Function simplicity_readBitstring

src/simplicity/bitstream.c:214–233

Fills a 'bitstring' containing 'n' bits from 'stream'. * Returns 'SIMPLICITY_ERR_BITSTREAM_EOF' if not enough bits are available. * If successful, '*result' is set to a bitstring with 'n' bits read from 'stream' and 'SIMPLICITY_NO_ERROR' is returned. * * If an error is returned '*result' might be modified. * * Precondition: NULL != result * n <= 2^31 * NULL != s

 *               NULL != stream
 */
simplicity_err simplicity_readBitstring(bitstring* result, size_t n, bitstream* stream) {
  static_assert(0x80000000u + 2*(CHAR_BIT - 1) <= SIZE_MAX, "size_t needs to be at least 32-bits");
  simplicity_assert(n <= 0x80000000u);
  size_t total_offset = n + stream->offset;
  /* |= stream->len * CHAR_BIT < total_offset iff stream->len < (total_offset + (CHAR_BIT - 1)) / CHAR_BIT */
  if (stream->len < (total_offset + (CHAR_BIT - 1)) / CHAR_BIT) return SIMPLICITY_ERR_BITSTREAM_EOF;
  /* total_offset <= stream->len * CHAR_BIT */
  *result = (bitstring)
          { .arr = stream->arr
          , .offset = stream->offset
          , .len = n
          };
  {
    size_t delta = total_offset / CHAR_BIT;
    stream->arr += delta; stream->len -= delta;
    stream->offset = total_offset % CHAR_BIT;
    /* Note that if 0 == stream->len then 0 == stream->offset. */
  }
  return SIMPLICITY_NO_ERROR;
}