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1#include "cache.h"
2#include "diff.h"
3#include "diffcore.h"
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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)
65416758 30{
ba23bbc8 31 unsigned int accum1, accum2, i;
65416758 32
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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--;
65416758 50 }
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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,
58{
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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;