// lib/skills/skill-frontmatter.mjs
// Frontmatter reader for skill, command and agent files: the fields the skill
// workshop needs, with the line each one is on and the YAML traps it hits.
//
// Why not lib/util/frontmatter.mjs: that parser deliberately rejects
// multi-line strings, and Voyage's agents write `description: |`. This reader
// takes plain, quoted and block-scalar (`|`, `>`, with `-`/`+`) values, flow
// lists, and indented lists or maps (kept as raw text). It is not a YAML
// parser: it reads the subset a SKILL.md uses and reports what would break a
// real one. Claude Code loads a skill whose YAML does not parse "with no
// fields set", so the description is gone and the skill cannot match; a trap
// here is an error, not a style note.

const KEY_RE = /^([A-Za-z0-9_-]+):(.*)$/;
const BLOCK_RE = /^([|>])([+-]?)\s*$/;

/**
 * Split a file into its frontmatter lines and body.
 * @returns {{ ok: boolean, fmLines: string[], body: string, bodyStartLine: number }}
 */
export function splitFile(text) {
  const src = String(text).replace(/^/, '');
  const lines = src.split(/\r?\n/);
  if (lines[0] !== '---') return { ok: false, fmLines: [], body: src, bodyStartLine: 1 };
  const end = lines.indexOf('---', 1);
  if (end < 0) return { ok: false, fmLines: [], body: src, bodyStartLine: 1 };
  return { ok: true, fmLines: lines.slice(1, end), body: lines.slice(end + 1).join('\n'), bodyStartLine: end + 2 };
}

/** The YAML traps in one plain (unquoted, single-line) scalar value. */
export function plainScalarHazards(value) {
  const out = [];
  if (/:\s/.test(value) || /:$/.test(value)) out.push('unquoted ": " (YAML reads a mapping and the line fails)');
  if (/\s#/.test(value)) out.push('" #" starts a YAML comment: the rest of the value is cut');
  if (/^[`@%*&!|>,?{]/.test(value)) out.push(`a value may not start with "${value[0]}" unquoted`);
  if (/^\[/.test(value) && !/\]$/.test(value)) out.push('an unclosed flow list');
  if (/^-\s/.test(value)) out.push('"- " at the start reads as a list item');
  return out;
}

function unquote(v) {
  if (v.startsWith('"')) {
    if (v.length < 2 || !v.endsWith('"') || v.endsWith('\\"')) return { error: 'unclosed double quote' };
    try { return { value: JSON.parse(v) }; } catch { return { error: 'bad escape in a double-quoted value' }; }
  }
  if (v.startsWith("'")) {
    if (v.length < 2 || !v.endsWith("'")) return { error: 'unclosed single quote' };
    return { value: v.slice(1, -1).replace(/''/g, "'") };
  }
  return null;
}

function blockValue(lines, style, chomp) {
  const nonBlank = lines.filter((l) => l.trim() !== '');
  const indent = nonBlank.length ? Math.min(...nonBlank.map((l) => l.match(/^ */)[0].length)) : 0;
  const stripped = lines.map((l) => l.slice(indent));
  while (stripped.length && stripped[stripped.length - 1].trim() === '') stripped.pop();
  let text;
  if (style === '|') text = stripped.join('\n');
  else {
    // Folded: lines join with a space; a blank line is a newline.
    text = '';
    for (const l of stripped) {
      if (l === '') text += '\n';
      else text += (text === '' || text.endsWith('\n') ? '' : ' ') + l;
    }
  }
  if (chomp === '-') return text;
  return `${text}\n`;
}

/**
 * Read the frontmatter of a skill/command/agent file.
 * @returns {{ ok: boolean, fields: Object<string, {value: any, line: number, style: string}>,
 *             hazards: {line: number, message: string}[], error?: string }}
 */
export function readFrontmatterFields(text) {
  const split = splitFile(text);
  if (!split.ok) return { ok: false, fields: {}, hazards: [], error: 'no frontmatter: the file must start with a "---" line and close it' };
  const { fmLines } = split;
  const fields = {};
  const hazards = [];
  let i = 0;
  while (i < fmLines.length) {
    const line = fmLines[i];
    const lineNo = i + 2; // line 1 is the opening ---
    if (/\t/.test(line)) hazards.push({ line: lineNo, message: 'a tab in frontmatter: YAML forbids tabs for indentation' });
    if (line.trim() === '' || line.trimStart().startsWith('#')) { i++; continue; }
    if (/^\s/.test(line)) { hazards.push({ line: lineNo, message: 'an indented line that belongs to no key' }); i++; continue; }
    const m = KEY_RE.exec(line);
    if (!m) { hazards.push({ line: lineNo, message: `not a "key: value" line: ${line.slice(0, 60)}` }); i++; continue; }
    const key = m[1];
    const rest = m[2].replace(/^[ \t]+/, '');
    if (m[2] !== '' && !/^[ \t]/.test(m[2])) hazards.push({ line: lineNo, message: `"${key}:" needs a space after the colon` });
    if (key in fields) hazards.push({ line: lineNo, message: `duplicate key "${key}"` });

    // Collect the indented continuation lines (block scalar, list or map).
    let j = i + 1;
    const cont = [];
    while (j < fmLines.length && (fmLines[j].trim() === '' || /^\s/.test(fmLines[j]))) { cont.push(fmLines[j]); j++; }
    while (cont.length && cont[cont.length - 1].trim() === '') { cont.pop(); }

    const block = BLOCK_RE.exec(rest);
    if (block) {
      fields[key] = { value: blockValue(cont, block[1], block[2]), line: lineNo, style: 'block' };
    } else if (rest === '') {
      const items = cont.filter((l) => l.trim() !== '');
      if (items.length && items.every((l) => /^\s+-\s/.test(l))) {
        fields[key] = { value: items.map((l) => l.replace(/^\s+-\s+/, '').trim()), line: lineNo, style: 'list' };
      } else {
        fields[key] = { value: items.join('\n'), line: lineNo, style: items.length ? 'map' : 'empty' };
      }
    } else {
      if (cont.length) hazards.push({ line: lineNo, message: `"${key}" has a value and indented lines under it` });
      const q = unquote(rest);
      if (q && q.error) {
        hazards.push({ line: lineNo, message: q.error });
        fields[key] = { value: rest, line: lineNo, style: 'broken' };
      } else if (q) {
        fields[key] = { value: q.value, line: lineNo, style: 'quoted' };
      } else if (/^\[.*\]$/.test(rest)) {
        const inner = rest.slice(1, -1).trim();
        fields[key] = {
          value: inner === '' ? [] : inner.split(',').map((s) => { const t = s.trim(); const u = unquote(t); return u && !u.error ? u.value : t; }),
          line: lineNo, style: 'flow',
        };
      } else {
        for (const h of plainScalarHazards(rest)) hazards.push({ line: lineNo, message: `${key}: ${h}` });
        fields[key] = { value: rest, line: lineNo, style: 'plain' };
      }
    }
    i = j;
  }
  return { ok: hazards.length === 0, fields, hazards };
}

/** The description of a file as one string ('' when absent). */
export function descriptionOf(text) {
  const r = readFrontmatterFields(text);
  const d = r.fields.description;
  if (!d) return '';
  return Array.isArray(d.value) ? d.value.join(' ') : String(d.value);
}