Function simplicity_readNBits

src/simplicity/bitstream.c:33–50

Fetches up to 31 bits from 'stream' as the 'n' least significant bits of return value. * The 'n' bits are set from the MSB to the LSB. * Returns 'SIMPLICITY_ERR_BITSTREAM_EOF' if not enough bits are available. * * Precondition: 0 <= n < 32 * NULL != stream */

 *               NULL != stream
 */
int32_t simplicity_readNBits(int n, bitstream* stream) {
  simplicity_assert(0 <= n && n < 32);

  uint32_t result = 0;
  while (CHAR_BIT <= stream->offset + n) {
    if (!stream->len) return SIMPLICITY_ERR_BITSTREAM_EOF;
    n -= CHAR_BIT - stream->offset;
    result |= (uint32_t)(*stream->arr & (UCHAR_MAX >> stream->offset)) << n;
    stream->arr++; stream->len--; stream->offset = 0;
  }
  /* stream->offset + n < CHAR_BIT */
  if (n) {
    if (!stream->len) return SIMPLICITY_ERR_BITSTREAM_EOF;
    stream->offset += (unsigned char)n;
    result |= (*stream->arr >> (CHAR_BIT - stream->offset)) & ((UCHAR_MAX >> (CHAR_BIT - n)));
  }
  return (int32_t)result;
}
/* Decode an encoded bitstring up to length 1.
 * If successful returns the length of the bitstring and 'result' contains the decoded bits.
 * The decoded bitstring is stored in the LSBs of 'result', with the LSB being the last bit decoded.