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![Johannes Schindelin](/assets/img/avatar_default.png)
When calculating hashes from pointers, it actually makes sense to cut off the most significant bits. In that case, said warning does not make a whole lot of sense. So let's just work around it by casting the pointer first to intptr_t and then casting up/down to the final integral type. Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de> Signed-off-by: Junio C Hamano <gitster@pobox.com>
248 lines
7 KiB
C
248 lines
7 KiB
C
#include "cache.h"
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#include "pack-revindex.h"
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/*
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* Pack index for existing packs give us easy access to the offsets into
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* corresponding pack file where each object's data starts, but the entries
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* do not store the size of the compressed representation (uncompressed
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* size is easily available by examining the pack entry header). It is
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* also rather expensive to find the sha1 for an object given its offset.
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*
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* We build a hashtable of existing packs (pack_revindex), and keep reverse
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* index here -- pack index file is sorted by object name mapping to offset;
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* this pack_revindex[].revindex array is a list of offset/index_nr pairs
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* ordered by offset, so if you know the offset of an object, next offset
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* is where its packed representation ends and the index_nr can be used to
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* get the object sha1 from the main index.
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*/
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static struct pack_revindex *pack_revindex;
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static int pack_revindex_hashsz;
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static int pack_revindex_ix(struct packed_git *p)
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{
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unsigned long ui = (unsigned long)(intptr_t)p;
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int i;
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ui = ui ^ (ui >> 16); /* defeat structure alignment */
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i = (int)(ui % pack_revindex_hashsz);
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while (pack_revindex[i].p) {
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if (pack_revindex[i].p == p)
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return i;
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if (++i == pack_revindex_hashsz)
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i = 0;
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}
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return -1 - i;
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}
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static void init_pack_revindex(void)
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{
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int num;
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struct packed_git *p;
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for (num = 0, p = packed_git; p; p = p->next)
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num++;
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if (!num)
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return;
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pack_revindex_hashsz = num * 11;
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pack_revindex = xcalloc(pack_revindex_hashsz, sizeof(*pack_revindex));
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for (p = packed_git; p; p = p->next) {
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num = pack_revindex_ix(p);
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num = - 1 - num;
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pack_revindex[num].p = p;
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}
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/* revindex elements are lazily initialized */
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}
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/*
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* This is a least-significant-digit radix sort.
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*
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* It sorts each of the "n" items in "entries" by its offset field. The "max"
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* parameter must be at least as large as the largest offset in the array,
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* and lets us quit the sort early.
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*/
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static void sort_revindex(struct revindex_entry *entries, unsigned n, off_t max)
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{
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/*
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* We use a "digit" size of 16 bits. That keeps our memory
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* usage reasonable, and we can generally (for a 4G or smaller
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* packfile) quit after two rounds of radix-sorting.
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*/
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#define DIGIT_SIZE (16)
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#define BUCKETS (1 << DIGIT_SIZE)
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/*
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* We want to know the bucket that a[i] will go into when we are using
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* the digit that is N bits from the (least significant) end.
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*/
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#define BUCKET_FOR(a, i, bits) (((a)[(i)].offset >> (bits)) & (BUCKETS-1))
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/*
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* We need O(n) temporary storage. Rather than do an extra copy of the
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* partial results into "entries", we sort back and forth between the
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* real array and temporary storage. In each iteration of the loop, we
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* keep track of them with alias pointers, always sorting from "from"
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* to "to".
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*/
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struct revindex_entry *tmp = xmalloc(n * sizeof(*tmp));
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struct revindex_entry *from = entries, *to = tmp;
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int bits;
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unsigned *pos = xmalloc(BUCKETS * sizeof(*pos));
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/*
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* If (max >> bits) is zero, then we know that the radix digit we are
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* on (and any higher) will be zero for all entries, and our loop will
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* be a no-op, as everybody lands in the same zero-th bucket.
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*/
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for (bits = 0; max >> bits; bits += DIGIT_SIZE) {
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struct revindex_entry *swap;
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unsigned i;
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memset(pos, 0, BUCKETS * sizeof(*pos));
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/*
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* We want pos[i] to store the index of the last element that
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* will go in bucket "i" (actually one past the last element).
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* To do this, we first count the items that will go in each
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* bucket, which gives us a relative offset from the last
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* bucket. We can then cumulatively add the index from the
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* previous bucket to get the true index.
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*/
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for (i = 0; i < n; i++)
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pos[BUCKET_FOR(from, i, bits)]++;
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for (i = 1; i < BUCKETS; i++)
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pos[i] += pos[i-1];
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/*
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* Now we can drop the elements into their correct buckets (in
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* our temporary array). We iterate the pos counter backwards
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* to avoid using an extra index to count up. And since we are
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* going backwards there, we must also go backwards through the
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* array itself, to keep the sort stable.
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*
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* Note that we use an unsigned iterator to make sure we can
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* handle 2^32-1 objects, even on a 32-bit system. But this
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* means we cannot use the more obvious "i >= 0" loop condition
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* for counting backwards, and must instead check for
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* wrap-around with UINT_MAX.
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*/
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for (i = n - 1; i != UINT_MAX; i--)
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to[--pos[BUCKET_FOR(from, i, bits)]] = from[i];
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/*
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* Now "to" contains the most sorted list, so we swap "from" and
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* "to" for the next iteration.
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*/
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swap = from;
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from = to;
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to = swap;
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}
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/*
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* If we ended with our data in the original array, great. If not,
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* we have to move it back from the temporary storage.
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*/
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if (from != entries)
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memcpy(entries, tmp, n * sizeof(*entries));
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free(tmp);
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free(pos);
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#undef BUCKET_FOR
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#undef BUCKETS
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#undef DIGIT_SIZE
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}
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/*
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* Ordered list of offsets of objects in the pack.
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*/
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static void create_pack_revindex(struct pack_revindex *rix)
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{
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struct packed_git *p = rix->p;
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unsigned num_ent = p->num_objects;
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unsigned i;
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const char *index = p->index_data;
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rix->revindex = xmalloc(sizeof(*rix->revindex) * (num_ent + 1));
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index += 4 * 256;
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if (p->index_version > 1) {
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const uint32_t *off_32 =
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(uint32_t *)(index + 8 + p->num_objects * (20 + 4));
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const uint32_t *off_64 = off_32 + p->num_objects;
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for (i = 0; i < num_ent; i++) {
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uint32_t off = ntohl(*off_32++);
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if (!(off & 0x80000000)) {
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rix->revindex[i].offset = off;
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} else {
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rix->revindex[i].offset =
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((uint64_t)ntohl(*off_64++)) << 32;
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rix->revindex[i].offset |=
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ntohl(*off_64++);
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}
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rix->revindex[i].nr = i;
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}
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} else {
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for (i = 0; i < num_ent; i++) {
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uint32_t hl = *((uint32_t *)(index + 24 * i));
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rix->revindex[i].offset = ntohl(hl);
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rix->revindex[i].nr = i;
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}
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}
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/* This knows the pack format -- the 20-byte trailer
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* follows immediately after the last object data.
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*/
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rix->revindex[num_ent].offset = p->pack_size - 20;
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rix->revindex[num_ent].nr = -1;
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sort_revindex(rix->revindex, num_ent, p->pack_size);
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}
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struct pack_revindex *revindex_for_pack(struct packed_git *p)
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{
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int num;
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struct pack_revindex *rix;
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if (!pack_revindex_hashsz)
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init_pack_revindex();
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num = pack_revindex_ix(p);
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if (num < 0)
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die("internal error: pack revindex fubar");
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rix = &pack_revindex[num];
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if (!rix->revindex)
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create_pack_revindex(rix);
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return rix;
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}
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int find_revindex_position(struct pack_revindex *pridx, off_t ofs)
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{
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int lo = 0;
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int hi = pridx->p->num_objects + 1;
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struct revindex_entry *revindex = pridx->revindex;
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do {
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unsigned mi = lo + (hi - lo) / 2;
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if (revindex[mi].offset == ofs) {
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return mi;
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} else if (ofs < revindex[mi].offset)
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hi = mi;
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else
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lo = mi + 1;
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} while (lo < hi);
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error("bad offset for revindex");
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return -1;
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}
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struct revindex_entry *find_pack_revindex(struct packed_git *p, off_t ofs)
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{
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struct pack_revindex *pridx = revindex_for_pack(p);
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int pos = find_revindex_position(pridx, ofs);
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if (pos < 0)
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return NULL;
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return pridx->revindex + pos;
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}
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