diffcore-delta.c

   1 #include "cache.h"
   2 #include "diff.h"
   3 #include "diffcore.h"
   4
   5 /*
   6  * Idea here is very simple.
   7  *
   8  * Almost all data we are interested in are text, but sometimes we have
   9  * to deal with binary data.  So we cut them into chunks delimited by
  10  * LF byte, or 64-byte sequence, whichever comes first, and hash them.
  11  *
  12  * For those chunks, if the source buffer has more instances of it
  13  * than the destination buffer, that means the difference are the
  14  * number of bytes not copied from source to destination.  If the
  15  * counts are the same, everything was copied from source to
  16  * destination.  If the destination has more, everything was copied,
  17  * and destination added more.
  18  *
  19  * We are doing an approximation so we do not really have to waste
  20  * memory by actually storing the sequence.  We just hash them into
  21  * somewhere around 2^16 hashbuckets and count the occurrences.
  22  */
  23
  24 /* Wild guess at the initial hash size */
  25 #define INITIAL_HASH_SIZE 9
  26
  27 /* We leave more room in smaller hash but do not let it
  28  * grow to have unused hole too much.
  29  */
  30 #define INITIAL_FREE(sz_log2) ((1<<(sz_log2))*(sz_log2-3)/(sz_log2))
  31
  32 /* A prime rather carefully chosen between 2^16..2^17, so that
  33  * HASHBASE < INITIAL_FREE(17).  We want to keep the maximum hashtable
  34  * size under the current 2<<17 maximum, which can hold this many
  35  * different values before overflowing to hashtable of size 2<<18.
  36  */
  37 #define HASHBASE 107927
  38
  39 struct spanhash {
  40         unsigned int hashval;
  41         unsigned int cnt;
  42 };
  43 struct spanhash_top {
  44         int alloc_log2;
  45         int free;
  46         struct spanhash data[FLEX_ARRAY];
  47 };
  48
  49 static struct spanhash_top *spanhash_rehash(struct spanhash_top *orig)
  50 {
  51         struct spanhash_top *new;
  52         int i;
  53         int osz = 1 << orig->alloc_log2;
  54         int sz = osz << 1;
  55
  56         new = xmalloc(sizeof(*orig) + sizeof(struct spanhash) * sz);
  57         new->alloc_log2 = orig->alloc_log2 + 1;
  58         new->free = INITIAL_FREE(new->alloc_log2);
  59         memset(new->data, 0, sizeof(struct spanhash) * sz);
  60         for (i = 0; i < osz; i++) {
  61                 struct spanhash *o = &(orig->data[i]);
  62                 int bucket;
  63                 if (!o->cnt)
  64                         continue;
  65                 bucket = o->hashval & (sz - 1);
  66                 while (1) {
  67                         struct spanhash *h = &(new->data[bucket++]);
  68                         if (!h->cnt) {
  69                                 h->hashval = o->hashval;
  70                                 h->cnt = o->cnt;
  71                                 new->free--;
  72                                 break;
  73                         }
  74                         if (sz <= bucket)
  75                                 bucket = 0;
  76                 }
  77         }
  78         free(orig);
  79         return new;
  80 }
  81
  82 static struct spanhash_top *add_spanhash(struct spanhash_top *top,
  83                                          unsigned int hashval, int cnt)
  84 {
  85         int bucket, lim;
  86         struct spanhash *h;
  87
  88         lim = (1 << top->alloc_log2);
  89         bucket = hashval & (lim - 1);
  90         while (1) {
  91                 h = &(top->data[bucket++]);
  92                 if (!h->cnt) {
  93                         h->hashval = hashval;
  94                         h->cnt = cnt;
  95                         top->free--;
  96                         if (top->free < 0)
  97                                 return spanhash_rehash(top);
  98                         return top;
  99                 }
 100                 if (h->hashval == hashval) {
 101                         h->cnt += cnt;
 102                         return top;
 103                 }
 104                 if (lim <= bucket)
 105                         bucket = 0;
 106         }
 107 }
 108
 109 static int spanhash_cmp(const void *a_, const void *b_)
 110 {
 111         const struct spanhash *a = a_;
 112         const struct spanhash *b = b_;
 113
 114         /* A count of zero compares at the end.. */
 115         if (!a->cnt)
 116                 return !b->cnt ? 0 : 1;
 117         if (!b->cnt)
 118                 return -1;
 119         return a->hashval < b->hashval ? -1 :
 120                 a->hashval > b->hashval ? 1 : 0;
 121 }
 122
 123 static struct spanhash_top *hash_chars(struct diff_filespec *one)
 124 {
 125         int i, n;
 126         unsigned int accum1, accum2, hashval;
 127         struct spanhash_top *hash;
 128         unsigned char *buf = one->data;
 129         unsigned int sz = one->size;
 130         int is_text = !diff_filespec_is_binary(one);
 131
 132         i = INITIAL_HASH_SIZE;
 133         hash = xmalloc(sizeof(*hash) + sizeof(struct spanhash) * (1<<i));
 134         hash->alloc_log2 = i;
 135         hash->free = INITIAL_FREE(i);
 136         memset(hash->data, 0, sizeof(struct spanhash) * (1<<i));
 137
 138         n = 0;
 139         accum1 = accum2 = 0;
 140         while (sz) {
 141                 unsigned int c = *buf++;
 142                 unsigned int old_1 = accum1;
 143                 sz--;
 144
 145                 /* Ignore CR in CRLF sequence if text */
 146                 if (is_text && c == '\r' && sz && *buf == '\n')
 147                         continue;
 148
 149                 accum1 = (accum1 << 7) ^ (accum2 >> 25);
 150                 accum2 = (accum2 << 7) ^ (old_1 >> 25);
 151                 accum1 += c;
 152                 if (++n < 64 && c != '\n')
 153                         continue;
 154                 hashval = (accum1 + accum2 * 0x61) % HASHBASE;
 155                 hash = add_spanhash(hash, hashval, n);
 156                 n = 0;
 157                 accum1 = accum2 = 0;
 158         }
 159         qsort(hash->data,
 160                 1ul << hash->alloc_log2,
 161                 sizeof(hash->data[0]),
 162                 spanhash_cmp);
 163         return hash;
 164 }
 165
 166 int diffcore_count_changes(struct diff_filespec *src,
 167                            struct diff_filespec *dst,
 168                            void **src_count_p,
 169                            void **dst_count_p,
 170                            unsigned long delta_limit,
 171                            unsigned long *src_copied,
 172                            unsigned long *literal_added)
 173 {
 174         struct spanhash *s, *d;
 175         struct spanhash_top *src_count, *dst_count;
 176         unsigned long sc, la;
 177
 178         src_count = dst_count = NULL;
 179         if (src_count_p)
 180                 src_count = *src_count_p;
 181         if (!src_count) {
 182                 src_count = hash_chars(src);
 183                 if (src_count_p)
 184                         *src_count_p = src_count;
 185         }
 186         if (dst_count_p)
 187                 dst_count = *dst_count_p;
 188         if (!dst_count) {
 189                 dst_count = hash_chars(dst);
 190                 if (dst_count_p)
 191                         *dst_count_p = dst_count;
 192         }
 193         sc = la = 0;
 194
 195         s = src_count->data;
 196         d = dst_count->data;
 197         for (;;) {
 198                 unsigned dst_cnt, src_cnt;
 199                 if (!s->cnt)
 200                         break; /* we checked all in src */
 201                 while (d->cnt) {
 202                         if (d->hashval >= s->hashval)
 203                                 break;
 204                         d++;
 205                 }
 206                 src_cnt = s->cnt;
 207                 dst_cnt = d->hashval == s->hashval ? d->cnt : 0;
 208                 if (src_cnt < dst_cnt) {
 209                         la += dst_cnt - src_cnt;
 210                         sc += src_cnt;
 211                 }
 212                 else
 213                         sc += dst_cnt;
 214                 s++;
 215         }
 216
 217         if (!src_count_p)
 218                 free(src_count);
 219         if (!dst_count_p)
 220                 free(dst_count);
 221         *src_copied = sc;
 222         *literal_added = la;
 223         return 0;
 224 }