Fastest way to scan for bit pattern in a stream of bits

Question

I need to scan for a 16 bit word in a bit stream. It is not guaranteed to be aligned on byte or word boundaries.

What is the fastest way of achieving this

Answer 1

For a general-purpose, non-SIMD algorithm you are unlikely to be able to do much better than something like this:

unsigned int const pattern = pattern to search for
unsigned int accumulator = first three input bytes

do
{
  bool const found = ( ((accumulator   ) & ((1<<16)-1)) == pattern )
                   | ( ((accumulator>>1) & ((1<<16)-1)) == pattern );
                   | ( ((accumulator>>2) & ((1<<16)-1)) == pattern );
                   | ( ((accumulator>>3) & ((1<<16)-1)) == pattern );
                   | ( ((accumulator>>4) & ((1<<16)-1)) == pattern );
                   | ( ((accumulator>>5) & ((1<<16)-1)) == pattern );
                   | ( ((accumulator>>6) & ((1<<16)-1)) == pattern );
                   | ( ((accumulator>>7) & ((1<<16)-1)) == pattern );
  if( found ) { /* pattern found */ }
  accumulator >>= 8;

  unsigned int const data = next input byte
  accumulator |= (data<<8);
} while( there is input data left );