Start preparing for 1.8.4.2
[alt-git.git] / pack-revindex.c
blobb4d2b35bb37120f97ec000524d12a85d91949119
1 #include "cache.h"
2 #include "pack-revindex.h"
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.
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.
19 struct pack_revindex {
20 struct packed_git *p;
21 struct revindex_entry *revindex;
24 static struct pack_revindex *pack_revindex;
25 static int pack_revindex_hashsz;
27 static int pack_revindex_ix(struct packed_git *p)
29 unsigned long ui = (unsigned long)p;
30 int i;
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;
40 return -1 - i;
43 static void init_pack_revindex(void)
45 int num;
46 struct packed_git *p;
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;
59 /* revindex elements are lazily initialized */
63 * This is a least-significant-digit radix sort.
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.
69 static void sort_revindex(struct revindex_entry *entries, unsigned n, off_t max)
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.
76 #define DIGIT_SIZE (16)
77 #define BUCKETS (1 << DIGIT_SIZE)
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.
82 #define BUCKET_FOR(a, i, bits) (((a)[(i)].offset >> (bits)) & (BUCKETS-1))
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".
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));
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.
101 for (bits = 0; max >> bits; bits += DIGIT_SIZE) {
102 struct revindex_entry *swap;
103 unsigned i;
105 memset(pos, 0, BUCKETS * sizeof(*pos));
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.
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];
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.
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.
133 for (i = n - 1; i != UINT_MAX; i--)
134 to[--pos[BUCKET_FOR(from, i, bits)]] = from[i];
137 * Now "to" contains the most sorted list, so we swap "from" and
138 * "to" for the next iteration.
140 swap = from;
141 from = to;
142 to = swap;
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.
149 if (from != entries)
150 memcpy(entries, tmp, n * sizeof(*entries));
151 free(tmp);
152 free(pos);
154 #undef BUCKET_FOR
155 #undef BUCKETS
156 #undef DIGIT_SIZE
160 * Ordered list of offsets of objects in the pack.
162 static void create_pack_revindex(struct pack_revindex *rix)
164 struct packed_git *p = rix->p;
165 unsigned num_ent = p->num_objects;
166 unsigned i;
167 const char *index = p->index_data;
169 rix->revindex = xmalloc(sizeof(*rix->revindex) * (num_ent + 1));
170 index += 4 * 256;
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++);
186 rix->revindex[i].nr = i;
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;
196 /* This knows the pack format -- the 20-byte trailer
197 * follows immediately after the last object data.
199 rix->revindex[num_ent].offset = p->pack_size - 20;
200 rix->revindex[num_ent].nr = -1;
201 sort_revindex(rix->revindex, num_ent, p->pack_size);
204 struct revindex_entry *find_pack_revindex(struct packed_git *p, off_t ofs)
206 int num;
207 unsigned lo, hi;
208 struct pack_revindex *rix;
209 struct revindex_entry *revindex;
211 if (!pack_revindex_hashsz)
212 init_pack_revindex();
213 num = pack_revindex_ix(p);
214 if (num < 0)
215 die("internal error: pack revindex fubar");
217 rix = &pack_revindex[num];
218 if (!rix->revindex)
219 create_pack_revindex(rix);
220 revindex = rix->revindex;
222 lo = 0;
223 hi = p->num_objects + 1;
224 do {
225 unsigned mi = lo + (hi - lo) / 2;
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);
233 error("bad offset for revindex");
234 return NULL;
237 void discard_revindex(void)
239 if (pack_revindex_hashsz) {
240 int i;
241 for (i = 0; i < pack_revindex_hashsz; i++)
242 free(pack_revindex[i].revindex);
243 free(pack_revindex);
244 pack_revindex_hashsz = 0;