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BitTwiddle - Your daily bit twiddle fortune. |
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___[ Twiddle ] |
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Compute parity by lookup table |
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static const bool ParityTable256[256] = |
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{ |
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# define P2(n) n, n^1, n^1, n |
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# define P4(n) P2(n), P2(n^1), P2(n^1), P2(n) |
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# define P6(n) P4(n), P4(n^1), P4(n^1), P4(n) |
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P6(0), P6(1), P6(1), P6(0) |
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}; |
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unsigned char b; // byte value to compute the parity of |
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bool parity = ParityTable256[b]; |
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// OR, for 32-bit words: |
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unsigned int v; |
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v ^= v >> 16; |
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v ^= v >> 8; |
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bool parity = ParityTable256[v & 0xff]; |
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// Variation: |
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unsigned char * p = (unsigned char *) &v; |
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parity = ParityTable256[p[0] ^ p[1] ^ p[2] ^ p[3]]; |
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Randal E. Bryant encouraged the addition of the (admittedly) obvious last |
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variation with variable p on May 3, 2005. Bruce Rawles found a typo in an |
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instance of the table variable's name on September 27, 2005, and he received a |
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$10 bug bounty. On October 9, 2006, Fabrice Bellard suggested the 32-bit |
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variations above, which require only one table lookup; the previous version had |
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four lookups (one per byte) and were slower. On July 14, 2009 Hallvard Furuseth |
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suggested the macro compacted table. |
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