/*
 * Copyright 2011 Avery Pennarun. All rights reserved.
 *
 * (This license applies to bupsplit.c and bupsplit.h only.)
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions are
 * met:
 *
 *    1. Redistributions of source code must retain the above copyright
 *       notice, this list of conditions and the following disclaimer.
 *
 *    2. Redistributions in binary form must reproduce the above copyright
 *       notice, this list of conditions and the following disclaimer in
 *       the documentation and/or other materials provided with the
 *       distribution.
 *
 * THIS SOFTWARE IS PROVIDED BY AVERY PENNARUN ``AS IS'' AND ANY
 * EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
 * PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL <COPYRIGHT HOLDER> OR
 * CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL,
 * EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,
 * PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR
 * PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF
 * LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING
 * NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
 * SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 */
#include "bupsplit.h"
#include <memory.h>
#include <stdint.h>
#include <stdio.h>
#include <stdlib.h>

// According to librsync/rollsum.h:
// "We should make this something other than zero to improve the
// checksum algorithm: tridge suggests a prime number."
// apenwarr: I unscientifically tried 0 and 7919, and they both ended up
// slightly worse than the librsync value of 31 for my arbitrary test data.
#define ROLLSUM_CHAR_OFFSET 31

typedef struct
{
  unsigned s1, s2;
  uint8_t window[BUP_WINDOWSIZE];
  int wofs;
} Rollsum;

// These formulas are based on rollsum.h in the librsync project.
static void
rollsum_add (Rollsum *r, uint8_t drop, uint8_t add)
{
  r->s1 += add - drop;
  r->s2 += r->s1 - (BUP_WINDOWSIZE * (drop + ROLLSUM_CHAR_OFFSET));
}

static void
rollsum_init (Rollsum *r)
{
  r->s1 = BUP_WINDOWSIZE * ROLLSUM_CHAR_OFFSET;
  r->s2 = BUP_WINDOWSIZE * (BUP_WINDOWSIZE - 1) * ROLLSUM_CHAR_OFFSET;
  r->wofs = 0;
  memset (r->window, 0, BUP_WINDOWSIZE);
}

// For some reason, gcc 4.3 (at least) optimizes badly if find_ofs()
// is static and rollsum_roll is an inline function.  Let's use a macro
// here instead to help out the optimizer.
#define rollsum_roll(r, ch) \
  do \
    { \
      rollsum_add ((r), (r)->window[(r)->wofs], (ch)); \
      (r)->window[(r)->wofs] = (ch); \
      (r)->wofs = ((r)->wofs + 1) % BUP_WINDOWSIZE; \
    } \
  while (0)

static uint32_t
rollsum_digest (Rollsum *r)
{
  return (r->s1 << 16) | (r->s2 & 0xffff);
}

uint32_t
bupsplit_sum (uint8_t *buf, size_t ofs, size_t len)
{
  size_t count;
  Rollsum r;
  rollsum_init (&r);
  for (count = ofs; count < len; count++)
    rollsum_roll (&r, buf[count]);
  return rollsum_digest (&r);
}

int
bupsplit_find_ofs (const unsigned char *buf, int len, int *bits)
{
  Rollsum r;
  int count;

  rollsum_init (&r);
  for (count = 0; count < len; count++)
    {
      rollsum_roll (&r, buf[count]);
      if ((r.s2 & (BUP_BLOBSIZE - 1)) == ((~0) & (BUP_BLOBSIZE - 1)))
        {
          if (bits)
            {
              unsigned rsum = rollsum_digest (&r);
              *bits = BUP_BLOBBITS;
              rsum >>= BUP_BLOBBITS;
              for (*bits = BUP_BLOBBITS; (rsum >>= 1) & 1; (*bits)++)
                ;
            }
          return count + 1;
        }
    }
  return 0;
}