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![Jeff King](/assets/img/avatar_default.png)
If our size computation overflows size_t, we may allocate a much smaller buffer than we expected and overflow it. It's probably impossible to trigger an overflow in most of these sites in practice, but it is easy enough convert their additions and multiplications into overflow-checking variants. This may be fixing real bugs, and it makes auditing the code easier. Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>
236 lines
5.4 KiB
C
236 lines
5.4 KiB
C
#include "cache.h"
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#include "diff.h"
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#include "diffcore.h"
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/*
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* Idea here is very simple.
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*
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* Almost all data we are interested in are text, but sometimes we have
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* to deal with binary data. So we cut them into chunks delimited by
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* LF byte, or 64-byte sequence, whichever comes first, and hash them.
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*
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* For those chunks, if the source buffer has more instances of it
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* than the destination buffer, that means the difference are the
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* number of bytes not copied from source to destination. If the
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* counts are the same, everything was copied from source to
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* destination. If the destination has more, everything was copied,
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* and destination added more.
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*
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* We are doing an approximation so we do not really have to waste
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* memory by actually storing the sequence. We just hash them into
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* somewhere around 2^16 hashbuckets and count the occurrences.
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*/
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/* Wild guess at the initial hash size */
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#define INITIAL_HASH_SIZE 9
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/* We leave more room in smaller hash but do not let it
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* grow to have unused hole too much.
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*/
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#define INITIAL_FREE(sz_log2) ((1<<(sz_log2))*(sz_log2-3)/(sz_log2))
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/* A prime rather carefully chosen between 2^16..2^17, so that
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* HASHBASE < INITIAL_FREE(17). We want to keep the maximum hashtable
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* size under the current 2<<17 maximum, which can hold this many
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* different values before overflowing to hashtable of size 2<<18.
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*/
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#define HASHBASE 107927
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struct spanhash {
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unsigned int hashval;
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unsigned int cnt;
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};
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struct spanhash_top {
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int alloc_log2;
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int free;
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struct spanhash data[FLEX_ARRAY];
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};
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static struct spanhash_top *spanhash_rehash(struct spanhash_top *orig)
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{
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struct spanhash_top *new;
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int i;
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int osz = 1 << orig->alloc_log2;
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int sz = osz << 1;
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new = xmalloc(st_add(sizeof(*orig),
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st_mult(sizeof(struct spanhash), sz)));
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new->alloc_log2 = orig->alloc_log2 + 1;
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new->free = INITIAL_FREE(new->alloc_log2);
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memset(new->data, 0, sizeof(struct spanhash) * sz);
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for (i = 0; i < osz; i++) {
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struct spanhash *o = &(orig->data[i]);
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int bucket;
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if (!o->cnt)
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continue;
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bucket = o->hashval & (sz - 1);
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while (1) {
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struct spanhash *h = &(new->data[bucket++]);
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if (!h->cnt) {
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h->hashval = o->hashval;
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h->cnt = o->cnt;
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new->free--;
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break;
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}
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if (sz <= bucket)
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bucket = 0;
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}
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}
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free(orig);
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return new;
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}
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static struct spanhash_top *add_spanhash(struct spanhash_top *top,
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unsigned int hashval, int cnt)
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{
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int bucket, lim;
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struct spanhash *h;
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lim = (1 << top->alloc_log2);
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bucket = hashval & (lim - 1);
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while (1) {
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h = &(top->data[bucket++]);
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if (!h->cnt) {
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h->hashval = hashval;
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h->cnt = cnt;
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top->free--;
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if (top->free < 0)
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return spanhash_rehash(top);
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return top;
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}
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if (h->hashval == hashval) {
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h->cnt += cnt;
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return top;
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}
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if (lim <= bucket)
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bucket = 0;
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}
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}
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static int spanhash_cmp(const void *a_, const void *b_)
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{
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const struct spanhash *a = a_;
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const struct spanhash *b = b_;
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/* A count of zero compares at the end.. */
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if (!a->cnt)
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return !b->cnt ? 0 : 1;
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if (!b->cnt)
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return -1;
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return a->hashval < b->hashval ? -1 :
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a->hashval > b->hashval ? 1 : 0;
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}
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static struct spanhash_top *hash_chars(struct diff_filespec *one)
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{
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int i, n;
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unsigned int accum1, accum2, hashval;
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struct spanhash_top *hash;
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unsigned char *buf = one->data;
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unsigned int sz = one->size;
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int is_text = !diff_filespec_is_binary(one);
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i = INITIAL_HASH_SIZE;
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hash = xmalloc(st_add(sizeof(*hash),
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st_mult(sizeof(struct spanhash), 1<<i)));
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hash->alloc_log2 = i;
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hash->free = INITIAL_FREE(i);
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memset(hash->data, 0, sizeof(struct spanhash) * (1<<i));
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n = 0;
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accum1 = accum2 = 0;
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while (sz) {
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unsigned int c = *buf++;
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unsigned int old_1 = accum1;
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sz--;
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/* Ignore CR in CRLF sequence if text */
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if (is_text && c == '\r' && sz && *buf == '\n')
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continue;
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accum1 = (accum1 << 7) ^ (accum2 >> 25);
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accum2 = (accum2 << 7) ^ (old_1 >> 25);
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accum1 += c;
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if (++n < 64 && c != '\n')
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continue;
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hashval = (accum1 + accum2 * 0x61) % HASHBASE;
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hash = add_spanhash(hash, hashval, n);
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n = 0;
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accum1 = accum2 = 0;
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}
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qsort(hash->data,
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1ul << hash->alloc_log2,
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sizeof(hash->data[0]),
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spanhash_cmp);
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return hash;
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}
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int diffcore_count_changes(struct diff_filespec *src,
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struct diff_filespec *dst,
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void **src_count_p,
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void **dst_count_p,
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unsigned long delta_limit,
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unsigned long *src_copied,
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unsigned long *literal_added)
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{
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struct spanhash *s, *d;
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struct spanhash_top *src_count, *dst_count;
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unsigned long sc, la;
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src_count = dst_count = NULL;
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if (src_count_p)
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src_count = *src_count_p;
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if (!src_count) {
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src_count = hash_chars(src);
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if (src_count_p)
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*src_count_p = src_count;
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}
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if (dst_count_p)
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dst_count = *dst_count_p;
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if (!dst_count) {
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dst_count = hash_chars(dst);
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if (dst_count_p)
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*dst_count_p = dst_count;
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}
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sc = la = 0;
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s = src_count->data;
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d = dst_count->data;
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for (;;) {
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unsigned dst_cnt, src_cnt;
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if (!s->cnt)
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break; /* we checked all in src */
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while (d->cnt) {
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if (d->hashval >= s->hashval)
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break;
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la += d->cnt;
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d++;
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}
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src_cnt = s->cnt;
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dst_cnt = 0;
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if (d->cnt && d->hashval == s->hashval) {
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dst_cnt = d->cnt;
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d++;
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}
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if (src_cnt < dst_cnt) {
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la += dst_cnt - src_cnt;
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sc += src_cnt;
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}
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else
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sc += dst_cnt;
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s++;
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}
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while (d->cnt) {
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la += d->cnt;
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d++;
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}
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if (!src_count_p)
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free(src_count);
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if (!dst_count_p)
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free(dst_count);
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*src_copied = sc;
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*literal_added = la;
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return 0;
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}
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