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38 lines
855 B
C++
38 lines
855 B
C++
#include "base/basictypes.h"
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#include "util/hash/hash.h"
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namespace hash {
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// Implementation from Wikipedia
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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_t Adler32(const uint8_t *data, size_t len) {
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uint32_t 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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