GIT 1.5.3-rc4
[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  * Almost all data we are interested in are text, but sometimes we have
9  * to deal with binary data.  So we cut them into chunks delimited by
10  * LF byte, or 64-byte sequence, whichever comes first, and hash them.
11  *
12  * For those chunks, if the source buffer has more instances of it
13  * than the destination buffer, that means the difference are the
14  * number of bytes not copied from source to destination.  If the
15  * counts are the same, everything was copied from source to
16  * destination.  If the destination has more, everything was copied,
17  * and destination added more.
18  *
19  * We are doing an approximation so we do not really have to waste
20  * memory by actually storing the sequence.  We just hash them into
21  * somewhere around 2^16 hashbuckets and count the occurrences.
22  */
23
24 /* Wild guess at the initial hash size */
25 #define INITIAL_HASH_SIZE 9
26
27 /* We leave more room in smaller hash but do not let it
28  * grow to have unused hole too much.
29  */
30 #define INITIAL_FREE(sz_log2) ((1<<(sz_log2))*(sz_log2-3)/(sz_log2))
31
32 /* A prime rather carefully chosen between 2^16..2^17, so that
33  * HASHBASE < INITIAL_FREE(17).  We want to keep the maximum hashtable
34  * size under the current 2<<17 maximum, which can hold this many
35  * different values before overflowing to hashtable of size 2<<18.
36  */
37 #define HASHBASE 107927
38
39 struct spanhash {
40         unsigned int hashval;
41         unsigned int cnt;
42 };
43 struct spanhash_top {
44         int alloc_log2;
45         int free;
46         struct spanhash data[FLEX_ARRAY];
47 };
48
49 static struct spanhash *spanhash_find(struct spanhash_top *top,
50                                       unsigned int hashval)
51 {
52         int sz = 1 << top->alloc_log2;
53         int bucket = hashval & (sz - 1);
54         while (1) {
55                 struct spanhash *h = &(top->data[bucket++]);
56                 if (!h->cnt)
57                         return NULL;
58                 if (h->hashval == hashval)
59                         return h;
60                 if (sz <= bucket)
61                         bucket = 0;
62         }
63 }
64
65 static struct spanhash_top *spanhash_rehash(struct spanhash_top *orig)
66 {
67         struct spanhash_top *new;
68         int i;
69         int osz = 1 << orig->alloc_log2;
70         int sz = osz << 1;
71
72         new = xmalloc(sizeof(*orig) + sizeof(struct spanhash) * sz);
73         new->alloc_log2 = orig->alloc_log2 + 1;
74         new->free = INITIAL_FREE(new->alloc_log2);
75         memset(new->data, 0, sizeof(struct spanhash) * sz);
76         for (i = 0; i < osz; i++) {
77                 struct spanhash *o = &(orig->data[i]);
78                 int bucket;
79                 if (!o->cnt)
80                         continue;
81                 bucket = o->hashval & (sz - 1);
82                 while (1) {
83                         struct spanhash *h = &(new->data[bucket++]);
84                         if (!h->cnt) {
85                                 h->hashval = o->hashval;
86                                 h->cnt = o->cnt;
87                                 new->free--;
88                                 break;
89                         }
90                         if (sz <= bucket)
91                                 bucket = 0;
92                 }
93         }
94         free(orig);
95         return new;
96 }
97
98 static struct spanhash_top *add_spanhash(struct spanhash_top *top,
99                                          unsigned int hashval, int cnt)
100 {
101         int bucket, lim;
102         struct spanhash *h;
103
104         lim = (1 << top->alloc_log2);
105         bucket = hashval & (lim - 1);
106         while (1) {
107                 h = &(top->data[bucket++]);
108                 if (!h->cnt) {
109                         h->hashval = hashval;
110                         h->cnt = cnt;
111                         top->free--;
112                         if (top->free < 0)
113                                 return spanhash_rehash(top);
114                         return top;
115                 }
116                 if (h->hashval == hashval) {
117                         h->cnt += cnt;
118                         return top;
119                 }
120                 if (lim <= bucket)
121                         bucket = 0;
122         }
123 }
124
125 static struct spanhash_top *hash_chars(struct diff_filespec *one)
126 {
127         int i, n;
128         unsigned int accum1, accum2, hashval;
129         struct spanhash_top *hash;
130         unsigned char *buf = one->data;
131         unsigned int sz = one->size;
132         int is_text = !diff_filespec_is_binary(one);
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         n = 0;
141         accum1 = accum2 = 0;
142         while (sz) {
143                 unsigned int c = *buf++;
144                 unsigned int old_1 = accum1;
145                 sz--;
146
147                 /* Ignore CR in CRLF sequence if text */
148                 if (is_text && c == '\r' && sz && *buf == '\n')
149                         continue;
150
151                 accum1 = (accum1 << 7) ^ (accum2 >> 25);
152                 accum2 = (accum2 << 7) ^ (old_1 >> 25);
153                 accum1 += c;
154                 if (++n < 64 && c != '\n')
155                         continue;
156                 hashval = (accum1 + accum2 * 0x61) % HASHBASE;
157                 hash = add_spanhash(hash, hashval, n);
158                 n = 0;
159                 accum1 = accum2 = 0;
160         }
161         return hash;
162 }
163
164 int diffcore_count_changes(struct diff_filespec *src,
165                            struct diff_filespec *dst,
166                            void **src_count_p,
167                            void **dst_count_p,
168                            unsigned long delta_limit,
169                            unsigned long *src_copied,
170                            unsigned long *literal_added)
171 {
172         int i, ssz;
173         struct spanhash_top *src_count, *dst_count;
174         unsigned long sc, la;
175
176         src_count = dst_count = NULL;
177         if (src_count_p)
178                 src_count = *src_count_p;
179         if (!src_count) {
180                 src_count = hash_chars(src);
181                 if (src_count_p)
182                         *src_count_p = src_count;
183         }
184         if (dst_count_p)
185                 dst_count = *dst_count_p;
186         if (!dst_count) {
187                 dst_count = hash_chars(dst);
188                 if (dst_count_p)
189                         *dst_count_p = dst_count;
190         }
191         sc = la = 0;
192
193         ssz = 1 << src_count->alloc_log2;
194         for (i = 0; i < ssz; i++) {
195                 struct spanhash *s = &(src_count->data[i]);
196                 struct spanhash *d;
197                 unsigned dst_cnt, src_cnt;
198                 if (!s->cnt)
199                         continue;
200                 src_cnt = s->cnt;
201                 d = spanhash_find(dst_count, s->hashval);
202                 dst_cnt = d ? d->cnt : 0;
203                 if (src_cnt < dst_cnt) {
204                         la += dst_cnt - src_cnt;
205                         sc += src_cnt;
206                 }
207                 else
208                         sc += dst_cnt;
209         }
210
211         if (!src_count_p)
212                 free(src_count);
213         if (!dst_count_p)
214                 free(dst_count);
215         *src_copied = sc;
216         *literal_added = la;
217         return 0;
218 }