Declare and document @conn_fd@.

[mLib] / unihash.c

/* -*-c-*-
 *
 * $Id: unihash.c,v 1.1 2003/10/12 14:43:24 mdw Exp $
 *
 * Simple and efficient universal hashing for hashtables
 *
 * (c) 2003 Straylight/Edgeware
 */

/*----- Licensing notice --------------------------------------------------* 
 *
 * This file is part of the mLib utilities library.
 *
 * mLib 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.
 * 
 * mLib 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 mLib; if not, write to the Free
 * Software Foundation, Inc., 59 Temple Place - Suite 330, Boston,
 * MA 02111-1307, USA.
 */

/*----- Revision history --------------------------------------------------* 
 *
 * $Log: unihash.c,v $
 * Revision 1.1  2003/10/12 14:43:24  mdw
 * Universal hashing.
 *
 */

/*----- Header files ------------------------------------------------------*/

#include <assert.h>
#include <stdlib.h>

#include "unihash.h"

/*----- Main code ---------------------------------------------------------*/

/* --- @unihash_setkey@ --- *
 *
 * Arguments:	@unihash_info *i@ = where to store the precomputed tables
 *		@uint32 k@ = the key to set, randomly chosen
 *
 * Returns:	---
 *
 * Use:		Calculates the tables required for efficient hashing.
 */

static uint32 mul(uint32 x, uint32 y)
{
  uint32 z = 0;
  while (y) {
    if (y & 1) z ^= x;
    if (x & (1 << 31))
      x = U32(x << 1) ^ UNIHASH_POLY;
    else
      x = U32(x << 1);
    y = U32(y >> 1);
  }
  return (z);
}

void unihash_setkey(unihash_info *i, uint32 k)
{
  size_t a;
  size_t b;
  uint32 x = 1;

  for (a = 0; a < UNIHASH_NBATCH; a++) {
    x = mul(x, k);
    for (b = 0; b < 256; b++) {
      i->s[a][0][b] = mul(x, b <<  0);
      i->s[a][1][b] = mul(x, b <<  8);
      i->s[a][2][b] = mul(x, b << 16);
      i->s[a][3][b] = mul(x, b << 24);
    }
  }
}

/* --- @unihash_hash@ --- *
 *
 * Arguments:	@const unihash_info *i@ = pointer to precomputed table
 *		@uint32 a@ = @i->[0][0][1]@ or value from previous call
 *		@const void *p@ = pointer to data to hash
 *		@size_t sz@ = size of the data
 *
 * Returns:	---
 *
 * Use:		Hashes data.  Call this as many times as needed.  
 */

uint32 unihash_hash(const unihash_info *i, uint32 a,
		    const void *p, size_t sz)
{
  const octet *pp = p;

  assert(UNIHASH_NBATCH == 4);

#define FULLMULT(u, x)							\
  (i->s[u][0][U8((x) >>  0)] ^ i->s[u][1][U8((x) >>  8)] ^		\
   i->s[u][2][U8((x) >> 16)] ^ i->s[u][3][U8((x) >> 24)]);

#define BYTEMULT(u, x) i->s[u][0][x]

  /* --- Do the main bulk in batches of %$n$% bytes --- *
   *
   * We have %$a$% and %$m_{n-1}, \ldots, m_1, m_0$%; we want
   *
   * %$a' = (a + m_{n-1}) k^n + m_{n-2} k^{n-1} + \cdots + m_1 k^2 + m_0 k$%
   */

  while (sz >= UNIHASH_NBATCH) {
    a ^= *pp++;
    a = FULLMULT(3, a);
    a ^= BYTEMULT(2, *pp++);
    a ^= BYTEMULT(1, *pp++);
    a ^= BYTEMULT(0, *pp++);
  }

  /* --- The tail end is a smaller batch --- */

  switch (sz) {
    case  3: a ^= *pp++; a = FULLMULT(2, a); goto batch_2;
    case  2: a ^= *pp++; a = FULLMULT(1, a); goto batch_1;
    case  1: a ^= *pp++; a = FULLMULT(0, a); goto batch_0;
    batch_2: a ^= BYTEMULT(1, *pp++);
    batch_1: a ^= BYTEMULT(0, *pp++);
    batch_0: break;
  }

  return (a);
}

/* --- @unihash@ --- *
 *
 * Arguments:	@const unihash_info *i@ = precomputed tables
 *		@const void *p@ = pointer to data to hash
 *		@size_t sz@ = size of the data
 *
 * Returns:	The hash value computed.
 *
 * Use:		All-in-one hashing function.  No faster than using the
 * 		separate calls, but more convenient.
 */

uint32 unihash(const unihash_info *i, const void *p, size_t sz)
{
  return (UNIHASH(i, p, sz));
}

/*----- That's all, folks -------------------------------------------------*/