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3449f8c4 NP |
1 | #include "cache.h" |
2 | #include "pack-revindex.h" | |
3 | ||
4 | /* | |
5 | * Pack index for existing packs give us easy access to the offsets into | |
6 | * corresponding pack file where each object's data starts, but the entries | |
7 | * do not store the size of the compressed representation (uncompressed | |
8 | * size is easily available by examining the pack entry header). It is | |
9 | * also rather expensive to find the sha1 for an object given its offset. | |
10 | * | |
11 | * We build a hashtable of existing packs (pack_revindex), and keep reverse | |
12 | * index here -- pack index file is sorted by object name mapping to offset; | |
13 | * this pack_revindex[].revindex array is a list of offset/index_nr pairs | |
14 | * ordered by offset, so if you know the offset of an object, next offset | |
15 | * is where its packed representation ends and the index_nr can be used to | |
16 | * get the object sha1 from the main index. | |
17 | */ | |
18 | ||
19 | struct pack_revindex { | |
20 | struct packed_git *p; | |
21 | struct revindex_entry *revindex; | |
22 | }; | |
23 | ||
24 | static struct pack_revindex *pack_revindex; | |
25 | static int pack_revindex_hashsz; | |
26 | ||
27 | static int pack_revindex_ix(struct packed_git *p) | |
28 | { | |
29 | unsigned long ui = (unsigned long)p; | |
30 | int i; | |
31 | ||
32 | ui = ui ^ (ui >> 16); /* defeat structure alignment */ | |
33 | i = (int)(ui % pack_revindex_hashsz); | |
34 | while (pack_revindex[i].p) { | |
35 | if (pack_revindex[i].p == p) | |
36 | return i; | |
37 | if (++i == pack_revindex_hashsz) | |
38 | i = 0; | |
39 | } | |
40 | return -1 - i; | |
41 | } | |
42 | ||
1f5c74f6 | 43 | static void init_pack_revindex(void) |
3449f8c4 NP |
44 | { |
45 | int num; | |
46 | struct packed_git *p; | |
47 | ||
48 | for (num = 0, p = packed_git; p; p = p->next) | |
49 | num++; | |
50 | if (!num) | |
51 | return; | |
52 | pack_revindex_hashsz = num * 11; | |
53 | pack_revindex = xcalloc(sizeof(*pack_revindex), pack_revindex_hashsz); | |
54 | for (p = packed_git; p; p = p->next) { | |
55 | num = pack_revindex_ix(p); | |
56 | num = - 1 - num; | |
57 | pack_revindex[num].p = p; | |
58 | } | |
59 | /* revindex elements are lazily initialized */ | |
60 | } | |
61 | ||
8b8dfd51 JK |
62 | /* |
63 | * This is a least-significant-digit radix sort. | |
64 | * | |
65 | * It sorts each of the "n" items in "entries" by its offset field. The "max" | |
66 | * parameter must be at least as large as the largest offset in the array, | |
67 | * and lets us quit the sort early. | |
68 | */ | |
69 | static void sort_revindex(struct revindex_entry *entries, unsigned n, off_t max) | |
3449f8c4 | 70 | { |
8b8dfd51 JK |
71 | /* |
72 | * We use a "digit" size of 16 bits. That keeps our memory | |
73 | * usage reasonable, and we can generally (for a 4G or smaller | |
74 | * packfile) quit after two rounds of radix-sorting. | |
75 | */ | |
76 | #define DIGIT_SIZE (16) | |
77 | #define BUCKETS (1 << DIGIT_SIZE) | |
78 | /* | |
79 | * We want to know the bucket that a[i] will go into when we are using | |
80 | * the digit that is N bits from the (least significant) end. | |
81 | */ | |
82 | #define BUCKET_FOR(a, i, bits) (((a)[(i)].offset >> (bits)) & (BUCKETS-1)) | |
83 | ||
84 | /* | |
85 | * We need O(n) temporary storage. Rather than do an extra copy of the | |
86 | * partial results into "entries", we sort back and forth between the | |
87 | * real array and temporary storage. In each iteration of the loop, we | |
88 | * keep track of them with alias pointers, always sorting from "from" | |
89 | * to "to". | |
90 | */ | |
91 | struct revindex_entry *tmp = xmalloc(n * sizeof(*tmp)); | |
92 | struct revindex_entry *from = entries, *to = tmp; | |
93 | int bits; | |
94 | unsigned *pos = xmalloc(BUCKETS * sizeof(*pos)); | |
95 | ||
96 | /* | |
97 | * If (max >> bits) is zero, then we know that the radix digit we are | |
98 | * on (and any higher) will be zero for all entries, and our loop will | |
99 | * be a no-op, as everybody lands in the same zero-th bucket. | |
100 | */ | |
101 | for (bits = 0; max >> bits; bits += DIGIT_SIZE) { | |
102 | struct revindex_entry *swap; | |
103 | unsigned i; | |
104 | ||
105 | memset(pos, 0, BUCKETS * sizeof(*pos)); | |
106 | ||
107 | /* | |
108 | * We want pos[i] to store the index of the last element that | |
109 | * will go in bucket "i" (actually one past the last element). | |
110 | * To do this, we first count the items that will go in each | |
111 | * bucket, which gives us a relative offset from the last | |
112 | * bucket. We can then cumulatively add the index from the | |
113 | * previous bucket to get the true index. | |
114 | */ | |
115 | for (i = 0; i < n; i++) | |
116 | pos[BUCKET_FOR(from, i, bits)]++; | |
117 | for (i = 1; i < BUCKETS; i++) | |
118 | pos[i] += pos[i-1]; | |
119 | ||
120 | /* | |
121 | * Now we can drop the elements into their correct buckets (in | |
122 | * our temporary array). We iterate the pos counter backwards | |
123 | * to avoid using an extra index to count up. And since we are | |
124 | * going backwards there, we must also go backwards through the | |
125 | * array itself, to keep the sort stable. | |
126 | * | |
127 | * Note that we use an unsigned iterator to make sure we can | |
128 | * handle 2^32-1 objects, even on a 32-bit system. But this | |
129 | * means we cannot use the more obvious "i >= 0" loop condition | |
130 | * for counting backwards, and must instead check for | |
131 | * wrap-around with UINT_MAX. | |
132 | */ | |
133 | for (i = n - 1; i != UINT_MAX; i--) | |
134 | to[--pos[BUCKET_FOR(from, i, bits)]] = from[i]; | |
135 | ||
136 | /* | |
137 | * Now "to" contains the most sorted list, so we swap "from" and | |
138 | * "to" for the next iteration. | |
139 | */ | |
140 | swap = from; | |
141 | from = to; | |
142 | to = swap; | |
143 | } | |
144 | ||
145 | /* | |
146 | * If we ended with our data in the original array, great. If not, | |
147 | * we have to move it back from the temporary storage. | |
148 | */ | |
149 | if (from != entries) | |
150 | memcpy(entries, tmp, n * sizeof(*entries)); | |
151 | free(tmp); | |
152 | free(pos); | |
153 | ||
154 | #undef BUCKET_FOR | |
155 | #undef BUCKETS | |
156 | #undef DIGIT_SIZE | |
3449f8c4 NP |
157 | } |
158 | ||
159 | /* | |
160 | * Ordered list of offsets of objects in the pack. | |
161 | */ | |
162 | static void create_pack_revindex(struct pack_revindex *rix) | |
163 | { | |
164 | struct packed_git *p = rix->p; | |
012b32bb JK |
165 | unsigned num_ent = p->num_objects; |
166 | unsigned i; | |
3449f8c4 NP |
167 | const char *index = p->index_data; |
168 | ||
169 | rix->revindex = xmalloc(sizeof(*rix->revindex) * (num_ent + 1)); | |
170 | index += 4 * 256; | |
171 | ||
172 | if (p->index_version > 1) { | |
173 | const uint32_t *off_32 = | |
174 | (uint32_t *)(index + 8 + p->num_objects * (20 + 4)); | |
175 | const uint32_t *off_64 = off_32 + p->num_objects; | |
176 | for (i = 0; i < num_ent; i++) { | |
177 | uint32_t off = ntohl(*off_32++); | |
178 | if (!(off & 0x80000000)) { | |
179 | rix->revindex[i].offset = off; | |
180 | } else { | |
181 | rix->revindex[i].offset = | |
182 | ((uint64_t)ntohl(*off_64++)) << 32; | |
183 | rix->revindex[i].offset |= | |
184 | ntohl(*off_64++); | |
185 | } | |
186 | rix->revindex[i].nr = i; | |
187 | } | |
188 | } else { | |
189 | for (i = 0; i < num_ent; i++) { | |
190 | uint32_t hl = *((uint32_t *)(index + 24 * i)); | |
191 | rix->revindex[i].offset = ntohl(hl); | |
192 | rix->revindex[i].nr = i; | |
193 | } | |
194 | } | |
195 | ||
196 | /* This knows the pack format -- the 20-byte trailer | |
197 | * follows immediately after the last object data. | |
198 | */ | |
199 | rix->revindex[num_ent].offset = p->pack_size - 20; | |
200 | rix->revindex[num_ent].nr = -1; | |
8b8dfd51 | 201 | sort_revindex(rix->revindex, num_ent, p->pack_size); |
3449f8c4 NP |
202 | } |
203 | ||
204 | struct revindex_entry *find_pack_revindex(struct packed_git *p, off_t ofs) | |
205 | { | |
206 | int num; | |
012b32bb | 207 | unsigned lo, hi; |
3449f8c4 NP |
208 | struct pack_revindex *rix; |
209 | struct revindex_entry *revindex; | |
210 | ||
1f5c74f6 NP |
211 | if (!pack_revindex_hashsz) |
212 | init_pack_revindex(); | |
3449f8c4 NP |
213 | num = pack_revindex_ix(p); |
214 | if (num < 0) | |
1f5c74f6 | 215 | die("internal error: pack revindex fubar"); |
3449f8c4 NP |
216 | |
217 | rix = &pack_revindex[num]; | |
218 | if (!rix->revindex) | |
219 | create_pack_revindex(rix); | |
220 | revindex = rix->revindex; | |
221 | ||
222 | lo = 0; | |
223 | hi = p->num_objects + 1; | |
224 | do { | |
012b32bb | 225 | unsigned mi = lo + (hi - lo) / 2; |
3449f8c4 NP |
226 | if (revindex[mi].offset == ofs) { |
227 | return revindex + mi; | |
228 | } else if (ofs < revindex[mi].offset) | |
229 | hi = mi; | |
230 | else | |
231 | lo = mi + 1; | |
232 | } while (lo < hi); | |
08698b1e NP |
233 | error("bad offset for revindex"); |
234 | return NULL; | |
3449f8c4 | 235 | } |
4b480c67 NP |
236 | |
237 | void discard_revindex(void) | |
238 | { | |
239 | if (pack_revindex_hashsz) { | |
240 | int i; | |
241 | for (i = 0; i < pack_revindex_hashsz; i++) | |
24deea52 | 242 | free(pack_revindex[i].revindex); |
4b480c67 NP |
243 | free(pack_revindex); |
244 | pack_revindex_hashsz = 0; | |
245 | } | |
246 | } |