66 lines
1.6 KiB
C++
66 lines
1.6 KiB
C++
#include "base/basictypes.h"
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#include "util/hash/hash.h"
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namespace hash {
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// uint32_t
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// WARNING - may read one more byte!
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// Implementation from Wikipedia.
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uint32 HashFletcher(const uint8 *data_uint8, size_t length) {
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const uint16 *data = (const uint16 *)data_uint8;
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size_t len = (length + 1) / 2;
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uint32 sum1 = 0xffff, sum2 = 0xffff;
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while (len) {
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size_t tlen = len > 360 ? 360 : len;
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len -= tlen;
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do {
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sum1 += *data++;
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sum2 += sum1;
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} while (--tlen);
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sum1 = (sum1 & 0xffff) + (sum1 >> 16);
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sum2 = (sum2 & 0xffff) + (sum2 >> 16);
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}
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/* Second reduction step to reduce sums to 16 bits */
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sum1 = (sum1 & 0xffff) + (sum1 >> 16);
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sum2 = (sum2 & 0xffff) + (sum2 >> 16);
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return sum2 << 16 | sum1;
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}
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// Implementation from Wikipedia
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// Slightly slower than Fletcher above, but slighly more reliable.
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#define MOD_ADLER 65521
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// data: Pointer to the data to be summed; len is in bytes
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uint32 HashAdler32(const uint8 *data, size_t len) {
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uint32 a = 1, b = 0;
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while (len) {
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size_t tlen = len > 5550 ? 5550 : len;
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len -= tlen;
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do {
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a += *data++;
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b += a;
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} while (--tlen);
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a = (a & 0xffff) + (a >> 16) * (65536 - MOD_ADLER);
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b = (b & 0xffff) + (b >> 16) * (65536 - MOD_ADLER);
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}
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// It can be shown that a <= 0x1013a here, so a single subtract will do.
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if (a >= MOD_ADLER) {
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a -= MOD_ADLER;
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}
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// It can be shown that b can reach 0xfff87 here.
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b = (b & 0xffff) + (b >> 16) * (65536 - MOD_ADLER);
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if (b >= MOD_ADLER) {
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b -= MOD_ADLER;
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}
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return (b << 16) | a;
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}
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} // namespace hash
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