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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.
*/

/* Wild guess at the initial hash size */
#define INITIAL_HASH_SIZE 9

/* We leave more room in smaller hash but do not let it
* grow to have unused hole too much.
*/
#define INITIAL_FREE(sz_log2) ((1<<(sz_log2))*(sz_log2-3)/(sz_log2))

/* A prime rather carefully chosen between 2^16..2^17, so that
* HASHBASE < INITIAL_FREE(17). We want to keep the maximum hashtable
* size under the current 2<<17 maximum, which can hold this many
* different values before overflowing to hashtable of size 2<<18.
*/
#define HASHBASE 107927

struct spanhash {
unsigned int hashval;
unsigned int cnt;
};
struct spanhash_top {
int alloc_log2;
int free;
struct spanhash data[FLEX_ARRAY];
};

static struct spanhash *spanhash_find(struct spanhash_top *top,
unsigned int hashval)
{
int sz = 1 << top->alloc_log2;
int bucket = hashval & (sz - 1);
while (1) {
struct spanhash *h = &(top->data[bucket++]);
if (!h->cnt)
return NULL;
if (h->hashval == hashval)
return h;
if (sz <= bucket)
bucket = 0;
}
}

static struct spanhash_top *spanhash_rehash(struct spanhash_top *orig)
{
struct spanhash_top *new;
int i;
int osz = 1 << orig->alloc_log2;
int sz = osz << 1;

new = xmalloc(sizeof(*orig) + sizeof(struct spanhash) * sz);
new->alloc_log2 = orig->alloc_log2 + 1;
new->free = INITIAL_FREE(new->alloc_log2);
memset(new->data, 0, sizeof(struct spanhash) * sz);
for (i = 0; i < osz; i++) {
struct spanhash *o = &(orig->data[i]);
int bucket;
if (!o->cnt)
continue;
bucket = o->hashval & (sz - 1);
while (1) {
struct spanhash *h = &(new->data[bucket++]);
if (!h->cnt) {
h->hashval = o->hashval;
h->cnt = o->cnt;
new->free--;
break;
}
if (sz <= bucket)
bucket = 0;
}
}
free(orig);
return new;
}

static struct spanhash_top *add_spanhash(struct spanhash_top *top,
unsigned int hashval, int cnt)
{
int bucket, lim;
struct spanhash *h;

lim = (1 << top->alloc_log2);
bucket = hashval & (lim - 1);
while (1) {
h = &(top->data[bucket++]);
if (!h->cnt) {
h->hashval = hashval;
h->cnt = cnt;
top->free--;
if (top->free < 0)
return spanhash_rehash(top);
return top;
}
if (h->hashval == hashval) {
h->cnt += cnt;
return top;
}
if (lim <= bucket)
bucket = 0;
}
}

static struct spanhash_top *hash_chars(unsigned char *buf, unsigned int sz)
{
int i, n;
unsigned int accum1, accum2, hashval;
struct spanhash_top *hash;

i = INITIAL_HASH_SIZE;
hash = xmalloc(sizeof(*hash) + sizeof(struct spanhash) * (1<<i));
hash->alloc_log2 = i;
hash->free = INITIAL_FREE(i);
memset(hash->data, 0, sizeof(struct spanhash) * (1<<i));

n = 0;
accum1 = accum2 = 0;
while (sz) {
unsigned int c = *buf++;
unsigned int old_1 = accum1;
sz--;
accum1 = (accum1 << 7) ^ (accum2 >> 25);
accum2 = (accum2 << 7) ^ (old_1 >> 25);
accum1 += c;
if (++n < 64 && c != '\n')
continue;
hashval = (accum1 + accum2 * 0x61) % HASHBASE;
hash = add_spanhash(hash, hashval, n);
n = 0;
accum1 = accum2 = 0;
}
return hash;
}

int diffcore_count_changes(void *src, unsigned long src_size,
void *dst, unsigned long dst_size,
void **src_count_p,
void **dst_count_p,
unsigned long delta_limit,
unsigned long *src_copied,
unsigned long *literal_added)
{
int i, ssz;
struct spanhash_top *src_count, *dst_count;
unsigned long sc, la;

src_count = dst_count = NULL;
if (src_count_p)
src_count = *src_count_p;
if (!src_count) {
src_count = hash_chars(src, src_size);
if (src_count_p)
*src_count_p = src_count;
}
if (dst_count_p)
dst_count = *dst_count_p;
if (!dst_count) {
dst_count = hash_chars(dst, dst_size);
if (dst_count_p)
*dst_count_p = dst_count;
}
sc = la = 0;

ssz = 1 << src_count->alloc_log2;
for (i = 0; i < ssz; i++) {
struct spanhash *s = &(src_count->data[i]);
struct spanhash *d;
unsigned dst_cnt, src_cnt;
if (!s->cnt)
continue;
src_cnt = s->cnt;
d = spanhash_find(dst_count, s->hashval);
dst_cnt = d ? d->cnt : 0;
if (src_cnt < dst_cnt) {
la += dst_cnt - src_cnt;
sc += src_cnt;
}
else
sc += dst_cnt;
}

if (!src_count_p)
free(src_count);
if (!dst_count_p)
free(dst_count);
*src_copied = sc;
*literal_added = la;
return 0;
}
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