diffcore-delta: 64-byte-or-EOL ultrafast replacement.
[git/git.git] / diffcore-delta.c
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1#include "cache.h"
2#include "diff.h"
3#include "diffcore.h"
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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
c06c7966 27/* Wild guess at the initial hash size */
2821104d 28#define INITIAL_HASH_SIZE 9
fc66d213 29
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30/* We leave more room in smaller hash but do not let it
31 * grow to have unused hole too much.
32 */
33#define INITIAL_FREE(sz_log2) ((1<<(sz_log2))*(sz_log2-3)/(sz_log2))
ba23bbc8 34
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35/* A prime rather carefully chosen between 2^16..2^17, so that
36 * HASHBASE < INITIAL_FREE(17). We want to keep the maximum hashtable
37 * size under the current 2<<17 maximum, which can hold this many
38 * different values before overflowing to hashtable of size 2<<18.
39 */
40#define HASHBASE 107927
41
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42struct spanhash {
43 unsigned long hashval;
44 unsigned long cnt;
45};
46struct spanhash_top {
47 int alloc_log2;
48 int free;
49 struct spanhash data[FLEX_ARRAY];
50};
51
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52static struct spanhash *spanhash_find(struct spanhash_top *top,
53 unsigned long hashval)
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54{
55 int sz = 1 << top->alloc_log2;
56 int bucket = hashval & (sz - 1);
57 while (1) {
58 struct spanhash *h = &(top->data[bucket++]);
59 if (!h->cnt)
60 return NULL;
61 if (h->hashval == hashval)
62 return h;
63 if (sz <= bucket)
64 bucket = 0;
65 }
66}
67
68static struct spanhash_top *spanhash_rehash(struct spanhash_top *orig)
69{
70 struct spanhash_top *new;
71 int i;
72 int osz = 1 << orig->alloc_log2;
73 int sz = osz << 1;
74
75 new = xmalloc(sizeof(*orig) + sizeof(struct spanhash) * sz);
76 new->alloc_log2 = orig->alloc_log2 + 1;
2821104d 77 new->free = INITIAL_FREE(new->alloc_log2);
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78 memset(new->data, 0, sizeof(struct spanhash) * sz);
79 for (i = 0; i < osz; i++) {
80 struct spanhash *o = &(orig->data[i]);
81 int bucket;
82 if (!o->cnt)
83 continue;
84 bucket = o->hashval & (sz - 1);
85 while (1) {
86 struct spanhash *h = &(new->data[bucket++]);
87 if (!h->cnt) {
88 h->hashval = o->hashval;
89 h->cnt = o->cnt;
90 new->free--;
91 break;
92 }
93 if (sz <= bucket)
94 bucket = 0;
95 }
96 }
97 free(orig);
98 return new;
99}
100
101static struct spanhash_top *add_spanhash(struct spanhash_top *top,
3c7ceba4 102 unsigned long hashval, int cnt)
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103{
104 int bucket, lim;
105 struct spanhash *h;
106
107 lim = (1 << top->alloc_log2);
108 bucket = hashval & (lim - 1);
109 while (1) {
110 h = &(top->data[bucket++]);
111 if (!h->cnt) {
112 h->hashval = hashval;
3c7ceba4 113 h->cnt = cnt;
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114 top->free--;
115 if (top->free < 0)
116 return spanhash_rehash(top);
117 return top;
118 }
119 if (h->hashval == hashval) {
3c7ceba4 120 h->cnt += cnt;
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121 return top;
122 }
123 if (lim <= bucket)
124 bucket = 0;
125 }
126}
127
128static struct spanhash_top *hash_chars(unsigned char *buf, unsigned long sz)
65416758 129{
3c7ceba4 130 int i, n;
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131 unsigned long accum1, accum2, hashval;
132 struct spanhash_top *hash;
133
134 i = INITIAL_HASH_SIZE;
135 hash = xmalloc(sizeof(*hash) + sizeof(struct spanhash) * (1<<i));
136 hash->alloc_log2 = i;
2821104d 137 hash->free = INITIAL_FREE(i);
c06c7966 138 memset(hash->data, 0, sizeof(struct spanhash) * (1<<i));
65416758 139
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LT
140 n = 0;
141 accum1 = accum2 = 0;
ba23bbc8 142 while (sz) {
3c7ceba4 143 unsigned long c = *buf++;
ba23bbc8 144 sz--;
3c7ceba4
LT
145 accum1 = (accum1 << 7) | (accum2 >> 25);
146 accum2 = (accum2 << 7) | (accum1 >> 25);
147 accum1 += c;
148 if (++n < 64 && c != '\n')
149 continue;
150 hashval = (accum1 + accum2 * 0x61) % HASHBASE;
151 hash = add_spanhash(hash, hashval, n);
152 n = 0;
153 accum1 = accum2 = 0;
65416758 154 }
c06c7966 155 return hash;
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156}
157
158int diffcore_count_changes(void *src, unsigned long src_size,
159 void *dst, unsigned long dst_size,
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160 void **src_count_p,
161 void **dst_count_p,
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162 unsigned long delta_limit,
163 unsigned long *src_copied,
164 unsigned long *literal_added)
165{
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166 int i, ssz;
167 struct spanhash_top *src_count, *dst_count;
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168 unsigned long sc, la;
169
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170 src_count = dst_count = NULL;
171 if (src_count_p)
172 src_count = *src_count_p;
173 if (!src_count) {
174 src_count = hash_chars(src, src_size);
175 if (src_count_p)
176 *src_count_p = src_count;
177 }
178 if (dst_count_p)
179 dst_count = *dst_count_p;
180 if (!dst_count) {
181 dst_count = hash_chars(dst, dst_size);
182 if (dst_count_p)
183 *dst_count_p = dst_count;
184 }
ba23bbc8 185 sc = la = 0;
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186
187 ssz = 1 << src_count->alloc_log2;
188 for (i = 0; i < ssz; i++) {
189 struct spanhash *s = &(src_count->data[i]);
190 struct spanhash *d;
191 unsigned dst_cnt, src_cnt;
192 if (!s->cnt)
193 continue;
194 src_cnt = s->cnt;
195 d = spanhash_find(dst_count, s->hashval);
196 dst_cnt = d ? d->cnt : 0;
197 if (src_cnt < dst_cnt) {
198 la += dst_cnt - src_cnt;
199 sc += src_cnt;
ba23bbc8 200 }
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201 else
202 sc += dst_cnt;
ba23bbc8 203 }
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204
205 if (!src_count_p)
206 free(src_count);
207 if (!dst_count_p)
208 free(dst_count);
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209 *src_copied = sc;
210 *literal_added = la;
ba23bbc8 211 return 0;
65416758 212}