/**
 * French prose is full of « 185 000 $ ». remark-math would treat those dollars as inline-math
 * delimiters. We apply Pandoc's rule before parsing: a single `$` opens math only if followed by
 * a non-space, closes only if preceded by a non-space and not followed by a digit. Every `$`
 * that cannot play either role is escaped (`\$`). `$$` blocks and code fences are left untouched.
 */
export function guardCurrency(md: string): string {
  let out = '';
  let i = 0;
  let inFence = false;
  let openIdx = -1; // index in `out` of the current unmatched opener
  const n = md.length;
  while (i < n) {
    const ch = md[i];
    // code fences
    if ((ch === '`' && md.startsWith('```', i)) || (ch === '~' && md.startsWith('~~~', i))) {
      inFence = !inFence;
      out += md.slice(i, i + 3);
      i += 3;
      continue;
    }
    if (inFence) {
      out += ch;
      i++;
      continue;
    }
    if (ch === '`') {
      // inline code: copy through the closing backtick
      const j = md.indexOf('`', i + 1);
      if (j > 0) {
        out += md.slice(i, j + 1);
        i = j + 1;
        continue;
      }
    }
    if (ch === '\\' && md[i + 1] === '$') {
      out += '\\$';
      i += 2;
      continue;
    }
    if (ch === '$' && md[i + 1] === '$') {
      // display math: copy through the closing $$
      const j = md.indexOf('$$', i + 2);
      if (j > 0) {
        out += md.slice(i, j + 2);
        i = j + 2;
        continue;
      }
    }
    if (ch === '$') {
      const prev = i > 0 ? md[i - 1] : ' ';
      const next = i + 1 < n ? md[i + 1] : ' ';
      const canOpen = !/\s/.test(next) && next !== '$';
      const canClose = !/\s/.test(prev) && !/\d/.test(next);
      if (openIdx < 0) {
        if (canOpen) {
          openIdx = out.length;
          out += '$';
        } else {
          out += '\\$';
        }
      } else if (canClose) {
        out += '$';
        openIdx = -1;
      } else {
        out += '\\$';
      }
      i++;
      continue;
    }
    if (ch === '\n' && openIdx >= 0 && md[i + 1] === '\n') {
      // paragraph break with an unmatched opener: it was a currency sign
      out = out.slice(0, openIdx) + '\\$' + out.slice(openIdx + 1);
      openIdx = -1;
    }
    out += ch;
    i++;
  }
  if (openIdx >= 0) out = out.slice(0, openIdx) + '\\$' + out.slice(openIdx + 1);
  return out;
}