Andrew's git - gitweb.git/blob

   1#include "cache.h"
   2#include "diff.h"
   3#include "diffcore.h"
   4
   5/*
   6 * Idea here is very simple.
   7 *
   8 * We have total of (sz-N+1) N-byte overlapping sequences in buf whose
   9 * size is sz.  If the same N-byte sequence appears in both source and
  10 * destination, we say the byte that starts that sequence is shared
  11 * between them (i.e. copied from source to destination).
  12 *
  13 * For each possible N-byte sequence, if the source buffer has more
  14 * instances of it than the destination buffer, that means the
  15 * difference are the number of bytes not copied from source to
  16 * destination.  If the counts are the same, everything was copied
  17 * from source to destination.  If the destination has more,
  18 * everything was copied, and destination added more.
  19 *
  20 * We are doing an approximation so we do not really have to waste
  21 * memory by actually storing the sequence.  We just hash them into
  22 * somewhere around 2^16 hashbuckets and count the occurrences.
  23 *
  24 * The length of the sequence is arbitrarily set to 8 for now.
  25 */
  26
  27#define HASHBASE 65537 /* next_prime(2^16) */
  28
  29static void hash_chars(unsigned char *buf, unsigned long sz, int *count)
  30{
  31        unsigned int accum1, accum2, i;
  32
  33        /* an 8-byte shift register made of accum1 and accum2.  New
  34         * bytes come at LSB of accum2, and shifted up to accum1
  35         */
  36        for (i = accum1 = accum2 = 0; i < 7; i++, sz--) {
  37                accum1 = (accum1 << 8) | (accum2 >> 24);
  38                accum2 = (accum2 << 8) | *buf++;
  39        }
  40        while (sz) {
  41                accum1 = (accum1 << 8) | (accum2 >> 24);
  42                accum2 = (accum2 << 8) | *buf++;
  43                /* We want something that hashes permuted byte
  44                 * sequences nicely; simpler hash like (accum1 ^
  45                 * accum2) does not perform as well.
  46                 */
  47                i = (accum1 + accum2 * 0x61) % HASHBASE;
  48                count[i]++;
  49                sz--;
  50        }
  51}
  52
  53int diffcore_count_changes(void *src, unsigned long src_size,
  54                           void *dst, unsigned long dst_size,
  55                           unsigned long delta_limit,
  56                           unsigned long *src_copied,
  57                           unsigned long *literal_added)
  58{
  59        int *src_count, *dst_count, i;
  60        unsigned long sc, la;
  61
  62        if (src_size < 8 || dst_size < 8)
  63                return -1;
  64
  65        src_count = xcalloc(HASHBASE * 2, sizeof(int));
  66        dst_count = src_count + HASHBASE;
  67        hash_chars(src, src_size, src_count);
  68        hash_chars(dst, dst_size, dst_count);
  69
  70        sc = la = 0;
  71        for (i = 0; i < HASHBASE; i++) {
  72                if (src_count[i] < dst_count[i]) {
  73                        la += dst_count[i] - src_count[i];
  74                        sc += src_count[i];
  75                }
  76                else /* i.e. if (dst_count[i] <= src_count[i]) */
  77                        sc += dst_count[i];
  78        }
  79        *src_copied = sc;
  80        *literal_added = la;
  81        free(src_count);
  82        return 0;
  83}