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#include "cache.h"
#include "diff.h"
#include "diffcore.h"

/*
* Idea here is very simple.
*
* Almost all data we are interested in are text, but sometimes we have
* to deal with binary data. So we cut them into chunks delimited by
* LF byte, or 64-byte sequence, whichever comes first, and hash them.
*
* For those chunks, 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.
*/

/* 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_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 int spanhash_cmp(const void *a_, const void *b_)
{
const struct spanhash *a = a_;
const struct spanhash *b = b_;

/* A count of zero compares at the end.. */
if (!a->cnt)
return !b->cnt ? 0 : 1;
if (!b->cnt)
return -1;
return a->hashval < b->hashval ? -1 :
a->hashval > b->hashval ? 1 : 0;
}

static struct spanhash_top *hash_chars(struct diff_filespec *one)
{
int i, n;
unsigned int accum1, accum2, hashval;
struct spanhash_top *hash;
unsigned char *buf = one->data;
unsigned int sz = one->size;
int is_text = !diff_filespec_is_binary(one);

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--;

/* Ignore CR in CRLF sequence if text */
if (is_text && c == '\r' && sz && *buf == '\n')
continue;

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;
}
qsort(hash->data,
1ul << hash->alloc_log2,
sizeof(hash->data[0]),
spanhash_cmp);
return hash;
}

int diffcore_count_changes(struct diff_filespec *src,
struct diff_filespec *dst,
void **src_count_p,
void **dst_count_p,
unsigned long delta_limit,
unsigned long *src_copied,
unsigned long *literal_added)
{
struct spanhash *s, *d;
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);
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);
if (dst_count_p)
*dst_count_p = dst_count;
}
sc = la = 0;

s = src_count->data;
d = dst_count->data;
for (;;) {
unsigned dst_cnt, src_cnt;
if (!s->cnt)
break; /* we checked all in src */
while (d->cnt) {
if (d->hashval >= s->hashval)
break;
d++;
}
src_cnt = s->cnt;
dst_cnt = d->hashval == s->hashval ? d->cnt : 0;
if (src_cnt < dst_cnt) {
la += dst_cnt - src_cnt;
sc += src_cnt;
}
else
sc += dst_cnt;
s++;
}

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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