two === "|:" || two === "||" || two === "|]" || two === "[|") {
      sectionStart = time;
      sectionEventStart = events.length;
      cursor.at += 2;
      barAccidentals = {};
      continue;
    }
    if (ch === "|") {
      cursor.at++;
      barAccidentals = {};
      continue;
    }
    if (ch === ":") {
      cursor.at++;
      continue;
    }

    // Slur parentheses carry no duration. Tuplets were rejected earlier, so a
    // bare "(" here is safe to skip.
    if (ch === "(" || ch === ")") {
      cursor.at++;
      continue;
    }
    if (ch === "-") {
      const previous = events[events.length - 1];
      if (previous) previous.tied = true;
      cursor.at++;
      continue;
    }
    if (ch === "$" || ch === "\\") {
      cursor.at++;
      continue;
    }

    // A quoted string: a chord symbol, or an annotation when it opens with a
    // placement character. Both are pinned to the current time so they survive
    // a rewrite and can be drawn above the grid.
    if (ch === '"') {
      const close = cursor.text.indexOf('"', cursor.at + 1);
      if (close < 0) {
        return { ok: false, reason: "A quoted chord symbol is never closed.", offending: '"' };
      }
      const raw = cursor.text.slice(cursor.at + 1, close);
      cursor.at = close + 1;
      if (raw.trim()) {
        marks.push({ start: time, text: raw, annotation: /^[\^_<>@]/.test(raw) });
      }
      continue;
    }

    // A chord: every note inside shares one onset and one length.
    if (ch === "[") {
      const close = cursor.text.indexOf("]", cursor.at);
      if (close < 0) {
        return { ok: false, reason: "A chord bracket is never closed.", offending: "[" };
      }
      const inner = cursor.text.slice(cursor.at + 1, close);
      cursor.at = close + 1;
      const trailing = readLength(cursor);

      const chordCursor: Cursor = { text: inner, at: 0 };
      let chordLength = 1;
      const pitches: number[] = [];
      while (chordCursor.at < inner.length) {
        if (/\s/.test(inner[chordCursor.at])) {
          chordCursor.at++;
          continue;
        }
        const note = readNote(chordCursor, barAccidentals, signature);
        if (!note) {
          return { ok: false, reason: "A chord contains something that is not a note.", offending: inner };
        }
        if (note.midi !== null) pitches.push(note.midi);
        chordLength = note.length;
      }

      const duration = chordLength * trailing;
      for (const midi of pitches) {
        pushTiedOrNew({ start: time, duration, midi, tied: false });
      }
      time += duration;
      continue;
    }

    const note = readNote(cursor, barAccidentals, signature);
    if (note) {
      if (note.midi === null) {
        time += note.length; // a rest
      } else {
        pushTiedOrNew({ start: time, duration: note.length, midi: note.midi, tied: false });
        time += note.length;
      }
      continue;
    }

    return {
      ok: false,
      reason: "This score has notation the grid does not understand.",
      offending: cursor.text.slice(cursor.at, cursor.at + 8).trim() || "?",
    };
  }

  return { ok: true, events, marks, repeated };
}

interface ReadNote {
  midi: number | null;
  length: number;
}

function readNote(
  cursor: Cursor,
  barAccidentals: Record<string, number>,
  signature: Record<string, number>,
): ReadNote | null {
  const start = cursor.at;
  let accidental: number | null = null;

  while (cursor.at < cursor.text.length) {
    const ch = cursor.text[cursor.at];
    if (ch === "^") {
      accidental = (accidental ?? 0) + 1;
      cursor.at++;
    } else if (ch === "_") {
      accidental = (accidental ?? 0) - 1;
      cursor.at++;
    } else if (ch === "=") {
      accidental = 0;
      cursor.at++;
    } else {
      break;
    }
  }

  const letter = cursor.text[cursor.at];
  if (!letter || !/[A-Ga-gzZxX]/.test(letter)) {
    cursor.at = start;
    return null;
  }
  cursor.at++;

  if (/[zZxX]/.test(letter)) {
    return { midi: null, length: readLength(cursor) };
  }

  // Lower case is the octave above upper case, then , and ' shift further.
  const upper = letter.toUpperCase();
  let octave = letter === upper ? 4 : 5;
  while (cursor.at < cursor.text.length) {
    const ch = cursor.text[cursor.at];
    if (ch === ",") {
      octave--;
      cursor.at++;
    } else if (ch === "'") {
      octave++;
      cursor.at++;
    } else {
      break;
    }
  }

  const slot = `${upper}${octave}`;
  if (accidental !== null) barAccidentals[slot] = accidental;
  const applied = slot in barAccidentals ? barAccidentals[slot] : (signature[upper] ?? 0);

  const midi = (octave + 1) * 12 + STEP[upper] + applied;
  return { midi, length: readLength(cursor) };
}

/** Reads the multiplier after a note: `2`, `/2`, `/`, `3/2`, or nothing. */
function readLength(cursor: Cursor): number {
  let numerator = "";
  while (/\d/.test(cursor.text[cursor.at] ?? "")) numerator += cursor.text[cursor.at++];

  let denominator = "";
  let slashes = 0;
  while (cursor.text[cursor.at] === "/") {
    slashes++;
    cursor.at++;
  }
  if (slashes) {
    while (/\d/.test(cursor.text[cursor.at] ?? "")) denominator += cursor.text[cursor.at++];
  }

  const top = numerator ? Number(numerator) : 1;
  // `/` alone halves, `//` quarters, and so on.
  const bottom = denominator ? Number(denominator) : slashes ? 2 ** slashes : 1;
  const value = top / bottom;
  return Number.isFinite(value) && value > 0 ? value : 1;
}

export function accidentalsFor(key: string): Record<string, number> {
  const match = /^([A-G][#b]?)/.exec(key.trim());
  const root = match ? match[1] : "C";
  const minor = /m(?!aj)/i.test(key) && !/major/i.test(key);
  // A minor key carries the signature of its relative major.
  const relative = (minor ? RELATIVE_MAJOR[root] : undefined) ?? root;
  const letters = KEY_SIGNATURES[relative] ?? [];
  const sharp = !relative.includes("b") && relative !== "F";
  const out: Record<string, number> = {};
  for (const letter of letters) out[letter] = sharp ? 1 : -1;
  return out;
}

/** A minor key carries the signature of its relative major. */
const RELATIVE_MAJOR: Record<string, string> = {
  A: "C", E: "G", B: "D", "F#": "A", "C#": "E", D: "F", G: "Bb", C: "Eb", F: "Ab", Bb: "Db",
};