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#include "cache.h"
#include "pack-revindex.h"

/*
* Pack index for existing packs give us easy access to the offsets into
* corresponding pack file where each object's data starts, but the entries
* do not store the size of the compressed representation (uncompressed
* size is easily available by examining the pack entry header). It is
* also rather expensive to find the sha1 for an object given its offset.
*
* We build a hashtable of existing packs (pack_revindex), and keep reverse
* index here -- pack index file is sorted by object name mapping to offset;
* this pack_revindex[].revindex array is a list of offset/index_nr pairs
* ordered by offset, so if you know the offset of an object, next offset
* is where its packed representation ends and the index_nr can be used to
* get the object sha1 from the main index.
*/

struct pack_revindex {
struct packed_git *p;
struct revindex_entry *revindex;
};

static struct pack_revindex *pack_revindex;
static int pack_revindex_hashsz;

static int pack_revindex_ix(struct packed_git *p)
{
unsigned long ui = (unsigned long)p;
int i;

ui = ui ^ (ui >> 16); /* defeat structure alignment */
i = (int)(ui % pack_revindex_hashsz);
while (pack_revindex[i].p) {
if (pack_revindex[i].p == p)
return i;
if (++i == pack_revindex_hashsz)
i = 0;
}
return -1 - i;
}

static void init_pack_revindex(void)
{
int num;
struct packed_git *p;

for (num = 0, p = packed_git; p; p = p->next)
num++;
if (!num)
return;
pack_revindex_hashsz = num * 11;
pack_revindex = xcalloc(sizeof(*pack_revindex), pack_revindex_hashsz);
for (p = packed_git; p; p = p->next) {
num = pack_revindex_ix(p);
num = - 1 - num;
pack_revindex[num].p = p;
}
/* revindex elements are lazily initialized */
}

/*
* This is a least-significant-digit radix sort.
*
* It sorts each of the "n" items in "entries" by its offset field. The "max"
* parameter must be at least as large as the largest offset in the array,
* and lets us quit the sort early.
*/
static void sort_revindex(struct revindex_entry *entries, unsigned n, off_t max)
{
/*
* We use a "digit" size of 16 bits. That keeps our memory
* usage reasonable, and we can generally (for a 4G or smaller
* packfile) quit after two rounds of radix-sorting.
*/
#define DIGIT_SIZE (16)
#define BUCKETS (1 << DIGIT_SIZE)
/*
* We want to know the bucket that a[i] will go into when we are using
* the digit that is N bits from the (least significant) end.
*/
#define BUCKET_FOR(a, i, bits) (((a)[(i)].offset >> (bits)) & (BUCKETS-1))

/*
* We need O(n) temporary storage. Rather than do an extra copy of the
* partial results into "entries", we sort back and forth between the
* real array and temporary storage. In each iteration of the loop, we
* keep track of them with alias pointers, always sorting from "from"
* to "to".
*/
struct revindex_entry *tmp = xmalloc(n * sizeof(*tmp));
struct revindex_entry *from = entries, *to = tmp;
int bits;
unsigned *pos = xmalloc(BUCKETS * sizeof(*pos));

/*
* If (max >> bits) is zero, then we know that the radix digit we are
* on (and any higher) will be zero for all entries, and our loop will
* be a no-op, as everybody lands in the same zero-th bucket.
*/
for (bits = 0; max >> bits; bits += DIGIT_SIZE) {
struct revindex_entry *swap;
unsigned i;

memset(pos, 0, BUCKETS * sizeof(*pos));

/*
* We want pos[i] to store the index of the last element that
* will go in bucket "i" (actually one past the last element).
* To do this, we first count the items that will go in each
* bucket, which gives us a relative offset from the last
* bucket. We can then cumulatively add the index from the
* previous bucket to get the true index.
*/
for (i = 0; i < n; i++)
pos[BUCKET_FOR(from, i, bits)]++;
for (i = 1; i < BUCKETS; i++)
pos[i] += pos[i-1];

/*
* Now we can drop the elements into their correct buckets (in
* our temporary array). We iterate the pos counter backwards
* to avoid using an extra index to count up. And since we are
* going backwards there, we must also go backwards through the
* array itself, to keep the sort stable.
*
* Note that we use an unsigned iterator to make sure we can
* handle 2^32-1 objects, even on a 32-bit system. But this
* means we cannot use the more obvious "i >= 0" loop condition
* for counting backwards, and must instead check for
* wrap-around with UINT_MAX.
*/
for (i = n - 1; i != UINT_MAX; i--)
to[--pos[BUCKET_FOR(from, i, bits)]] = from[i];

/*
* Now "to" contains the most sorted list, so we swap "from" and
* "to" for the next iteration.
*/
swap = from;
from = to;
to = swap;
}

/*
* If we ended with our data in the original array, great. If not,
* we have to move it back from the temporary storage.
*/
if (from != entries)
memcpy(entries, tmp, n * sizeof(*entries));
free(tmp);
free(pos);

#undef BUCKET_FOR
#undef BUCKETS
#undef DIGIT_SIZE
}

/*
* Ordered list of offsets of objects in the pack.
*/
static void create_pack_revindex(struct pack_revindex *rix)
{
struct packed_git *p = rix->p;
unsigned num_ent = p->num_objects;
unsigned i;
const char *index = p->index_data;

rix->revindex = xmalloc(sizeof(*rix->revindex) * (num_ent + 1));
index += 4 * 256;

if (p->index_version > 1) {
const uint32_t *off_32 =
(uint32_t *)(index + 8 + p->num_objects * (20 + 4));
const uint32_t *off_64 = off_32 + p->num_objects;
for (i = 0; i < num_ent; i++) {
uint32_t off = ntohl(*off_32++);
if (!(off & 0x80000000)) {
rix->revindex[i].offset = off;
} else {
rix->revindex[i].offset =
((uint64_t)ntohl(*off_64++)) << 32;
rix->revindex[i].offset |=
ntohl(*off_64++);
}
rix->revindex[i].nr = i;
}
} else {
for (i = 0; i < num_ent; i++) {
uint32_t hl = *((uint32_t *)(index + 24 * i));
rix->revindex[i].offset = ntohl(hl);
rix->revindex[i].nr = i;
}
}

/* This knows the pack format -- the 20-byte trailer
* follows immediately after the last object data.
*/
rix->revindex[num_ent].offset = p->pack_size - 20;
rix->revindex[num_ent].nr = -1;
sort_revindex(rix->revindex, num_ent, p->pack_size);
}

struct revindex_entry *find_pack_revindex(struct packed_git *p, off_t ofs)
{
int num;
unsigned lo, hi;
struct pack_revindex *rix;
struct revindex_entry *revindex;

if (!pack_revindex_hashsz)
init_pack_revindex();
num = pack_revindex_ix(p);
if (num < 0)
die("internal error: pack revindex fubar");

rix = &pack_revindex[num];
if (!rix->revindex)
create_pack_revindex(rix);
revindex = rix->revindex;

lo = 0;
hi = p->num_objects + 1;
do {
unsigned mi = lo + (hi - lo) / 2;
if (revindex[mi].offset == ofs) {
return revindex + mi;
} else if (ofs < revindex[mi].offset)
hi = mi;
else
lo = mi + 1;
} while (lo < hi);
error("bad offset for revindex");
return NULL;
}

void discard_revindex(void)
{
if (pack_revindex_hashsz) {
int i;
for (i = 0; i < pack_revindex_hashsz; i++)
free(pack_revindex[i].revindex);
free(pack_revindex);
pack_revindex_hashsz = 0;
}
}
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