hash.cc

早期freebsd实现

/* 
Copyright (C) 1990 Free Software Foundation
    written by Doug Lea ([e-mail])

This file is part of the GNU C++ Library.  This library is free
software; you can redistribute it and/or modify it under the terms of
the GNU Library General Public License as published by the Free
Software Foundation; either version 2 of the License, or (at your
option) any later version.  This library is distributed in the hope
that it will be useful, but WITHOUT ANY WARRANTY; without even the
implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR
PURPOSE.  See the GNU Library General Public License for more details.
You should have received a copy of the GNU Library General Public
License along with this library; if not, write to the Free Software
Foundation, 675 Mass Ave, Cambridge, MA 02139, USA.
*/

#ifdef __GNUG__
#pragma implementation
#endif
#include <builtin.h>

/*
 some useful hash functions
*/

unsigned int hashpjw(const char* x) // From Dragon book, p436
{
  unsigned int h = 0;
  unsigned int g;

  while (*x != 0)
  {
    h = (h << 4) + *x++;
    if ((g = h & 0xf0000000) != 0)
      h = (h ^ (g >> 24)) ^ g;
  }
  return h;
}

unsigned int multiplicativehash(int x)
{
  // uses a const close to golden ratio * pow(2,32)
  return ((unsigned)x) * 2654435767;
}


unsigned int foldhash(double x)
{
  union { unsigned int i[2]; double d; } u;
  u.d = x;
  unsigned int u0 = u.i[0];
  unsigned int u1 = u.i[1]; 
  return u0 ^ u1;
}