Git 2.0
[git/git.git] / pack-revindex.c
CommitLineData
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
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NP
19static struct pack_revindex *pack_revindex;
20static int pack_revindex_hashsz;
21
22static int pack_revindex_ix(struct packed_git *p)
23{
24 unsigned long ui = (unsigned long)p;
25 int i;
26
27 ui = ui ^ (ui >> 16); /* defeat structure alignment */
28 i = (int)(ui % pack_revindex_hashsz);
29 while (pack_revindex[i].p) {
30 if (pack_revindex[i].p == p)
31 return i;
32 if (++i == pack_revindex_hashsz)
33 i = 0;
34 }
35 return -1 - i;
36}
37
1f5c74f6 38static void init_pack_revindex(void)
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39{
40 int num;
41 struct packed_git *p;
42
43 for (num = 0, p = packed_git; p; p = p->next)
44 num++;
45 if (!num)
46 return;
47 pack_revindex_hashsz = num * 11;
48 pack_revindex = xcalloc(sizeof(*pack_revindex), pack_revindex_hashsz);
49 for (p = packed_git; p; p = p->next) {
50 num = pack_revindex_ix(p);
51 num = - 1 - num;
52 pack_revindex[num].p = p;
53 }
54 /* revindex elements are lazily initialized */
55}
56
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JK
57/*
58 * This is a least-significant-digit radix sort.
59 *
60 * It sorts each of the "n" items in "entries" by its offset field. The "max"
61 * parameter must be at least as large as the largest offset in the array,
62 * and lets us quit the sort early.
63 */
64static void sort_revindex(struct revindex_entry *entries, unsigned n, off_t max)
3449f8c4 65{
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JK
66 /*
67 * We use a "digit" size of 16 bits. That keeps our memory
68 * usage reasonable, and we can generally (for a 4G or smaller
69 * packfile) quit after two rounds of radix-sorting.
70 */
71#define DIGIT_SIZE (16)
72#define BUCKETS (1 << DIGIT_SIZE)
73 /*
74 * We want to know the bucket that a[i] will go into when we are using
75 * the digit that is N bits from the (least significant) end.
76 */
77#define BUCKET_FOR(a, i, bits) (((a)[(i)].offset >> (bits)) & (BUCKETS-1))
78
79 /*
80 * We need O(n) temporary storage. Rather than do an extra copy of the
81 * partial results into "entries", we sort back and forth between the
82 * real array and temporary storage. In each iteration of the loop, we
83 * keep track of them with alias pointers, always sorting from "from"
84 * to "to".
85 */
86 struct revindex_entry *tmp = xmalloc(n * sizeof(*tmp));
87 struct revindex_entry *from = entries, *to = tmp;
88 int bits;
89 unsigned *pos = xmalloc(BUCKETS * sizeof(*pos));
90
91 /*
92 * If (max >> bits) is zero, then we know that the radix digit we are
93 * on (and any higher) will be zero for all entries, and our loop will
94 * be a no-op, as everybody lands in the same zero-th bucket.
95 */
96 for (bits = 0; max >> bits; bits += DIGIT_SIZE) {
97 struct revindex_entry *swap;
98 unsigned i;
99
100 memset(pos, 0, BUCKETS * sizeof(*pos));
101
102 /*
103 * We want pos[i] to store the index of the last element that
104 * will go in bucket "i" (actually one past the last element).
105 * To do this, we first count the items that will go in each
106 * bucket, which gives us a relative offset from the last
107 * bucket. We can then cumulatively add the index from the
108 * previous bucket to get the true index.
109 */
110 for (i = 0; i < n; i++)
111 pos[BUCKET_FOR(from, i, bits)]++;
112 for (i = 1; i < BUCKETS; i++)
113 pos[i] += pos[i-1];
114
115 /*
116 * Now we can drop the elements into their correct buckets (in
117 * our temporary array). We iterate the pos counter backwards
118 * to avoid using an extra index to count up. And since we are
119 * going backwards there, we must also go backwards through the
120 * array itself, to keep the sort stable.
121 *
122 * Note that we use an unsigned iterator to make sure we can
123 * handle 2^32-1 objects, even on a 32-bit system. But this
124 * means we cannot use the more obvious "i >= 0" loop condition
125 * for counting backwards, and must instead check for
126 * wrap-around with UINT_MAX.
127 */
128 for (i = n - 1; i != UINT_MAX; i--)
129 to[--pos[BUCKET_FOR(from, i, bits)]] = from[i];
130
131 /*
132 * Now "to" contains the most sorted list, so we swap "from" and
133 * "to" for the next iteration.
134 */
135 swap = from;
136 from = to;
137 to = swap;
138 }
139
140 /*
141 * If we ended with our data in the original array, great. If not,
142 * we have to move it back from the temporary storage.
143 */
144 if (from != entries)
145 memcpy(entries, tmp, n * sizeof(*entries));
146 free(tmp);
147 free(pos);
148
149#undef BUCKET_FOR
150#undef BUCKETS
151#undef DIGIT_SIZE
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152}
153
154/*
155 * Ordered list of offsets of objects in the pack.
156 */
157static void create_pack_revindex(struct pack_revindex *rix)
158{
159 struct packed_git *p = rix->p;
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160 unsigned num_ent = p->num_objects;
161 unsigned i;
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162 const char *index = p->index_data;
163
164 rix->revindex = xmalloc(sizeof(*rix->revindex) * (num_ent + 1));
165 index += 4 * 256;
166
167 if (p->index_version > 1) {
168 const uint32_t *off_32 =
169 (uint32_t *)(index + 8 + p->num_objects * (20 + 4));
170 const uint32_t *off_64 = off_32 + p->num_objects;
171 for (i = 0; i < num_ent; i++) {
172 uint32_t off = ntohl(*off_32++);
173 if (!(off & 0x80000000)) {
174 rix->revindex[i].offset = off;
175 } else {
176 rix->revindex[i].offset =
177 ((uint64_t)ntohl(*off_64++)) << 32;
178 rix->revindex[i].offset |=
179 ntohl(*off_64++);
180 }
181 rix->revindex[i].nr = i;
182 }
183 } else {
184 for (i = 0; i < num_ent; i++) {
185 uint32_t hl = *((uint32_t *)(index + 24 * i));
186 rix->revindex[i].offset = ntohl(hl);
187 rix->revindex[i].nr = i;
188 }
189 }
190
191 /* This knows the pack format -- the 20-byte trailer
192 * follows immediately after the last object data.
193 */
194 rix->revindex[num_ent].offset = p->pack_size - 20;
195 rix->revindex[num_ent].nr = -1;
8b8dfd51 196 sort_revindex(rix->revindex, num_ent, p->pack_size);
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197}
198
92e5c77c 199struct pack_revindex *revindex_for_pack(struct packed_git *p)
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200{
201 int num;
3449f8c4 202 struct pack_revindex *rix;
3449f8c4 203
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NP
204 if (!pack_revindex_hashsz)
205 init_pack_revindex();
92e5c77c 206
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207 num = pack_revindex_ix(p);
208 if (num < 0)
1f5c74f6 209 die("internal error: pack revindex fubar");
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210
211 rix = &pack_revindex[num];
212 if (!rix->revindex)
213 create_pack_revindex(rix);
3449f8c4 214
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215 return rix;
216}
217
218int find_revindex_position(struct pack_revindex *pridx, off_t ofs)
219{
220 int lo = 0;
221 int hi = pridx->p->num_objects + 1;
222 struct revindex_entry *revindex = pridx->revindex;
223
3449f8c4 224 do {
012b32bb 225 unsigned mi = lo + (hi - lo) / 2;
3449f8c4 226 if (revindex[mi].offset == ofs) {
92e5c77c 227 return mi;
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NP
228 } else if (ofs < revindex[mi].offset)
229 hi = mi;
230 else
231 lo = mi + 1;
232 } while (lo < hi);
92e5c77c 233
08698b1e 234 error("bad offset for revindex");
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VM
235 return -1;
236}
237
238struct revindex_entry *find_pack_revindex(struct packed_git *p, off_t ofs)
239{
240 struct pack_revindex *pridx = revindex_for_pack(p);
241 int pos = find_revindex_position(pridx, ofs);
242
243 if (pos < 0)
244 return NULL;
245
246 return pridx->revindex + pos;
3449f8c4 247}