Merge branch 'rb/hpe'
[git.git] / pack-revindex.c
blob3c58784a5f4dedd0738ecc564f1945a10320c772
1 #include "cache.h"
2 #include "pack-revindex.h"
3 #include "object-store.h"
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.
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
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.
20 * This is a least-significant-digit radix sort.
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.
26 static void sort_revindex(struct revindex_entry *entries, unsigned n, off_t max)
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.
33 #define DIGIT_SIZE (16)
34 #define BUCKETS (1 << DIGIT_SIZE)
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.
39 #define BUCKET_FOR(a, i, bits) (((a)[(i)].offset >> (bits)) & (BUCKETS-1))
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".
48 struct revindex_entry *tmp, *from, *to;
49 int bits;
50 unsigned *pos;
52 ALLOC_ARRAY(pos, BUCKETS);
53 ALLOC_ARRAY(tmp, n);
54 from = entries;
55 to = tmp;
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.
62 for (bits = 0; max >> bits; bits += DIGIT_SIZE) {
63 unsigned i;
65 memset(pos, 0, BUCKETS * sizeof(*pos));
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.
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];
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.
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.
93 for (i = n - 1; i != UINT_MAX; i--)
94 to[--pos[BUCKET_FOR(from, i, bits)]] = from[i];
97 * Now "to" contains the most sorted list, so we swap "from" and
98 * "to" for the next iteration.
100 SWAP(from, to);
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.
107 if (from != entries)
108 COPY_ARRAY(entries, tmp, n);
109 free(tmp);
110 free(pos);
112 #undef BUCKET_FOR
113 #undef BUCKETS
114 #undef DIGIT_SIZE
118 * Ordered list of offsets of objects in the pack.
120 static void create_pack_revindex(struct packed_git *p)
122 unsigned num_ent = p->num_objects;
123 unsigned i;
124 const char *index = p->index_data;
125 const unsigned hashsz = the_hash_algo->rawsz;
127 ALLOC_ARRAY(p->revindex, num_ent + 1);
128 index += 4 * 256;
130 if (p->index_version > 1) {
131 const uint32_t *off_32 =
132 (uint32_t *)(index + 8 + p->num_objects * (hashsz + 4));
133 const uint32_t *off_64 = off_32 + p->num_objects;
134 for (i = 0; i < num_ent; i++) {
135 uint32_t off = ntohl(*off_32++);
136 if (!(off & 0x80000000)) {
137 p->revindex[i].offset = off;
138 } else {
139 p->revindex[i].offset = get_be64(off_64);
140 off_64 += 2;
142 p->revindex[i].nr = i;
144 } else {
145 for (i = 0; i < num_ent; i++) {
146 uint32_t hl = *((uint32_t *)(index + (hashsz + 4) * i));
147 p->revindex[i].offset = ntohl(hl);
148 p->revindex[i].nr = i;
153 * This knows the pack format -- the hash trailer
154 * follows immediately after the last object data.
156 p->revindex[num_ent].offset = p->pack_size - hashsz;
157 p->revindex[num_ent].nr = -1;
158 sort_revindex(p->revindex, num_ent, p->pack_size);
161 void load_pack_revindex(struct packed_git *p)
163 if (!p->revindex)
164 create_pack_revindex(p);
167 int find_revindex_position(struct packed_git *p, off_t ofs)
169 int lo = 0;
170 int hi = p->num_objects + 1;
171 struct revindex_entry *revindex = p->revindex;
173 do {
174 unsigned mi = lo + (hi - lo) / 2;
175 if (revindex[mi].offset == ofs) {
176 return mi;
177 } else if (ofs < revindex[mi].offset)
178 hi = mi;
179 else
180 lo = mi + 1;
181 } while (lo < hi);
183 error("bad offset for revindex");
184 return -1;
187 struct revindex_entry *find_pack_revindex(struct packed_git *p, off_t ofs)
189 int pos;
191 load_pack_revindex(p);
192 pos = find_revindex_position(p, ofs);
194 if (pos < 0)
195 return NULL;
197 return p->revindex + pos;