/*++
Copyright (c) 1998-2000 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__
#include <math.h>
#include <limits.h>
#ifndef __HASHFN_NO_NAMESPACE__
namespace HashFn {
#endif // !__HASHFN_NO_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 DWORD
HashScramble(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 ;
}
enum {
// No number in 0..2^31-1 maps to this number after it has been
// scrambled by HashFn::HashRandomizeBits
HASH_INVALID_SIGNATURE = 31678523,
// Given M = A % B, A and B unsigned 32-bit integers greater than zero,
// there are no values of A or B which yield M = 2^32-1. Why? Because
// M must be less than B. (For numbers scrambled by HashScramble)
// HASH_INVALID_SIGNATURE = ULONG_MAX
};
// Faster scrambling function suggested by Eric Jacobsen
inline DWORD
HashRandomizeBits(DWORD dw)
{
return (((dw * 1103515245 + 12345) >> 16)
| ((dw * 69069 + 1) & 0xffff0000));
}
#undef HASH_SHIFT_MULTIPLY
#ifdef HASH_SHIFT_MULTIPLY
inline DWORD
HASH_MULTIPLY(
DWORD dw)
{
// 127 = 2^7 - 1 is prime
return (dw << 7) - dw;
}
#else // !HASH_SHIFT_MULTIPLY
inline DWORD
HASH_MULTIPLY(
DWORD dw)
{
// Small prime number used as a multiplier in the supplied hash functions
const DWORD HASH_MULTIPLIER = 101;
return dw * HASH_MULTIPLIER;
}
#endif // !HASH_SHIFT_MULTIPLY
// 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 DWORD
HashString(
const char* psz,
DWORD dwHash = 0)
{
// force compiler to use unsigned arithmetic
const unsigned char* upsz = (const unsigned char*) psz;
for ( ; *upsz != '\0'; ++upsz)
dwHash = HASH_MULTIPLY(dwHash) + *upsz;
return dwHash;
}
// --------------------------------------------------------
// Compute a hash value from an input string of any type, i.e.
// the input is just treated as a sequence of bytes.
// Based on a hash function originally proposed by J. Zobel.
// Author: Paul Larson, 1999, palarson@microsoft.com
// --------------------------------------------------------
inline DWORD
HashString2(
const char* pszInputKey, // ptr to input - any type is OK
DWORD dwHash = 314159269) // Initial seed for hash function
{
// Initialize dwHash to a reasonably large constant so very
// short keys won't get mapped to small values. Virtually any
// large odd constant will do.
const unsigned char* upsz = (const unsigned char*) pszInputKey;
for ( ; *upsz != '\0'; ++upsz)
dwHash ^= (dwHash << 11) + (dwHash << 5) + (dwHash >> 2) + *upsz;
return (dwHash & 0x7FFFFFFF);
}
// Unicode version of above
inline DWORD
HashString(
const wchar_t* pwsz,
DWORD dwHash = 0)
{
for ( ; *pwsz != L'\0'; ++pwsz)
dwHash = HASH_MULTIPLY(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 DWORD
HashStringNoCase(
const char* psz,
DWORD dwHash = 0)
{
const unsigned char* upsz = (const unsigned char*) psz;
for ( ; *upsz != '\0'; ++upsz)
dwHash = HASH_MULTIPLY(dwHash)
+ (*upsz & 0xDF); // strip off lowercase bit
return dwHash;
}
// Unicode version of above
inline DWORD
HashStringNoCase(
const wchar_t* pwsz,
DWORD dwHash = 0)
{
for ( ; *pwsz != L'\0'; ++pwsz)
dwHash = HASH_MULTIPLY(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,
// HASH_MULTIPLIER * Hash(rFoo.m_ch)
// + Hash(rFoo.m_d));
// }
//
// Again, apply HashScramble if using with something other than LKRhash.
inline DWORD
HashBlob(
const void* pv,
size_t cb,
DWORD dwHash = 0)
{
const BYTE* pb = static_cast<const BYTE*>(pv);
while (cb-- > 0)
dwHash = HASH_MULTIPLY(dwHash) + *pb++;
return dwHash;
}
// --------------------------------------------------------
// Compute a hash value from an input string of any type, i.e.
// the input is just treated as a sequence of bytes.
// Based on a hash function originally proposed by J. Zobel.
// Author: Paul Larson, 1999, palarson@microsoft.com
// --------------------------------------------------------
inline DWORD
HashBlob2(
const void* pvInputKey, // ptr to input - any type is OK
size_t cbKeyLen, // length of input key in bytes
DWORD dwHash = 314159269) // Initial seed for hash function
{
// Initialize dwHash to a reasonably large constant so very
// short keys won't get mapped to small values. Virtually any
// large odd constant will do.
const BYTE* pb = static_cast<const BYTE*>(pvInputKey);
const BYTE* pbSentinel = pb + cbKeyLen;
for ( ; pb < pbSentinel; ++pb)
dwHash ^= (dwHash << 11) + (dwHash << 5) + (dwHash >> 2) + *pb;
return (dwHash & 0x7FFFFFFF);
}
//
// 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 DWORD
Hash(
const GUID* pguid,
DWORD dwHash = 0)
{
dwHash += * reinterpret_cast<const DWORD*>(pguid);
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(double dbl)
{
if (dbl == 0.0)
return 0;
int nExponent;
double dblMantissa = frexp(dbl, &nExponent);
// 0.5 <= |mantissa| < 1.0
return (DWORD) ((2.0 * fabs(dblMantissa) - 1.0) * ULONG_MAX);
}
inline DWORD Hash(float f)
{ return Hash((double) f); }
inline DWORD Hash(unsigned __int64 ull)
{
union {
unsigned __int64 _ull;
DWORD dw[2];
} u = {ull};
return HASH_MULTIPLY(u.dw[0]) + u.dw[1];
}
inline DWORD Hash(__int64 ll)
{ return Hash((unsigned __int64) ll); }
#ifndef __HASHFN_NO_NAMESPACE__
}
#endif // !__HASHFN_NO_NAMESPACE__
#endif // __HASHFN_H__