Merge branch 'jc/diff' into next
[git] / 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 9
29
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))
34
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
42 struct spanhash {
43         unsigned long hashval;
44         unsigned long cnt;
45 };
46 struct spanhash_top {
47         int alloc_log2;
48         int free;
49         struct spanhash data[FLEX_ARRAY];
50 };
51
52 static struct spanhash *spanhash_find(struct spanhash_top *top,
53                                       unsigned long hashval)
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
68 static 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;
77         new->free = INITIAL_FREE(new->alloc_log2);
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
101 static struct spanhash_top *add_spanhash(struct spanhash_top *top,
102                                          unsigned long hashval)
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;
113                         h->cnt = 1;
114                         top->free--;
115                         if (top->free < 0)
116                                 return spanhash_rehash(top);
117                         return top;
118                 }
119                 if (h->hashval == hashval) {
120                         h->cnt++;
121                         return top;
122                 }
123                 if (lim <= bucket)
124                         bucket = 0;
125         }
126 }
127
128 static struct spanhash_top *hash_chars(unsigned char *buf, unsigned long sz)
129 {
130         int i;
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;
137         hash->free = INITIAL_FREE(i);
138         memset(hash->data, 0, sizeof(struct spanhash) * (1<<i));
139
140         /* an 8-byte shift register made of accum1 and accum2.  New
141          * bytes come at LSB of accum2, and shifted up to accum1
142          */
143         for (i = accum1 = accum2 = 0; i < 7; i++, sz--) {
144                 accum1 = (accum1 << 8) | (accum2 >> 24);
145                 accum2 = (accum2 << 8) | *buf++;
146         }
147         while (sz) {
148                 accum1 = (accum1 << 8) | (accum2 >> 24);
149                 accum2 = (accum2 << 8) | *buf++;
150                 hashval = (accum1 + accum2 * 0x61) % HASHBASE;
151                 hash = add_spanhash(hash, hashval);
152                 sz--;
153         }
154         return hash;
155 }
156
157 int diffcore_count_changes(void *src, unsigned long src_size,
158                            void *dst, unsigned long dst_size,
159                            void **src_count_p,
160                            void **dst_count_p,
161                            unsigned long delta_limit,
162                            unsigned long *src_copied,
163                            unsigned long *literal_added)
164 {
165         int i, ssz;
166         struct spanhash_top *src_count, *dst_count;
167         unsigned long sc, la;
168
169         if (src_size < 8 || dst_size < 8)
170                 return -1;
171
172         src_count = dst_count = NULL;
173         if (src_count_p)
174                 src_count = *src_count_p;
175         if (!src_count) {
176                 src_count = hash_chars(src, src_size);
177                 if (src_count_p)
178                         *src_count_p = src_count;
179         }
180         if (dst_count_p)
181                 dst_count = *dst_count_p;
182         if (!dst_count) {
183                 dst_count = hash_chars(dst, dst_size);
184                 if (dst_count_p)
185                         *dst_count_p = dst_count;
186         }
187         sc = la = 0;
188
189         ssz = 1 << src_count->alloc_log2;
190         for (i = 0; i < ssz; i++) {
191                 struct spanhash *s = &(src_count->data[i]);
192                 struct spanhash *d;
193                 unsigned dst_cnt, src_cnt;
194                 if (!s->cnt)
195                         continue;
196                 src_cnt = s->cnt;
197                 d = spanhash_find(dst_count, s->hashval);
198                 dst_cnt = d ? d->cnt : 0;
199                 if (src_cnt < dst_cnt) {
200                         la += dst_cnt - src_cnt;
201                         sc += src_cnt;
202                 }
203                 else
204                         sc += dst_cnt;
205         }
206
207         if (!src_count_p)
208                 free(src_count);
209         if (!dst_count_p)
210                 free(dst_count);
211         *src_copied = sc;
212         *literal_added = la;
213         return 0;
214 }