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/*++
Copyright (c) 1998 Microsoft Corporation
Module Name : hashfn.h
Abstract: Declares and defines a collection of overloaded hash functions. It is strongly suggested that you use these functions with LKRHash.
Author: George V. Reilly (GeorgeRe) 06-Jan-1998
Environment: Win32 - User Mode
Project: Internet Information Server RunTime Library
Revision History:
--*/
#ifndef __HASHFN_H__
#define __HASHFN_H__
#ifdef __HASHFN_NAMESPACE__
namespace HashFn {#endif // __HASHFN_NAMESPACE__
// Produce a scrambled, randomish number in the range 0 to RANDOM_PRIME-1.
// Applying this to the results of the other hash functions is likely to
// produce a much better distribution, especially for the identity hash
// functions such as Hash(char c), where records will tend to cluster at
// the low end of the hashtable otherwise. LKRHash applies this internally
// to all hash signatures for exactly this reason.
inline DWORDHashScramble(DWORD dwHash){ // Here are 10 primes slightly greater than 10^9
// 1000000007, 1000000009, 1000000021, 1000000033, 1000000087,
// 1000000093, 1000000097, 1000000103, 1000000123, 1000000181.
// default value for "scrambling constant"
const DWORD RANDOM_CONSTANT = 314159269UL; // large prime number, also used for scrambling
const DWORD RANDOM_PRIME = 1000000007UL;
return (RANDOM_CONSTANT * dwHash) % RANDOM_PRIME ;}
// Fast, simple hash function that tends to give a good distribution.
// Apply HashScramble to the result if you're using this for something
// other than LKRHash.
inline DWORDHashString( const char* psz, DWORD dwHash = 0){ // force compiler to use unsigned arithmetic
const unsigned char* upsz = (const unsigned char*) psz;
for ( ; *upsz; ++upsz) dwHash = 37 * dwHash + *upsz;
return dwHash;}
// Unicode version of above
inline DWORDHashString( const wchar_t* pwsz, DWORD dwHash = 0){ for ( ; *pwsz; ++pwsz) dwHash = 37 * dwHash + *pwsz;
return dwHash;}
// Quick-'n'-dirty case-insensitive string hash function.
// Make sure that you follow up with _stricmp or _mbsicmp. You should
// also cache the length of strings and check those first. Caching
// an uppercase version of a string can help too.
// Again, apply HashScramble to the result if using with something other
// than LKRHash.
// Note: this is not really adequate for MBCS strings.
inline DWORDHashStringNoCase( const char* psz, DWORD dwHash = 0){ const unsigned char* upsz = (const unsigned char*) psz;
for ( ; *upsz; ++upsz) dwHash = 37 * dwHash + (*upsz & 0xDF); // strip off lowercase bit
return dwHash;}
// Unicode version of above
inline DWORDHashStringNoCase( const wchar_t* pwsz, DWORD dwHash = 0){ for ( ; *pwsz; ++pwsz) dwHash = 37 * dwHash + (*pwsz & 0xFFDF);
return dwHash;}
// HashBlob returns the hash of a blob of arbitrary binary data.
//
// Warning: HashBlob is generally not the right way to hash a class object.
// Consider:
// class CFoo {
// public:
// char m_ch;
// double m_d;
// char* m_psz;
// };
//
// inline DWORD Hash(const CFoo& rFoo)
// { return HashBlob(&rFoo, sizeof(CFoo)); }
//
// This is the wrong way to hash a CFoo for two reasons: (a) there will be
// a 7-byte gap between m_ch and m_d imposed by the alignment restrictions
// of doubles, which will be filled with random data (usually non-zero for
// stack variables), and (b) it hashes the address (rather than the
// contents) of the string m_psz. Similarly,
//
// bool operator==(const CFoo& rFoo1, const CFoo& rFoo2)
// { return memcmp(&rFoo1, &rFoo2, sizeof(CFoo)) == 0; }
//
// does the wrong thing. Much better to do this:
//
// DWORD Hash(const CFoo& rFoo)
// {
// return HashString(rFoo.m_psz,
// 37 * Hash(rFoo.m_ch) + Hash(rFoo.m_d));
// }
//
// Again, apply HashScramble if using with something other than LKRHash.
inline DWORDHashBlob( const void* pv, size_t cb, DWORD dwHash = 0){ LPBYTE pb = static_cast<LPBYTE>(const_cast<void*>(pv));
while (cb-- > 0) dwHash = 37 * dwHash + *pb++;
return dwHash;}
//
// Overloaded hash functions for all the major builtin types.
// Again, apply HashScramble to result if using with something other than
// LKRHash.
//
inline DWORD Hash(const char* psz){ return HashString(psz); }
inline DWORD Hash(const unsigned char* pusz){ return HashString(reinterpret_cast<const char*>(pusz)); }
inline DWORD Hash(const signed char* pssz){ return HashString(reinterpret_cast<const char*>(pssz)); }
inline DWORD Hash(const wchar_t* pwsz){ return HashString(pwsz); }
inline DWORDHash( const GUID* pguid, DWORD dwHash = 0){ DWORD* pdw = reinterpret_cast<DWORD*>(const_cast<GUID*>(pguid)); dwHash = 37 * dwHash + *pdw++; dwHash = 37 * dwHash + *pdw++; dwHash = 37 * dwHash + *pdw++; dwHash = 37 * dwHash + *pdw;
return dwHash;}
// Identity hash functions: scalar values map to themselves
inline DWORD Hash(char c){ return c; }
inline DWORD Hash(unsigned char uc){ return uc; }
inline DWORD Hash(signed char sc){ return sc; }
inline DWORD Hash(short sh){ return sh; }
inline DWORD Hash(unsigned short ush){ return ush; }
inline DWORD Hash(int i){ return i; }
inline DWORD Hash(unsigned int u){ return u; }
inline DWORD Hash(long l){ return l; }
inline DWORD Hash(unsigned long ul){ return ul; }
inline DWORD Hash(float f){ // be careful of rounding errors when computing keys
union { float f; DWORD dw; } u; u.f = f; return u.dw;}
inline DWORD Hash(double dbl){ // be careful of rounding errors when computing keys
union { double dbl; DWORD dw[2]; } u; u.dbl = dbl; return u.dw[0] * 37 + u.dw[1];}
#ifdef __HASHFN_NAMESPACE__
}#endif // __HASHFN_NAMESPACE__
#endif // __HASHFN_H__
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