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65416758 JH |
1 | #include "cache.h" |
2 | #include "diff.h" | |
3 | #include "diffcore.h" | |
ba23bbc8 JH |
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 JH |
27 | /* Wild guess at the initial hash size */ |
28 | #define INITIAL_HASH_SIZE 10 | |
ba23bbc8 JH |
29 | #define HASHBASE 65537 /* next_prime(2^16) */ |
30 | ||
c06c7966 JH |
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) | |
65416758 | 117 | { |
c06c7966 JH |
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)); | |
65416758 | 127 | |
ba23bbc8 JH |
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++; | |
c06c7966 JH |
138 | hashval = (accum1 + accum2 * 0x61) % HASHBASE; |
139 | hash = add_spanhash(hash, hashval); | |
ba23bbc8 | 140 | sz--; |
65416758 | 141 | } |
c06c7966 | 142 | return hash; |
65416758 JH |
143 | } |
144 | ||
145 | int diffcore_count_changes(void *src, unsigned long src_size, | |
146 | void *dst, unsigned long dst_size, | |
c06c7966 JH |
147 | void **src_count_p, |
148 | void **dst_count_p, | |
65416758 JH |
149 | unsigned long delta_limit, |
150 | unsigned long *src_copied, | |
151 | unsigned long *literal_added) | |
152 | { | |
c06c7966 JH |
153 | int i, ssz; |
154 | struct spanhash_top *src_count, *dst_count; | |
ba23bbc8 JH |
155 | unsigned long sc, la; |
156 | ||
157 | if (src_size < 8 || dst_size < 8) | |
158 | return -1; | |
159 | ||
c06c7966 JH |
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 | } | |
ba23bbc8 | 175 | sc = la = 0; |
c06c7966 JH |
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; | |
ba23bbc8 | 190 | } |
c06c7966 JH |
191 | else |
192 | sc += dst_cnt; | |
ba23bbc8 | 193 | } |
c06c7966 JH |
194 | |
195 | if (!src_count_p) | |
196 | free(src_count); | |
197 | if (!dst_count_p) | |
198 | free(dst_count); | |
ba23bbc8 JH |
199 | *src_copied = sc; |
200 | *literal_added = la; | |
ba23bbc8 | 201 | return 0; |
65416758 | 202 | } |