decodeTag function

Map<String, int> decodeTag(List<int> buffer, int offset)

Decodes a protobuf field tag from buffer starting at offset.

Returns a map with three entries:

• 'fieldNumber': the field number extracted from the tag.
• 'wireType': the wire type (lowest 3 bits of the tag value).
• 'bytesRead': the number of bytes consumed (the varint length).

The varint decoding is inlined here to avoid cross-file imports.

Throws a FormatException if the varint exceeds 10 bytes or the buffer ends before the varint is complete.

Implementation

Map<String, int> decodeTag(List<int> buffer, int offset) {
  // Inline varint decoding.
  int result = 0;
  int shift = 0;
  int bytesRead = 0;

  while (true) {
    if (offset + bytesRead >= buffer.length) {
      throw FormatException(
        'Unexpected end of buffer while decoding tag varint at offset $offset',
      );
    }

    int byte = buffer[offset + bytesRead];
    bytesRead++;

    result = result | ((byte & 0x7F) << shift);
    shift += 7;

    if ((byte & 0x80) == 0) {
      // Reject a non-minimal (overlong) tag: a multi-byte varint whose final
      // byte is 0x00 carries no bits and is redundant. A minimal encoding never
      // ends in a zero continuation (the sole exception, value 0, is one byte).
      if (bytesRead > 1 && byte == 0) {
        throw FormatException('Overlong (non-minimal) tag at offset $offset');
      }
      break;
    }

    if (bytesRead >= 10) {
      throw FormatException(
        'Tag varint exceeds maximum length of 10 bytes at offset $offset',
      );
    }
  }

  int wireType = result & 0x07;
  int fieldNumber = result >>> 3;
  // Reject structurally illegal tags so malformed input is a parse error rather
  // than silently-skipped unknown fields: field 0 is never valid, and field
  // numbers must fit in 29 bits (max 2^29 - 1 = 536870911).
  if (fieldNumber == 0) {
    throw FormatException('Illegal field number 0 in tag at offset $offset');
  }
  if (fieldNumber > 536870911) {
    throw FormatException(
      'Field number $fieldNumber exceeds the maximum 2^29-1 at offset $offset',
    );
  }
  return {
    'fieldNumber': fieldNumber,
    'wireType': wireType,
    'bytesRead': bytesRead,
  };
}