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  * We have total of (sz-N+1) N-byte overlapping sequences in buf whose
   9  * size is sz.  If the same N-byte sequence appears in both source and
  10  * destination, we say the byte that starts that sequence is shared
  11  * between them (i.e. copied from source to destination).
  12  *
  13  * For each possible N-byte sequence, if the source buffer has more
  14  * instances of it than the destination buffer, that means the
  15  * difference are the number of bytes not copied from source to
  16  * destination.  If the counts are the same, everything was copied
  17  * from source to destination.  If the destination has more,
  18  * everything was copied, and destination added more.
  19  *
  20  * We are doing an approximation so we do not really have to waste
  21  * memory by actually storing the sequence.  We just hash them into
  22  * somewhere around 2^16 hashbuckets and count the occurrences.
  23  *
  24  * The length of the sequence is arbitrarily set to 8 for now.
  25  */
  26
  27 /* Wild guess at the initial hash size */
  28 #define INITIAL_HASH_SIZE 10
  29 #define HASHBASE 65537 /* next_prime(2^16) */
  30
  31 struct spanhash {
  32         unsigned long hashval;
  33         unsigned long cnt;
  34 };
  35 struct spanhash_top {
  36         int alloc_log2;
  37         int free;
  38         struct spanhash data[FLEX_ARRAY];
  39 };
  40
  41 static struct spanhash *spanhash_find(struct spanhash_top *top, unsigned long hashval)
  42 {
  43         int sz = 1 << top->alloc_log2;
  44         int bucket = hashval & (sz - 1);
  45         while (1) {
  46                 struct spanhash *h = &(top->data[bucket++]);
  47                 if (!h->cnt)
  48                         return NULL;
  49                 if (h->hashval == hashval)
  50                         return h;
  51                 if (sz <= bucket)
  52                         bucket = 0;
  53         }
  54 }
  55
  56 static struct spanhash_top *spanhash_rehash(struct spanhash_top *orig)
  57 {
  58         struct spanhash_top *new;
  59         int i;
  60         int osz = 1 << orig->alloc_log2;
  61         int sz = osz << 1;
  62
  63         new = xmalloc(sizeof(*orig) + sizeof(struct spanhash) * sz);
  64         new->alloc_log2 = orig->alloc_log2 + 1;
  65         new->free = osz;
  66         memset(new->data, 0, sizeof(struct spanhash) * sz);
  67         for (i = 0; i < osz; i++) {
  68                 struct spanhash *o = &(orig->data[i]);
  69                 int bucket;
  70                 if (!o->cnt)
  71                         continue;
  72                 bucket = o->hashval & (sz - 1);
  73                 while (1) {
  74                         struct spanhash *h = &(new->data[bucket++]);
  75                         if (!h->cnt) {
  76                                 h->hashval = o->hashval;
  77                                 h->cnt = o->cnt;
  78                                 new->free--;
  79                                 break;
  80                         }
  81                         if (sz <= bucket)
  82                                 bucket = 0;
  83                 }
  84         }
  85         free(orig);
  86         return new;
  87 }
  88
  89 static struct spanhash_top *add_spanhash(struct spanhash_top *top,
  90                                          unsigned long hashval)
  91 {
  92         int bucket, lim;
  93         struct spanhash *h;
  94
  95         lim = (1 << top->alloc_log2);
  96         bucket = hashval & (lim - 1);
  97         while (1) {
  98                 h = &(top->data[bucket++]);
  99                 if (!h->cnt) {
 100                         h->hashval = hashval;
 101                         h->cnt = 1;
 102                         top->free--;
 103                         if (top->free < 0)
 104                                 return spanhash_rehash(top);
 105                         return top;
 106                 }
 107                 if (h->hashval == hashval) {
 108                         h->cnt++;
 109                         return top;
 110                 }
 111                 if (lim <= bucket)
 112                         bucket = 0;
 113         }
 114 }
 115
 116 static struct spanhash_top *hash_chars(unsigned char *buf, unsigned long sz)
 117 {
 118         int i;
 119         unsigned long accum1, accum2, hashval;
 120         struct spanhash_top *hash;
 121
 122         i = INITIAL_HASH_SIZE;
 123         hash = xmalloc(sizeof(*hash) + sizeof(struct spanhash) * (1<<i));
 124         hash->alloc_log2 = i;
 125         hash->free = (1<<i)/2;
 126         memset(hash->data, 0, sizeof(struct spanhash) * (1<<i));
 127
 128         /* an 8-byte shift register made of accum1 and accum2.  New
 129          * bytes come at LSB of accum2, and shifted up to accum1
 130          */
 131         for (i = accum1 = accum2 = 0; i < 7; i++, sz--) {
 132                 accum1 = (accum1 << 8) | (accum2 >> 24);
 133                 accum2 = (accum2 << 8) | *buf++;
 134         }
 135         while (sz) {
 136                 accum1 = (accum1 << 8) | (accum2 >> 24);
 137                 accum2 = (accum2 << 8) | *buf++;
 138                 hashval = (accum1 + accum2 * 0x61) % HASHBASE;
 139                 hash = add_spanhash(hash, hashval);
 140                 sz--;
 141         }
 142         return hash;
 143 }
 144
 145 int diffcore_count_changes(void *src, unsigned long src_size,
 146                            void *dst, unsigned long dst_size,
 147                            void **src_count_p,
 148                            void **dst_count_p,
 149                            unsigned long delta_limit,
 150                            unsigned long *src_copied,
 151                            unsigned long *literal_added)
 152 {
 153         int i, ssz;
 154         struct spanhash_top *src_count, *dst_count;
 155         unsigned long sc, la;
 156
 157         if (src_size < 8 || dst_size < 8)
 158                 return -1;
 159
 160         src_count = dst_count = NULL;
 161         if (src_count_p)
 162                 src_count = *src_count_p;
 163         if (!src_count) {
 164                 src_count = hash_chars(src, src_size);
 165                 if (src_count_p)
 166                         *src_count_p = src_count;
 167         }
 168         if (dst_count_p)
 169                 dst_count = *dst_count_p;
 170         if (!dst_count) {
 171                 dst_count = hash_chars(dst, dst_size);
 172                 if (dst_count_p)
 173                         *dst_count_p = dst_count;
 174         }
 175         sc = la = 0;
 176
 177         ssz = 1 << src_count->alloc_log2;
 178         for (i = 0; i < ssz; i++) {
 179                 struct spanhash *s = &(src_count->data[i]);
 180                 struct spanhash *d;
 181                 unsigned dst_cnt, src_cnt;
 182                 if (!s->cnt)
 183                         continue;
 184                 src_cnt = s->cnt;
 185                 d = spanhash_find(dst_count, s->hashval);
 186                 dst_cnt = d ? d->cnt : 0;
 187                 if (src_cnt < dst_cnt) {
 188                         la += dst_cnt - src_cnt;
 189                         sc += src_cnt;
 190                 }
 191                 else
 192                         sc += dst_cnt;
 193         }
 194
 195         if (!src_count_p)
 196                 free(src_count);
 197         if (!dst_count_p)
 198                 free(dst_count);
 199         *src_copied = sc;
 200         *literal_added = la;
 201         return 0;
 202 }