unescapeFluentString function

String unescapeFluentString(String input)

Decodes escape sequences inside a Fluent string-literal value.

Per the Fluent Syntax 1.0 spec, the parser preserves escape sequences in StringLiteral.value as their literal characters (\\ stays as \\, \uXXXX stays as \uXXXX). The runtime resolver decodes them when the literal is rendered into a pattern. This function is that decoder.

Recognized sequences:

• \\ → \ (single backslash)
• \" → " (double quote)
• \uXXXX → BMP codepoint named by 4 hex digits
• \UXXXXXX → full-range codepoint named by 6 hex digits

Malformed input is decoded best-effort: the parser already validated every sequence, so a value reaching this function is well-formed by construction. Unknown bytes after \ are passed through unchanged to keep the function total.

Implementation

String unescapeFluentString(String input) {
  if (!input.contains(r'\')) return input;

  final buffer = StringBuffer();
  final length = input.length;
  var i = 0;
  while (i < length) {
    final ch = input.codeUnitAt(i);
    if (ch != _backslash) {
      buffer.writeCharCode(ch);
      i++;
      continue;
    }
    if (i + 1 >= length) {
      // Trailing backslash — preserve as literal.
      buffer.writeCharCode(_backslash);
      i++;
      continue;
    }
    final kind = input.codeUnitAt(i + 1);
    if (kind == _backslash) {
      buffer.writeCharCode(_backslash);
      i += 2;
    } else if (kind == _quote) {
      buffer.writeCharCode(_quote);
      i += 2;
    } else if (kind == _lowerU) {
      // `\uXXXX` — 4 hex digits.
      buffer.writeCharCode(int.parse(input.substring(i + 2, i + 6), radix: 16));
      i += 6;
    } else if (kind == _upperU) {
      // `\UXXXXXX` — 6 hex digits.
      buffer.writeCharCode(int.parse(input.substring(i + 2, i + 8), radix: 16));
      i += 8;
    } else {
      // Unknown sequence — should never reach here on parser-validated
      // input, but be tolerant.
      buffer.writeCharCode(_backslash);
      i++;
    }
  }
  return buffer.toString();
}