279 lines
6.8 KiB
C++
279 lines
6.8 KiB
C++
/*++
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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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#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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// 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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// 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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#undef HASH_SHIFT_MULTIPLY
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#ifdef HASH_SHIFT_MULTIPLY
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# define HASH_MULTIPLY(dw) (((dw) << 7) - (dw))
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#else
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# define HASH_MULTIPLY(dw) ((dw) * HASH_MULTIPLIER)
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#endif
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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; ++upsz)
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dwHash = HASH_MULTIPLY(dwHash) + *upsz;
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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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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; ++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; ++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; ++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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LPBYTE pb = static_cast<LPBYTE>(const_cast<void*>(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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// 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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return * reinterpret_cast<DWORD*>(const_cast<GUID*>(pguid)) + 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(float f)
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{
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// be careful of rounding errors when computing keys
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union {
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float f;
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DWORD dw;
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} u;
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u.f = f;
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return u.dw;
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}
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inline DWORD Hash(double dbl)
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{
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// be careful of rounding errors when computing keys
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union {
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double dbl;
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DWORD dw[2];
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} u;
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u.dbl = dbl;
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return u.dw[0] * HASH_MULTIPLIER + u.dw[1];
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}
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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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