root/diffcore-delta.c

DEFINITIONS

1. spanhash_rehash
2. add_spanhash
3. spanhash_cmp
4. hash_chars
5. diffcore_count_changes

#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;
                        la += d->cnt;
                        d++;
                }
                src_cnt = s->cnt;
                dst_cnt = 0;
                if (d->cnt && d->hashval == s->hashval) {
                        dst_cnt = d->cnt;
                        d++;
                }
                if (src_cnt < dst_cnt) {
                        la += dst_cnt - src_cnt;
                        sc += src_cnt;
                }
                else
                        sc += dst_cnt;
                s++;
        }
        while (d->cnt) {
                la += d->cnt;
                d++;
        }

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