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 ```1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 151 152 153 154 155 156 157 158 159 160 161 162 163 164 165 166 167 168 169 170 171 172 173 174 175 176 177 178 179 180 181 182 183 184 185 186 187 188 189 190 191 192 193 194 195 196 197 198 199 200 201 202 203 204 205 206 207 208 209 210 211 212 213 214 ``` ``````#include "cache.h" #include "diff.h" #include "diffcore.h"   /* * Idea here is very simple. * * We have total of (sz-N+1) N-byte overlapping sequences in buf whose * size is sz. If the same N-byte sequence appears in both source and * destination, we say the byte that starts that sequence is shared * between them (i.e. copied from source to destination). * * For each possible N-byte sequence, 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. * * The length of the sequence is arbitrarily set to 8 for now. */   /* 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 long hashval; unsigned long cnt; }; struct spanhash_top { int alloc_log2; int free; struct spanhash data[FLEX_ARRAY]; };   static struct spanhash *spanhash_find(struct spanhash_top *top, unsigned long hashval) { int sz = 1 << top->alloc_log2; int bucket = hashval & (sz - 1); while (1) { struct spanhash *h = &(top->data[bucket++]); if (!h->cnt) return NULL; if (h->hashval == hashval) return h; if (sz <= bucket) bucket = 0; } }   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 long hashval) { 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 = 1; top->free--; if (top->free < 0) return spanhash_rehash(top); return top; } if (h->hashval == hashval) { h->cnt++; return top; } if (lim <= bucket) bucket = 0; } }   static struct spanhash_top *hash_chars(unsigned char *buf, unsigned long sz) { int i; unsigned long accum1, accum2, hashval; struct spanhash_top *hash;   i = INITIAL_HASH_SIZE; hash = xmalloc(sizeof(*hash) + sizeof(struct spanhash) * (1<alloc_log2 = i; hash->free = INITIAL_FREE(i); memset(hash->data, 0, sizeof(struct spanhash) * (1<> 24); accum2 = (accum2 << 8) | *buf++; } while (sz) { accum1 = (accum1 << 8) | (accum2 >> 24); accum2 = (accum2 << 8) | *buf++; hashval = (accum1 + accum2 * 0x61) % HASHBASE; hash = add_spanhash(hash, hashval); sz--; } return hash; }   int diffcore_count_changes(void *src, unsigned long src_size, void *dst, unsigned long dst_size, void **src_count_p, void **dst_count_p, unsigned long delta_limit, unsigned long *src_copied, unsigned long *literal_added) { int i, ssz; struct spanhash_top *src_count, *dst_count; unsigned long sc, la;   if (src_size < 8 || dst_size < 8) return -1;   src_count = dst_count = NULL; if (src_count_p) src_count = *src_count_p; if (!src_count) { src_count = hash_chars(src, src_size); 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, dst_size); if (dst_count_p) *dst_count_p = dst_count; } sc = la = 0;   ssz = 1 << src_count->alloc_log2; for (i = 0; i < ssz; i++) { struct spanhash *s = &(src_count->data[i]); struct spanhash *d; unsigned dst_cnt, src_cnt; if (!s->cnt) continue; src_cnt = s->cnt; d = spanhash_find(dst_count, s->hashval); dst_cnt = d ? d->cnt : 0; if (src_cnt < dst_cnt) { la += dst_cnt - src_cnt; sc += src_cnt; } else sc += dst_cnt; }   if (!src_count_p) free(src_count); if (!dst_count_p) free(dst_count); *src_copied = sc; *literal_added = la; return 0; }  ``````