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361 lines
9.1 KiB
361 lines
9.1 KiB
/*++
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Copyright (c) 1998-2000 Microsoft Corporation
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Module Name :
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HashFn.h
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Abstract:
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Declares and defines a collection of overloaded hash functions.
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It is strongly suggested that you use these functions with LKRhash.
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Author:
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George V. Reilly (GeorgeRe) 06-Jan-1998
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Environment:
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Win32 - User Mode
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Project:
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Internet Information Server RunTime Library
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Revision History:
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--*/
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#ifndef __HASHFN_H__
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#define __HASHFN_H__
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#include <math.h>
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#include <limits.h>
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#ifndef __HASHFN_NO_NAMESPACE__
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namespace HashFn {
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#endif // !__HASHFN_NO_NAMESPACE__
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// Produce a scrambled, randomish number in the range 0 to RANDOM_PRIME-1.
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// Applying this to the results of the other hash functions is likely to
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// produce a much better distribution, especially for the identity hash
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// functions such as Hash(char c), where records will tend to cluster at
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// the low end of the hashtable otherwise. LKRhash applies this internally
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// to all hash signatures for exactly this reason.
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inline DWORD
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HashScramble(DWORD dwHash)
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{
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// Here are 10 primes slightly greater than 10^9
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// 1000000007, 1000000009, 1000000021, 1000000033, 1000000087,
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// 1000000093, 1000000097, 1000000103, 1000000123, 1000000181.
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// default value for "scrambling constant"
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const DWORD RANDOM_CONSTANT = 314159269UL;
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// large prime number, also used for scrambling
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const DWORD RANDOM_PRIME = 1000000007UL;
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return (RANDOM_CONSTANT * dwHash) % RANDOM_PRIME ;
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}
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enum {
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// No number in 0..2^31-1 maps to this number after it has been
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// scrambled by HashFn::HashRandomizeBits
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HASH_INVALID_SIGNATURE = 31678523,
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// Given M = A % B, A and B unsigned 32-bit integers greater than zero,
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// there are no values of A or B which yield M = 2^32-1. Why? Because
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// M must be less than B. (For numbers scrambled by HashScramble)
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// HASH_INVALID_SIGNATURE = ULONG_MAX
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};
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// Faster scrambling function suggested by Eric Jacobsen
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inline DWORD
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HashRandomizeBits(DWORD dw)
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{
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return (((dw * 1103515245 + 12345) >> 16)
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| ((dw * 69069 + 1) & 0xffff0000));
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}
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#undef HASH_SHIFT_MULTIPLY
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#ifdef HASH_SHIFT_MULTIPLY
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inline DWORD
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HASH_MULTIPLY(
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DWORD dw)
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{
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// 127 = 2^7 - 1 is prime
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return (dw << 7) - dw;
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}
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#else // !HASH_SHIFT_MULTIPLY
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inline DWORD
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HASH_MULTIPLY(
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DWORD dw)
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{
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// Small prime number used as a multiplier in the supplied hash functions
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const DWORD HASH_MULTIPLIER = 101;
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return dw * HASH_MULTIPLIER;
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}
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#endif // !HASH_SHIFT_MULTIPLY
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// Fast, simple hash function that tends to give a good distribution.
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// Apply HashScramble to the result if you're using this for something
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// other than LKRhash.
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inline DWORD
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HashString(
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const char* psz,
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DWORD dwHash = 0)
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{
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// force compiler to use unsigned arithmetic
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const unsigned char* upsz = (const unsigned char*) psz;
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for ( ; *upsz != '\0'; ++upsz)
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dwHash = HASH_MULTIPLY(dwHash) + *upsz;
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return dwHash;
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}
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// --------------------------------------------------------
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// Compute a hash value from an input string of any type, i.e.
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// the input is just treated as a sequence of bytes.
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// Based on a hash function originally proposed by J. Zobel.
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// Author: Paul Larson, 1999, [email protected]
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// --------------------------------------------------------
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inline DWORD
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HashString2(
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const char* pszInputKey, // ptr to input - any type is OK
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DWORD dwHash = 314159269) // Initial seed for hash function
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{
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// Initialize dwHash to a reasonably large constant so very
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// short keys won't get mapped to small values. Virtually any
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// large odd constant will do.
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const unsigned char* upsz = (const unsigned char*) pszInputKey;
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for ( ; *upsz != '\0'; ++upsz)
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dwHash ^= (dwHash << 11) + (dwHash << 5) + (dwHash >> 2) + *upsz;
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return (dwHash & 0x7FFFFFFF);
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}
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// Unicode version of above
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inline DWORD
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HashString(
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const wchar_t* pwsz,
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DWORD dwHash = 0)
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{
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for ( ; *pwsz != L'\0'; ++pwsz)
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dwHash = HASH_MULTIPLY(dwHash) + *pwsz;
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return dwHash;
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}
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// Quick-'n'-dirty case-insensitive string hash function.
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// Make sure that you follow up with _stricmp or _mbsicmp. You should
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// also cache the length of strings and check those first. Caching
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// an uppercase version of a string can help too.
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// Again, apply HashScramble to the result if using with something other
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// than LKRhash.
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// Note: this is not really adequate for MBCS strings.
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inline DWORD
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HashStringNoCase(
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const char* psz,
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DWORD dwHash = 0)
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{
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const unsigned char* upsz = (const unsigned char*) psz;
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for ( ; *upsz != '\0'; ++upsz)
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dwHash = HASH_MULTIPLY(dwHash)
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+ (*upsz & 0xDF); // strip off lowercase bit
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return dwHash;
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}
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// Unicode version of above
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inline DWORD
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HashStringNoCase(
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const wchar_t* pwsz,
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DWORD dwHash = 0)
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{
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for ( ; *pwsz != L'\0'; ++pwsz)
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dwHash = HASH_MULTIPLY(dwHash) + (*pwsz & 0xFFDF);
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return dwHash;
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}
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// HashBlob returns the hash of a blob of arbitrary binary data.
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//
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// Warning: HashBlob is generally not the right way to hash a class object.
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// Consider:
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// class CFoo {
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// public:
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// char m_ch;
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// double m_d;
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// char* m_psz;
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// };
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//
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// inline DWORD Hash(const CFoo& rFoo)
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// { return HashBlob(&rFoo, sizeof(CFoo)); }
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//
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// This is the wrong way to hash a CFoo for two reasons: (a) there will be
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// a 7-byte gap between m_ch and m_d imposed by the alignment restrictions
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// of doubles, which will be filled with random data (usually non-zero for
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// stack variables), and (b) it hashes the address (rather than the
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// contents) of the string m_psz. Similarly,
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//
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// bool operator==(const CFoo& rFoo1, const CFoo& rFoo2)
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// { return memcmp(&rFoo1, &rFoo2, sizeof(CFoo)) == 0; }
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//
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// does the wrong thing. Much better to do this:
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//
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// DWORD Hash(const CFoo& rFoo)
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// {
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// return HashString(rFoo.m_psz,
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// HASH_MULTIPLIER * Hash(rFoo.m_ch)
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// + Hash(rFoo.m_d));
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// }
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//
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// Again, apply HashScramble if using with something other than LKRhash.
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inline DWORD
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HashBlob(
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const void* pv,
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size_t cb,
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DWORD dwHash = 0)
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{
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const BYTE* pb = static_cast<const BYTE*>(pv);
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while (cb-- > 0)
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dwHash = HASH_MULTIPLY(dwHash) + *pb++;
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return dwHash;
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}
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// --------------------------------------------------------
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// Compute a hash value from an input string of any type, i.e.
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// the input is just treated as a sequence of bytes.
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// Based on a hash function originally proposed by J. Zobel.
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// Author: Paul Larson, 1999, [email protected]
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// --------------------------------------------------------
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inline DWORD
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HashBlob2(
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const void* pvInputKey, // ptr to input - any type is OK
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size_t cbKeyLen, // length of input key in bytes
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DWORD dwHash = 314159269) // Initial seed for hash function
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{
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// Initialize dwHash to a reasonably large constant so very
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// short keys won't get mapped to small values. Virtually any
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// large odd constant will do.
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const BYTE* pb = static_cast<const BYTE*>(pvInputKey);
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const BYTE* pbSentinel = pb + cbKeyLen;
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for ( ; pb < pbSentinel; ++pb)
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dwHash ^= (dwHash << 11) + (dwHash << 5) + (dwHash >> 2) + *pb;
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return (dwHash & 0x7FFFFFFF);
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}
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//
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// Overloaded hash functions for all the major builtin types.
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// Again, apply HashScramble to result if using with something other than
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// LKRhash.
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//
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inline DWORD Hash(const char* psz)
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{ return HashString(psz); }
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inline DWORD Hash(const unsigned char* pusz)
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{ return HashString(reinterpret_cast<const char*>(pusz)); }
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inline DWORD Hash(const signed char* pssz)
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{ return HashString(reinterpret_cast<const char*>(pssz)); }
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inline DWORD Hash(const wchar_t* pwsz)
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{ return HashString(pwsz); }
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inline DWORD
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Hash(
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const GUID* pguid,
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DWORD dwHash = 0)
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{
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dwHash += * reinterpret_cast<const DWORD*>(pguid);
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return dwHash;
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}
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// Identity hash functions: scalar values map to themselves
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inline DWORD Hash(char c)
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{ return c; }
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inline DWORD Hash(unsigned char uc)
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{ return uc; }
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inline DWORD Hash(signed char sc)
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{ return sc; }
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inline DWORD Hash(short sh)
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{ return sh; }
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inline DWORD Hash(unsigned short ush)
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{ return ush; }
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inline DWORD Hash(int i)
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{ return i; }
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inline DWORD Hash(unsigned int u)
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{ return u; }
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inline DWORD Hash(long l)
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{ return l; }
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inline DWORD Hash(unsigned long ul)
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{ return ul; }
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inline DWORD Hash(double dbl)
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{
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if (dbl == 0.0)
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return 0;
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int nExponent;
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double dblMantissa = frexp(dbl, &nExponent);
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// 0.5 <= |mantissa| < 1.0
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return (DWORD) ((2.0 * fabs(dblMantissa) - 1.0) * ULONG_MAX);
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}
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inline DWORD Hash(float f)
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{ return Hash((double) f); }
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inline DWORD Hash(unsigned __int64 ull)
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{
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union {
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unsigned __int64 _ull;
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DWORD dw[2];
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} u = {ull};
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return HASH_MULTIPLY(u.dw[0]) + u.dw[1];
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}
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inline DWORD Hash(__int64 ll)
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{ return Hash((unsigned __int64) ll); }
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#ifndef __HASHFN_NO_NAMESPACE__
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}
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#endif // !__HASHFN_NO_NAMESPACE__
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#endif // __HASHFN_H__
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