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![Linus Torvalds](/assets/img/avatar_default.png)
The rotating 64-bit number was not really rotating, and worse yet ulong was longer than 64-bit on 64-bit architectures X-<. Signed-off-by: Junio C Hamano <junkio@cox.net>
214 lines
5.1 KiB
C
214 lines
5.1 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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* We have total of (sz-N+1) N-byte overlapping sequences in buf whose
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* size is sz. If the same N-byte sequence appears in both source and
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* destination, we say the byte that starts that sequence is shared
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* between them (i.e. copied from source to destination).
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*
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* For each possible N-byte sequence, if the source buffer has more
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* instances of it than the destination buffer, that means the
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* difference are the number of bytes not copied from source to
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* destination. If the counts are the same, everything was copied
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* from source to destination. If the destination has more,
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* everything was copied, 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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* The length of the sequence is arbitrarily set to 8 for now.
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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 *spanhash_find(struct spanhash_top *top,
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unsigned int hashval)
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{
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int sz = 1 << top->alloc_log2;
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int bucket = hashval & (sz - 1);
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while (1) {
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struct spanhash *h = &(top->data[bucket++]);
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if (!h->cnt)
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return NULL;
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if (h->hashval == hashval)
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return h;
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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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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(sizeof(*orig) + 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 struct spanhash_top *hash_chars(unsigned char *buf, unsigned int sz)
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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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i = INITIAL_HASH_SIZE;
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hash = xmalloc(sizeof(*hash) + 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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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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return hash;
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}
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int diffcore_count_changes(void *src, unsigned long src_size,
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void *dst, unsigned long dst_size,
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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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int i, ssz;
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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, src_size);
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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, dst_size);
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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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ssz = 1 << src_count->alloc_log2;
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for (i = 0; i < ssz; i++) {
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struct spanhash *s = &(src_count->data[i]);
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struct spanhash *d;
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unsigned dst_cnt, src_cnt;
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if (!s->cnt)
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continue;
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src_cnt = s->cnt;
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d = spanhash_find(dst_count, s->hashval);
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dst_cnt = d ? d->cnt : 0;
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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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}
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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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