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#include "cache.h"
#include "diff.h"
#include "diffcore.h"
/*
* Idea here is very simple.
*
* We have total of (sz-N+1) N-byte overlapping sequences in buf whose
* size is sz. If the same N-byte sequence appears in both source and
* destination, we say the byte that starts that sequence is shared
* between them (i.e. copied from source to destination).
*
* For each possible N-byte sequence, if the source buffer has more
* instances of it than the destination buffer, that means the
* difference are the number of bytes not copied from source to
* destination. If the counts are the same, everything was copied
* from source to destination. If the destination has more,
* everything was copied, and destination added more.
*
* We are doing an approximation so we do not really have to waste
* memory by actually storing the sequence. We just hash them into
* somewhere around 2^16 hashbuckets and count the occurrences.
*
* The length of the sequence is arbitrarily set to 8 for now.
*/
#define HASHBASE 65537 /* next_prime(2^16) */
static void hash_chars(unsigned char *buf, unsigned long sz, int *count)
{
unsigned int accum1, accum2, i;
/* an 8-byte shift register made of accum1 and accum2. New
* bytes come at LSB of accum2, and shifted up to accum1
*/
for (i = accum1 = accum2 = 0; i < 7; i++, sz--) {
accum1 = (accum1 << 8) | (accum2 >> 24);
accum2 = (accum2 << 8) | *buf++;
}
while (sz) {
accum1 = (accum1 << 8) | (accum2 >> 24);
accum2 = (accum2 << 8) | *buf++;
/* We want something that hashes permuted byte
* sequences nicely; simpler hash like (accum1 ^
* accum2) does not perform as well.
*/
i = (accum1 + accum2 * 0x61) % HASHBASE;
count[i]++;
sz--;
}
}
int diffcore_count_changes(void *src, unsigned long src_size,
void *dst, unsigned long dst_size,
unsigned long delta_limit,
unsigned long *src_copied,
unsigned long *literal_added)
{
int *src_count, *dst_count, i;
unsigned long sc, la;
if (src_size < 8 || dst_size < 8)
return -1;
src_count = xcalloc(HASHBASE * 2, sizeof(int));
dst_count = src_count + HASHBASE;
hash_chars(src, src_size, src_count);
hash_chars(dst, dst_size, dst_count);
sc = la = 0;
for (i = 0; i < HASHBASE; i++) {
if (src_count[i] < dst_count[i]) {
la += dst_count[i] - src_count[i];
sc += src_count[i];
}
else /* i.e. if (dst_count[i] <= src_count[i]) */
sc += dst_count[i];
}
*src_copied = sc;
*literal_added = la;
free(src_count);
return 0;
}
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