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3449f8c4 NP |
1 | #include "cache.h" |
2 | #include "pack-revindex.h" | |
a80d72db | 3 | #include "object-store.h" |
3449f8c4 NP |
4 | |
5 | /* | |
6 | * Pack index for existing packs give us easy access to the offsets into | |
7 | * corresponding pack file where each object's data starts, but the entries | |
8 | * do not store the size of the compressed representation (uncompressed | |
9 | * size is easily available by examining the pack entry header). It is | |
10 | * also rather expensive to find the sha1 for an object given its offset. | |
11 | * | |
f4015337 JK |
12 | * The pack index file is sorted by object name mapping to offset; |
13 | * this revindex array is a list of offset/index_nr pairs | |
3449f8c4 NP |
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 | ||
8b8dfd51 JK |
19 | /* |
20 | * This is a least-significant-digit radix sort. | |
21 | * | |
22 | * It sorts each of the "n" items in "entries" by its offset field. The "max" | |
23 | * parameter must be at least as large as the largest offset in the array, | |
24 | * and lets us quit the sort early. | |
25 | */ | |
26 | static void sort_revindex(struct revindex_entry *entries, unsigned n, off_t max) | |
3449f8c4 | 27 | { |
8b8dfd51 JK |
28 | /* |
29 | * We use a "digit" size of 16 bits. That keeps our memory | |
30 | * usage reasonable, and we can generally (for a 4G or smaller | |
31 | * packfile) quit after two rounds of radix-sorting. | |
32 | */ | |
33 | #define DIGIT_SIZE (16) | |
34 | #define BUCKETS (1 << DIGIT_SIZE) | |
35 | /* | |
36 | * We want to know the bucket that a[i] will go into when we are using | |
37 | * the digit that is N bits from the (least significant) end. | |
38 | */ | |
39 | #define BUCKET_FOR(a, i, bits) (((a)[(i)].offset >> (bits)) & (BUCKETS-1)) | |
40 | ||
41 | /* | |
42 | * We need O(n) temporary storage. Rather than do an extra copy of the | |
43 | * partial results into "entries", we sort back and forth between the | |
44 | * real array and temporary storage. In each iteration of the loop, we | |
45 | * keep track of them with alias pointers, always sorting from "from" | |
46 | * to "to". | |
47 | */ | |
b32fa95f | 48 | struct revindex_entry *tmp, *from, *to; |
8b8dfd51 | 49 | int bits; |
b32fa95f JK |
50 | unsigned *pos; |
51 | ||
52 | ALLOC_ARRAY(pos, BUCKETS); | |
53 | ALLOC_ARRAY(tmp, n); | |
54 | from = entries; | |
55 | to = tmp; | |
8b8dfd51 JK |
56 | |
57 | /* | |
58 | * If (max >> bits) is zero, then we know that the radix digit we are | |
59 | * on (and any higher) will be zero for all entries, and our loop will | |
60 | * be a no-op, as everybody lands in the same zero-th bucket. | |
61 | */ | |
62 | for (bits = 0; max >> bits; bits += DIGIT_SIZE) { | |
8b8dfd51 JK |
63 | unsigned i; |
64 | ||
65 | memset(pos, 0, BUCKETS * sizeof(*pos)); | |
66 | ||
67 | /* | |
68 | * We want pos[i] to store the index of the last element that | |
69 | * will go in bucket "i" (actually one past the last element). | |
70 | * To do this, we first count the items that will go in each | |
71 | * bucket, which gives us a relative offset from the last | |
72 | * bucket. We can then cumulatively add the index from the | |
73 | * previous bucket to get the true index. | |
74 | */ | |
75 | for (i = 0; i < n; i++) | |
76 | pos[BUCKET_FOR(from, i, bits)]++; | |
77 | for (i = 1; i < BUCKETS; i++) | |
78 | pos[i] += pos[i-1]; | |
79 | ||
80 | /* | |
81 | * Now we can drop the elements into their correct buckets (in | |
82 | * our temporary array). We iterate the pos counter backwards | |
83 | * to avoid using an extra index to count up. And since we are | |
84 | * going backwards there, we must also go backwards through the | |
85 | * array itself, to keep the sort stable. | |
86 | * | |
87 | * Note that we use an unsigned iterator to make sure we can | |
88 | * handle 2^32-1 objects, even on a 32-bit system. But this | |
89 | * means we cannot use the more obvious "i >= 0" loop condition | |
90 | * for counting backwards, and must instead check for | |
91 | * wrap-around with UINT_MAX. | |
92 | */ | |
93 | for (i = n - 1; i != UINT_MAX; i--) | |
94 | to[--pos[BUCKET_FOR(from, i, bits)]] = from[i]; | |
95 | ||
96 | /* | |
97 | * Now "to" contains the most sorted list, so we swap "from" and | |
98 | * "to" for the next iteration. | |
99 | */ | |
35d803bc | 100 | SWAP(from, to); |
8b8dfd51 JK |
101 | } |
102 | ||
103 | /* | |
104 | * If we ended with our data in the original array, great. If not, | |
105 | * we have to move it back from the temporary storage. | |
106 | */ | |
107 | if (from != entries) | |
45ccef87 | 108 | COPY_ARRAY(entries, tmp, n); |
8b8dfd51 JK |
109 | free(tmp); |
110 | free(pos); | |
111 | ||
112 | #undef BUCKET_FOR | |
113 | #undef BUCKETS | |
114 | #undef DIGIT_SIZE | |
3449f8c4 NP |
115 | } |
116 | ||
117 | /* | |
118 | * Ordered list of offsets of objects in the pack. | |
119 | */ | |
9d98bbf5 | 120 | static void create_pack_revindex(struct packed_git *p) |
3449f8c4 | 121 | { |
012b32bb JK |
122 | unsigned num_ent = p->num_objects; |
123 | unsigned i; | |
3449f8c4 NP |
124 | const char *index = p->index_data; |
125 | ||
11529ece | 126 | ALLOC_ARRAY(p->revindex, num_ent + 1); |
3449f8c4 NP |
127 | index += 4 * 256; |
128 | ||
129 | if (p->index_version > 1) { | |
130 | const uint32_t *off_32 = | |
131 | (uint32_t *)(index + 8 + p->num_objects * (20 + 4)); | |
132 | const uint32_t *off_64 = off_32 + p->num_objects; | |
133 | for (i = 0; i < num_ent; i++) { | |
134 | uint32_t off = ntohl(*off_32++); | |
135 | if (!(off & 0x80000000)) { | |
9d98bbf5 | 136 | p->revindex[i].offset = off; |
3449f8c4 | 137 | } else { |
ad622a25 DS |
138 | p->revindex[i].offset = get_be64(off_64); |
139 | off_64 += 2; | |
3449f8c4 | 140 | } |
9d98bbf5 | 141 | p->revindex[i].nr = i; |
3449f8c4 NP |
142 | } |
143 | } else { | |
144 | for (i = 0; i < num_ent; i++) { | |
145 | uint32_t hl = *((uint32_t *)(index + 24 * i)); | |
9d98bbf5 JK |
146 | p->revindex[i].offset = ntohl(hl); |
147 | p->revindex[i].nr = i; | |
3449f8c4 NP |
148 | } |
149 | } | |
150 | ||
151 | /* This knows the pack format -- the 20-byte trailer | |
152 | * follows immediately after the last object data. | |
153 | */ | |
9d98bbf5 JK |
154 | p->revindex[num_ent].offset = p->pack_size - 20; |
155 | p->revindex[num_ent].nr = -1; | |
156 | sort_revindex(p->revindex, num_ent, p->pack_size); | |
3449f8c4 NP |
157 | } |
158 | ||
9d98bbf5 | 159 | void load_pack_revindex(struct packed_git *p) |
3449f8c4 | 160 | { |
9d98bbf5 JK |
161 | if (!p->revindex) |
162 | create_pack_revindex(p); | |
92e5c77c VM |
163 | } |
164 | ||
9d98bbf5 | 165 | int find_revindex_position(struct packed_git *p, off_t ofs) |
92e5c77c VM |
166 | { |
167 | int lo = 0; | |
9d98bbf5 JK |
168 | int hi = p->num_objects + 1; |
169 | struct revindex_entry *revindex = p->revindex; | |
92e5c77c | 170 | |
3449f8c4 | 171 | do { |
012b32bb | 172 | unsigned mi = lo + (hi - lo) / 2; |
3449f8c4 | 173 | if (revindex[mi].offset == ofs) { |
92e5c77c | 174 | return mi; |
3449f8c4 NP |
175 | } else if (ofs < revindex[mi].offset) |
176 | hi = mi; | |
177 | else | |
178 | lo = mi + 1; | |
179 | } while (lo < hi); | |
92e5c77c | 180 | |
08698b1e | 181 | error("bad offset for revindex"); |
92e5c77c VM |
182 | return -1; |
183 | } | |
184 | ||
185 | struct revindex_entry *find_pack_revindex(struct packed_git *p, off_t ofs) | |
186 | { | |
9d98bbf5 JK |
187 | int pos; |
188 | ||
189 | load_pack_revindex(p); | |
190 | pos = find_revindex_position(p, ofs); | |
92e5c77c VM |
191 | |
192 | if (pos < 0) | |
193 | return NULL; | |
194 | ||
9d98bbf5 | 195 | return p->revindex + pos; |
3449f8c4 | 196 | } |