/** Decode the two Git path tokens in a `diff --git` header. */
function token(source: string, offset: number): { value: string; next: number } | undefined {
  if (source[offset] !== '"') {
    const end = source.indexOf(' ', offset),
      next = end < 0 ? source.length : end;
    if (next === offset) return;
    return { value: source.slice(offset, next), next };
  }
  const bytes: number[] = [];
  let at = offset + 1;
  const append = (value: string) => bytes.push(...new TextEncoder().encode(value));
  while (at < source.length) {
    const value = source[at++];
    if (value === '"')
      return { value: new TextDecoder('utf-8', { fatal: true }).decode(Uint8Array.from(bytes)), next: at };
    if (value !== '\\') {
      append(value);
      continue;
    }
    if (at >= source.length) return;
    const escaped = source[at++],
      mapped: { [key: string]: string } = {
        a: '\x07',
        b: '\b',
        f: '\f',
        n: '\n',
        r: '\r',
        t: '\t',
        v: '\v',
        '\\': '\\',
        '"': '"',
      };
    if (mapped[escaped] !== undefined) {
      append(mapped[escaped]);
      continue;
    }
    if (/[0-7]/.test(escaped) && /^[0-7]{2}/.test(source.slice(at, at + 2))) {
      bytes.push(Number.parseInt(escaped + source.slice(at, at + 2), 8));
      at += 2;
      continue;
    }
    return;
  }
}

export function gitDiffHeaderPaths(line: string): [string, string] | undefined {
  const prefix = 'diff --git ';
  if (!line.startsWith(prefix)) return;
  try {
    const left = token(line, prefix.length);
    if (!left || line[left.next] !== ' ') return;
    const right = token(line, left.next + 1);
    if (!right || right.next !== line.length) return;
    return [left.value, right.value];
  } catch {
    return;
  }
}

/** Return only exact path hunks, keeping one commit preamble per matching commit. */
export function historyForPath(raw: string, path: string): string | undefined {
  const expected = new Set([`a/${path}`, `b/${path}`]),
    output: string[] = [],
    lines = raw.split('\n');
  let preamble: string[] = [],
    section: string[] | undefined,
    include = false,
    preambleEmitted = false;
  const flush = () => {
    if (section && include) {
      if (!preambleEmitted) {
        output.push(...preamble);
        preambleEmitted = true;
      }
      output.push(...section);
    }
    section = undefined;
    include = false;
  };
  for (const line of lines) {
    if (line.startsWith('commit ')) {
      flush();
      preamble = [line];
      preambleEmitted = false;
      continue;
    }
    if (line.startsWith('diff --git ')) {
      flush();
      section = [line];
      const paths = gitDiffHeaderPaths(line);
      include = Boolean(paths && (expected.has(paths[0]) || expected.has(paths[1])));
      continue;
    }
    if (section) section.push(line);
    else preamble.push(line);
  }
  flush();
  while (output.at(-1) === '') output.pop();
  return output.length ? output.join('\n') : undefined;
}