KingOfYellowAndBlack's blog

By KingOfYellowAndBlack, history, 12 days ago, In English

Are there any non linear ways to reverse a bitset? For E.G the bitset 1001. Reverse from 1-3 and it would be 0011.

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12 days ago, # |
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Reverse an N-bit quantity in parallel in 5 * lg(N) operations:

unsigned int v; // 32-bit word to reverse bit order
// swap odd and even bits
v = ((v >> 1) & 0x55555555) | ((v & 0x55555555) << 1);
// swap consecutive pairs
v = ((v >> 2) & 0x33333333) | ((v & 0x33333333) << 2);
// swap nibbles ... 
v = ((v >> 4) & 0x0F0F0F0F) | ((v & 0x0F0F0F0F) << 4);
// swap bytes
v = ((v >> 8) & 0x00FF00FF) | ((v & 0x00FF00FF) << 8);
// swap 2-byte long pairs
v = ( v >> 16             ) | ( v               << 16);

Use this to flip all 32 integers in O(length*log w/w), then do a brute force flip with O(length/w).