Reverse bits in word by lookup table

reverse_int.cpp

static const unsigned char BitReverseTable256[256] = 
{
#   define R2(n)     n,     n + 2*64,     n + 1*64,     n + 3*64
#   define R4(n) R2(n), R2(n + 2*16), R2(n + 1*16), R2(n + 3*16)
#   define R6(n) R4(n), R4(n + 2*4 ), R4(n + 1*4 ), R4(n + 3*4 )
    R6(0), R6(2), R6(1), R6(3)
};

// reverse 32-bit value, 8 bits at time
unsigned int reverse_int(unsigned int v) {
  return (BitReverseTable256[v & 0xff] << 24) |
         (BitReverseTable256[(v >> 8) & 0xff] << 16) | 
         (BitReverseTable256[(v >> 16) & 0xff] << 8) |
         (BitReverseTable256[(v >> 24) & 0xff]);
}