simple euristic for further free packing improvements

[git/dscho.git] / pack-objects.c

#include "cache.h"
#include "object.h"
#include "blob.h"
#include "commit.h"
#include "tag.h"
#include "tree.h"
#include "delta.h"
#include "pack.h"
#include "csum-file.h"
#include "tree-walk.h"
#include <sys/time.h>
#include <signal.h>
#include <stdint.h>

static const char pack_usage[] = "git-pack-objects [-q] [--no-reuse-delta] [--non-empty] [--local] [--incremental] [--window=N] [--depth=N] {--stdout | base-name} < object-list";

struct object_entry {
        unsigned char sha1[20];
        unsigned long size;     /* uncompressed size */
        unsigned long offset;   /* offset into the final pack file;
                                 * nonzero if already written.
                                 */
        unsigned int depth;     /* delta depth */
        unsigned int delta_limit;       /* base adjustment for in-pack delta */
        unsigned int hash;      /* name hint hash */
        enum object_type type;
        enum object_type in_pack_type;  /* could be delta */
        unsigned long delta_size;       /* delta data size (uncompressed) */
        struct object_entry *delta;     /* delta base object */
        struct packed_git *in_pack;     /* already in pack */
        unsigned int in_pack_offset;
        struct object_entry *delta_child; /* delitified objects who bases me */
        struct object_entry *delta_sibling; /* other deltified objects who
                                             * uses the same base as me
                                             */
        int preferred_base;     /* we do not pack this, but is encouraged to
                                 * be used as the base objectto delta huge
                                 * objects against.
                                 */
};

/*
 * Objects we are going to pack are colected in objects array (dynamically
 * expanded).  nr_objects & nr_alloc controls this array.  They are stored
 * in the order we see -- typically rev-list --objects order that gives us
 * nice "minimum seek" order.
 *
 * sorted-by-sha ans sorted-by-type are arrays of pointers that point at
 * elements in the objects array.  The former is used to build the pack
 * index (lists object names in the ascending order to help offset lookup),
 * and the latter is used to group similar things together by try_delta()
 * heuristics.
 */

static unsigned char object_list_sha1[20];
static int non_empty = 0;
static int no_reuse_delta = 0;
static int local = 0;
static int incremental = 0;
static struct object_entry **sorted_by_sha, **sorted_by_type;
static struct object_entry *objects = NULL;
static int nr_objects = 0, nr_alloc = 0, nr_result = 0;
static const char *base_name;
static unsigned char pack_file_sha1[20];
static int progress = 1;
static volatile sig_atomic_t progress_update = 0;

/*
 * The object names in objects array are hashed with this hashtable,
 * to help looking up the entry by object name.  Binary search from
 * sorted_by_sha is also possible but this was easier to code and faster.
 * This hashtable is built after all the objects are seen.
 */
static int *object_ix = NULL;
static int object_ix_hashsz = 0;

/*
 * 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).  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 an ordered list of offsets, so if you
 * know the offset of an object, next offset is where its packed
 * representation ends.
 */
struct pack_revindex {
        struct packed_git *p;
        unsigned long *revindex;
} *pack_revindex = NULL;
static int pack_revindex_hashsz = 0;

/*
 * stats
 */
static int written = 0;
static int written_delta = 0;
static int reused = 0;
static int reused_delta = 0;

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 prepare_pack_ix(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 */
}

static int cmp_offset(const void *a_, const void *b_)
{
        unsigned long a = *(unsigned long *) a_;
        unsigned long b = *(unsigned long *) b_;
        if (a < b)
                return -1;
        else if (a == b)
                return 0;
        else
                return 1;
}

/*
 * Ordered list of offsets of objects in the pack.
 */
static void prepare_pack_revindex(struct pack_revindex *rix)
{
        struct packed_git *p = rix->p;
        int num_ent = num_packed_objects(p);
        int i;
        void *index = p->index_base + 256;

        rix->revindex = xmalloc(sizeof(unsigned long) * (num_ent + 1));
        for (i = 0; i < num_ent; i++) {
                uint32_t hl = *((uint32_t *)(index + 24 * i));
                rix->revindex[i] = ntohl(hl);
        }
        /* This knows the pack format -- the 20-byte trailer
         * follows immediately after the last object data.
         */
        rix->revindex[num_ent] = p->pack_size - 20;
        qsort(rix->revindex, num_ent, sizeof(unsigned long), cmp_offset);
}

static unsigned long find_packed_object_size(struct packed_git *p,
                                             unsigned long ofs)
{
        int num;
        int lo, hi;
        struct pack_revindex *rix;
        unsigned long *revindex;
        num = pack_revindex_ix(p);
        if (num < 0)
                die("internal error: pack revindex uninitialized");
        rix = &pack_revindex[num];
        if (!rix->revindex)
                prepare_pack_revindex(rix);
        revindex = rix->revindex;
        lo = 0;
        hi = num_packed_objects(p) + 1;
        do {
                int mi = (lo + hi) / 2;
                if (revindex[mi] == ofs) {
                        return revindex[mi+1] - ofs;
                }
                else if (ofs < revindex[mi])
                        hi = mi;
                else
                        lo = mi + 1;
        } while (lo < hi);
        die("internal error: pack revindex corrupt");
}

static void *delta_against(void *buf, unsigned long size, struct object_entry *entry)
{
        unsigned long othersize, delta_size;
        char type[10];
        void *otherbuf = read_sha1_file(entry->delta->sha1, type, &othersize);
        void *delta_buf;

        if (!otherbuf)
                die("unable to read %s", sha1_to_hex(entry->delta->sha1));
        delta_buf = diff_delta(otherbuf, othersize,
                               buf, size, &delta_size, 0);
        if (!delta_buf || delta_size != entry->delta_size)
                die("delta size changed");
        free(buf);
        free(otherbuf);
        return delta_buf;
}

/*
 * The per-object header is a pretty dense thing, which is
 *  - first byte: low four bits are "size", then three bits of "type",
 *    and the high bit is "size continues".
 *  - each byte afterwards: low seven bits are size continuation,
 *    with the high bit being "size continues"
 */
static int encode_header(enum object_type type, unsigned long size, unsigned char *hdr)
{
        int n = 1;
        unsigned char c;

        if (type < OBJ_COMMIT || type > OBJ_DELTA)
                die("bad type %d", type);

        c = (type << 4) | (size & 15);
        size >>= 4;
        while (size) {
                *hdr++ = c | 0x80;
                c = size & 0x7f;
                size >>= 7;
                n++;
        }
        *hdr = c;
        return n;
}

static unsigned long write_object(struct sha1file *f,
                                  struct object_entry *entry)
{
        unsigned long size;
        char type[10];
        void *buf;
        unsigned char header[10];
        unsigned hdrlen, datalen;
        enum object_type obj_type;
        int to_reuse = 0;

        if (entry->preferred_base)
                return 0;

        obj_type = entry->type;
        if (! entry->in_pack)
                to_reuse = 0;   /* can't reuse what we don't have */
        else if (obj_type == OBJ_DELTA)
                to_reuse = 1;   /* check_object() decided it for us */
        else if (obj_type != entry->in_pack_type)
                to_reuse = 0;   /* pack has delta which is unusable */
        else if (entry->delta)
                to_reuse = 0;   /* we want to pack afresh */
        else
                to_reuse = 1;   /* we have it in-pack undeltified,
                                 * and we do not need to deltify it.
                                 */

        if (! to_reuse) {
                buf = read_sha1_file(entry->sha1, type, &size);
                if (!buf)
                        die("unable to read %s", sha1_to_hex(entry->sha1));
                if (size != entry->size)
                        die("object %s size inconsistency (%lu vs %lu)",
                            sha1_to_hex(entry->sha1), size, entry->size);
                if (entry->delta) {
                        buf = delta_against(buf, size, entry);
                        size = entry->delta_size;
                        obj_type = OBJ_DELTA;
                }
                /*
                 * The object header is a byte of 'type' followed by zero or
                 * more bytes of length.  For deltas, the 20 bytes of delta
                 * sha1 follows that.
                 */
                hdrlen = encode_header(obj_type, size, header);
                sha1write(f, header, hdrlen);

                if (entry->delta) {
                        sha1write(f, entry->delta, 20);
                        hdrlen += 20;
                }
                datalen = sha1write_compressed(f, buf, size);
                free(buf);
        }
        else {
                struct packed_git *p = entry->in_pack;
                use_packed_git(p);

                datalen = find_packed_object_size(p, entry->in_pack_offset);
                buf = p->pack_base + entry->in_pack_offset;
                sha1write(f, buf, datalen);
                unuse_packed_git(p);
                hdrlen = 0; /* not really */
                if (obj_type == OBJ_DELTA)
                        reused_delta++;
                reused++;
        }
        if (obj_type == OBJ_DELTA)
                written_delta++;
        written++;
        return hdrlen + datalen;
}

static unsigned long write_one(struct sha1file *f,
                               struct object_entry *e,
                               unsigned long offset)
{
        if (e->offset)
                /* offset starts from header size and cannot be zero
                 * if it is written already.
                 */
                return offset;
        e->offset = offset;
        offset += write_object(f, e);
        /* if we are deltified, write out its base object. */
        if (e->delta)
                offset = write_one(f, e->delta, offset);
        return offset;
}

static void write_pack_file(void)
{
        int i;
        struct sha1file *f;
        unsigned long offset;
        struct pack_header hdr;
        unsigned last_percent = 999;
        int do_progress = 0;

        if (!base_name)
                f = sha1fd(1, "<stdout>");
        else {
                f = sha1create("%s-%s.%s", base_name,
                               sha1_to_hex(object_list_sha1), "pack");
                do_progress = progress;
        }
        if (do_progress)
                fprintf(stderr, "Writing %d objects.\n", nr_result);

        hdr.hdr_signature = htonl(PACK_SIGNATURE);
        hdr.hdr_version = htonl(PACK_VERSION);
        hdr.hdr_entries = htonl(nr_result);
        sha1write(f, &hdr, sizeof(hdr));
        offset = sizeof(hdr);
        if (!nr_result)
                goto done;
        for (i = 0; i < nr_objects; i++) {
                offset = write_one(f, objects + i, offset);
                if (do_progress) {
                        unsigned percent = written * 100 / nr_result;
                        if (progress_update || percent != last_percent) {
                                fprintf(stderr, "%4u%% (%u/%u) done\r",
                                        percent, written, nr_result);
                                progress_update = 0;
                                last_percent = percent;
                        }
                }
        }
        if (do_progress)
                fputc('\n', stderr);
 done:
        sha1close(f, pack_file_sha1, 1);
}

static void write_index_file(void)
{
        int i;
        struct sha1file *f = sha1create("%s-%s.%s", base_name,
                                        sha1_to_hex(object_list_sha1), "idx");
        struct object_entry **list = sorted_by_sha;
        struct object_entry **last = list + nr_result;
        unsigned int array[256];

        /*
         * Write the first-level table (the list is sorted,
         * but we use a 256-entry lookup to be able to avoid
         * having to do eight extra binary search iterations).
         */
        for (i = 0; i < 256; i++) {
                struct object_entry **next = list;
                while (next < last) {
                        struct object_entry *entry = *next;
                        if (entry->sha1[0] != i)
                                break;
                        next++;
                }
                array[i] = htonl(next - sorted_by_sha);
                list = next;
        }
        sha1write(f, array, 256 * sizeof(int));

        /*
         * Write the actual SHA1 entries..
         */
        list = sorted_by_sha;
        for (i = 0; i < nr_result; i++) {
                struct object_entry *entry = *list++;
                unsigned int offset = htonl(entry->offset);
                sha1write(f, &offset, 4);
                sha1write(f, entry->sha1, 20);
        }
        sha1write(f, pack_file_sha1, 20);
        sha1close(f, NULL, 1);
}

static int locate_object_entry_hash(const unsigned char *sha1)
{
        int i;
        unsigned int ui;
        memcpy(&ui, sha1, sizeof(unsigned int));
        i = ui % object_ix_hashsz;
        while (0 < object_ix[i]) {
                if (!memcmp(sha1, objects[object_ix[i]-1].sha1, 20))
                        return i;
                if (++i == object_ix_hashsz)
                        i = 0;
        }
        return -1 - i;
}

static struct object_entry *locate_object_entry(const unsigned char *sha1)
{
        int i;

        if (!object_ix_hashsz)
                return NULL;

        i = locate_object_entry_hash(sha1);
        if (0 <= i)
                return &objects[object_ix[i]-1];
        return NULL;
}

static void rehash_objects(void)
{
        int i;
        struct object_entry *oe;

        object_ix_hashsz = nr_objects * 3;
        if (object_ix_hashsz < 1024)
                object_ix_hashsz = 1024;
        object_ix = xrealloc(object_ix, sizeof(int) * object_ix_hashsz);
        memset(object_ix, 0, sizeof(int) * object_ix_hashsz);
        for (i = 0, oe = objects; i < nr_objects; i++, oe++) {
                int ix = locate_object_entry_hash(oe->sha1);
                if (0 <= ix)
                        continue;
                ix = -1 - ix;
                object_ix[ix] = i + 1;
        }
}

struct name_path {
        struct name_path *up;
        const char *elem;
        int len;
};

#define DIRBITS 12

static unsigned name_hash(struct name_path *path, const char *name)
{
        struct name_path *p = path;
        const char *n = name + strlen(name);
        unsigned hash = 0, name_hash = 0, name_done = 0;

        if (n != name && n[-1] == '\n')
                n--;
        while (name <= --n) {
                unsigned char c = *n;
                if (c == '/' && !name_done) {
                        name_hash = hash;
                        name_done = 1;
                        hash = 0;
                }
                hash = hash * 11 + c;
        }
        if (!name_done) {
                name_hash = hash;
                hash = 0;
        }
        for (p = path; p; p = p->up) {
                hash = hash * 11 + '/';
                n = p->elem + p->len;
                while (p->elem <= --n) {
                        unsigned char c = *n;
                        hash = hash * 11 + c;
                }
        }
        /*
         * Make sure "Makefile" and "t/Makefile" are hashed separately
         * but close enough.
         */
        hash = (name_hash<<DIRBITS) | (hash & ((1U<<DIRBITS )-1));
        return hash;
}

static int add_object_entry(const unsigned char *sha1, unsigned hash, int exclude)
{
        unsigned int idx = nr_objects;
        struct object_entry *entry;
        struct packed_git *p;
        unsigned int found_offset = 0;
        struct packed_git *found_pack = NULL;
        int ix, status = 0;

        if (!exclude) {
                for (p = packed_git; p; p = p->next) {
                        struct pack_entry e;
                        if (find_pack_entry_one(sha1, &e, p)) {
                                if (incremental)
                                        return 0;
                                if (local && !p->pack_local)
                                        return 0;
                                if (!found_pack) {
                                        found_offset = e.offset;
                                        found_pack = e.p;
                                }
                        }
                }
        }
        if ((entry = locate_object_entry(sha1)) != NULL)
                goto already_added;

        if (idx >= nr_alloc) {
                unsigned int needed = (idx + 1024) * 3 / 2;
                objects = xrealloc(objects, needed * sizeof(*entry));
                nr_alloc = needed;
        }
        entry = objects + idx;
        nr_objects = idx + 1;
        memset(entry, 0, sizeof(*entry));
        memcpy(entry->sha1, sha1, 20);
        entry->hash = hash;

        if (object_ix_hashsz * 3 <= nr_objects * 4)
                rehash_objects();
        else {
                ix = locate_object_entry_hash(entry->sha1);
                if (0 <= ix)
                        die("internal error in object hashing.");
                object_ix[-1 - ix] = idx + 1;
        }
        status = 1;

 already_added:
        if (progress_update) {
                fprintf(stderr, "Counting objects...%d\r", nr_objects);
                progress_update = 0;
        }
        if (exclude)
                entry->preferred_base = 1;
        else {
                if (found_pack) {
                        entry->in_pack = found_pack;
                        entry->in_pack_offset = found_offset;
                }
        }
        return status;
}

struct pbase_tree_cache {
        unsigned char sha1[20];
        int ref;
        int temporary;
        void *tree_data;
        unsigned long tree_size;
};

static struct pbase_tree_cache *(pbase_tree_cache[256]);
static int pbase_tree_cache_ix(const unsigned char *sha1)
{
        return sha1[0] % ARRAY_SIZE(pbase_tree_cache);
}
static int pbase_tree_cache_ix_incr(int ix)
{
        return (ix+1) % ARRAY_SIZE(pbase_tree_cache);
}

static struct pbase_tree {
        struct pbase_tree *next;
        /* This is a phony "cache" entry; we are not
         * going to evict it nor find it through _get()
         * mechanism -- this is for the toplevel node that
         * would almost always change with any commit.
         */
        struct pbase_tree_cache pcache;
} *pbase_tree;

static struct pbase_tree_cache *pbase_tree_get(const unsigned char *sha1)
{
        struct pbase_tree_cache *ent, *nent;
        void *data;
        unsigned long size;
        char type[20];
        int neigh;
        int my_ix = pbase_tree_cache_ix(sha1);
        int available_ix = -1;

        /* pbase-tree-cache acts as a limited hashtable.
         * your object will be found at your index or within a few
         * slots after that slot if it is cached.
         */
        for (neigh = 0; neigh < 8; neigh++) {
                ent = pbase_tree_cache[my_ix];
                if (ent && !memcmp(ent->sha1, sha1, 20)) {
                        ent->ref++;
                        return ent;
                }
                else if (((available_ix < 0) && (!ent || !ent->ref)) ||
                         ((0 <= available_ix) &&
                          (!ent && pbase_tree_cache[available_ix])))
                        available_ix = my_ix;
                if (!ent)
                        break;
                my_ix = pbase_tree_cache_ix_incr(my_ix);
        }

        /* Did not find one.  Either we got a bogus request or
         * we need to read and perhaps cache.
         */
        data = read_sha1_file(sha1, type, &size);
        if (!data)
                return NULL;
        if (strcmp(type, tree_type)) {
                free(data);
                return NULL;
        }

        /* We need to either cache or return a throwaway copy */

        if (available_ix < 0)
                ent = NULL;
        else {
                ent = pbase_tree_cache[available_ix];
                my_ix = available_ix;
        }

        if (!ent) {
                nent = xmalloc(sizeof(*nent));
                nent->temporary = (available_ix < 0);
        }
        else {
                /* evict and reuse */
                free(ent->tree_data);
                nent = ent;
        }
        memcpy(nent->sha1, sha1, 20);
        nent->tree_data = data;
        nent->tree_size = size;
        nent->ref = 1;
        if (!nent->temporary)
                pbase_tree_cache[my_ix] = nent;
        return nent;
}

static void pbase_tree_put(struct pbase_tree_cache *cache)
{
        if (!cache->temporary) {
                cache->ref--;
                return;
        }
        free(cache->tree_data);
        free(cache);
}

static int name_cmp_len(const char *name)
{
        int i;
        for (i = 0; name[i] && name[i] != '\n' && name[i] != '/'; i++)
                ;
        return i;
}

static void add_pbase_object(struct tree_desc *tree,
                             struct name_path *up,
                             const char *name,
                             int cmplen)
{
        while (tree->size) {
                const unsigned char *sha1;
                const char *entry_name;
                int entry_len;
                unsigned mode;
                unsigned long size;
                char type[20];

                sha1 = tree_entry_extract(tree, &entry_name, &mode);
                update_tree_entry(tree);
                entry_len = strlen(entry_name);
                if (entry_len != cmplen ||
                    memcmp(entry_name, name, cmplen) ||
                    !has_sha1_file(sha1) ||
                    sha1_object_info(sha1, type, &size))
                        continue;
                if (name[cmplen] != '/') {
                        unsigned hash = name_hash(up, name);
                        add_object_entry(sha1, hash, 1);
                        return;
                }
                if (!strcmp(type, tree_type)) {
                        struct tree_desc sub;
                        struct name_path me;
                        struct pbase_tree_cache *tree;
                        const char *down = name+cmplen+1;
                        int downlen = name_cmp_len(down);

                        tree = pbase_tree_get(sha1);
                        if (!tree)
                                return;
                        sub.buf = tree->tree_data;
                        sub.size = tree->tree_size;

                        me.up = up;
                        me.elem = entry_name;
                        me.len = entry_len;
                        add_pbase_object(&sub, &me, down, downlen);
                        pbase_tree_put(tree);
                }
        }
}

static unsigned *done_pbase_paths;
static int done_pbase_paths_num;
static int done_pbase_paths_alloc;
static int done_pbase_path_pos(unsigned hash)
{
        int lo = 0;
        int hi = done_pbase_paths_num;
        while (lo < hi) {
                int mi = (hi + lo) / 2;
                if (done_pbase_paths[mi] == hash)
                        return mi;
                if (done_pbase_paths[mi] < hash)
                        hi = mi;
                else
                        lo = mi + 1;
        }
        return -lo-1;
}

static int check_pbase_path(unsigned hash)
{
        int pos = (!done_pbase_paths) ? -1 : done_pbase_path_pos(hash);
        if (0 <= pos)
                return 1;
        pos = -pos - 1;
        if (done_pbase_paths_alloc <= done_pbase_paths_num) {
                done_pbase_paths_alloc = alloc_nr(done_pbase_paths_alloc);
                done_pbase_paths = xrealloc(done_pbase_paths,
                                            done_pbase_paths_alloc *
                                            sizeof(unsigned));
        }
        done_pbase_paths_num++;
        if (pos < done_pbase_paths_num)
                memmove(done_pbase_paths + pos + 1,
                        done_pbase_paths + pos,
                        (done_pbase_paths_num - pos - 1) * sizeof(unsigned));
        done_pbase_paths[pos] = hash;
        return 0;
}

static void add_preferred_base_object(char *name, unsigned hash)
{
        struct pbase_tree *it;
        int cmplen = name_cmp_len(name);

        if (check_pbase_path(hash))
                return;

        for (it = pbase_tree; it; it = it->next) {
                if (cmplen == 0) {
                        hash = name_hash(NULL, "");
                        add_object_entry(it->pcache.sha1, hash, 1);
                }
                else {
                        struct tree_desc tree;
                        tree.buf = it->pcache.tree_data;
                        tree.size = it->pcache.tree_size;
                        add_pbase_object(&tree, NULL, name, cmplen);
                }
        }
}

static void add_preferred_base(unsigned char *sha1)
{
        struct pbase_tree *it;
        void *data;
        unsigned long size;
        unsigned char tree_sha1[20];

        data = read_object_with_reference(sha1, tree_type, &size, tree_sha1);
        if (!data)
                return;

        for (it = pbase_tree; it; it = it->next) {
                if (!memcmp(it->pcache.sha1, tree_sha1, 20)) {
                        free(data);
                        return;
                }
        }

        it = xcalloc(1, sizeof(*it));
        it->next = pbase_tree;
        pbase_tree = it;

        memcpy(it->pcache.sha1, tree_sha1, 20);
        it->pcache.tree_data = data;
        it->pcache.tree_size = size;
}

static void check_object(struct object_entry *entry)
{
        char type[20];

        if (entry->in_pack && !entry->preferred_base) {
                unsigned char base[20];
                unsigned long size;
                struct object_entry *base_entry;

                /* We want in_pack_type even if we do not reuse delta.
                 * There is no point not reusing non-delta representations.
                 */
                check_reuse_pack_delta(entry->in_pack,
                                       entry->in_pack_offset,
                                       base, &size,
                                       &entry->in_pack_type);

                /* Check if it is delta, and the base is also an object
                 * we are going to pack.  If so we will reuse the existing
                 * delta.
                 */
                if (!no_reuse_delta &&
                    entry->in_pack_type == OBJ_DELTA &&
                    (base_entry = locate_object_entry(base)) &&
                    (!base_entry->preferred_base)) {

                        /* Depth value does not matter - find_deltas()
                         * will never consider reused delta as the
                         * base object to deltify other objects
                         * against, in order to avoid circular deltas.
                         */

                        /* uncompressed size of the delta data */
                        entry->size = entry->delta_size = size;
                        entry->delta = base_entry;
                        entry->type = OBJ_DELTA;

                        entry->delta_sibling = base_entry->delta_child;
                        base_entry->delta_child = entry;

                        return;
                }
                /* Otherwise we would do the usual */
        }

        if (sha1_object_info(entry->sha1, type, &entry->size))
                die("unable to get type of object %s",
                    sha1_to_hex(entry->sha1));

        if (!strcmp(type, commit_type)) {
                entry->type = OBJ_COMMIT;
        } else if (!strcmp(type, tree_type)) {
                entry->type = OBJ_TREE;
        } else if (!strcmp(type, blob_type)) {
                entry->type = OBJ_BLOB;
        } else if (!strcmp(type, tag_type)) {
                entry->type = OBJ_TAG;
        } else
                die("unable to pack object %s of type %s",
                    sha1_to_hex(entry->sha1), type);
}

static unsigned int check_delta_limit(struct object_entry *me, unsigned int n)
{
        struct object_entry *child = me->delta_child;
        unsigned int m = n;
        while (child) {
                unsigned int c = check_delta_limit(child, n + 1);
                if (m < c)
                        m = c;
                child = child->delta_sibling;
        }
        return m;
}

static void get_object_details(void)
{
        int i;
        struct object_entry *entry;

        prepare_pack_ix();
        for (i = 0, entry = objects; i < nr_objects; i++, entry++)
                check_object(entry);

        if (nr_objects == nr_result) {
                /*
                 * Depth of objects that depend on the entry -- this
                 * is subtracted from depth-max to break too deep
                 * delta chain because of delta data reusing.
                 * However, we loosen this restriction when we know we
                 * are creating a thin pack -- it will have to be
                 * expanded on the other end anyway, so do not
                 * artificially cut the delta chain and let it go as
                 * deep as it wants.
                 */
                for (i = 0, entry = objects; i < nr_objects; i++, entry++)
                        if (!entry->delta && entry->delta_child)
                                entry->delta_limit =
                                        check_delta_limit(entry, 1);
        }
}

typedef int (*entry_sort_t)(const struct object_entry *, const struct object_entry *);

static entry_sort_t current_sort;

static int sort_comparator(const void *_a, const void *_b)
{
        struct object_entry *a = *(struct object_entry **)_a;
        struct object_entry *b = *(struct object_entry **)_b;
        return current_sort(a,b);
}

static struct object_entry **create_sorted_list(entry_sort_t sort)
{
        struct object_entry **list = xmalloc(nr_objects * sizeof(struct object_entry *));
        int i;

        for (i = 0; i < nr_objects; i++)
                list[i] = objects + i;
        current_sort = sort;
        qsort(list, nr_objects, sizeof(struct object_entry *), sort_comparator);
        return list;
}

static int sha1_sort(const struct object_entry *a, const struct object_entry *b)
{
        return memcmp(a->sha1, b->sha1, 20);
}

static struct object_entry **create_final_object_list(void)
{
        struct object_entry **list;
        int i, j;

        for (i = nr_result = 0; i < nr_objects; i++)
                if (!objects[i].preferred_base)
                        nr_result++;
        list = xmalloc(nr_result * sizeof(struct object_entry *));
        for (i = j = 0; i < nr_objects; i++) {
                if (!objects[i].preferred_base)
                        list[j++] = objects + i;
        }
        current_sort = sha1_sort;
        qsort(list, nr_result, sizeof(struct object_entry *), sort_comparator);
        return list;
}

static int type_size_sort(const struct object_entry *a, const struct object_entry *b)
{
        if (a->type < b->type)
                return -1;
        if (a->type > b->type)
                return 1;
        if (a->hash < b->hash)
                return -1;
        if (a->hash > b->hash)
                return 1;
        if (a->preferred_base < b->preferred_base)
                return -1;
        if (a->preferred_base > b->preferred_base)
                return 1;
        if (a->size < b->size)
                return -1;
        if (a->size > b->size)
                return 1;
        return a < b ? -1 : (a > b);
}

struct unpacked {
        struct object_entry *entry;
        void *data;
        struct delta_index *index;
};

/*
 * We search for deltas _backwards_ in a list sorted by type and
 * by size, so that we see progressively smaller and smaller files.
 * That's because we prefer deltas to be from the bigger file
 * to the smaller - deletes are potentially cheaper, but perhaps
 * more importantly, the bigger file is likely the more recent
 * one.
 */
static int try_delta(struct unpacked *trg, struct unpacked *src,
                     struct delta_index *src_index, unsigned max_depth)
{
        struct object_entry *trg_entry = trg->entry;
        struct object_entry *src_entry = src->entry;
        unsigned long size, src_size, delta_size, sizediff, max_size;
        void *delta_buf;

        /* Don't bother doing diffs between different types */
        if (trg_entry->type != src_entry->type)
                return -1;

        /* We do not compute delta to *create* objects we are not
         * going to pack.
         */
        if (trg_entry->preferred_base)
                return -1;

        /*
         * If the current object is at pack edge, take the depth the
         * objects that depend on the current object into account --
         * otherwise they would become too deep.
         */
        if (trg_entry->delta_child) {
                if (max_depth <= trg_entry->delta_limit)
                        return 0;
                max_depth -= trg_entry->delta_limit;
        }
        if (src_entry->depth >= max_depth)
                return 0;

        /* Now some size filtering euristics. */
        size = trg_entry->size;
        max_size = (size/2 - 20) / (src_entry->depth + 1);
        if (trg_entry->delta && trg_entry->delta_size <= max_size)
                max_size = trg_entry->delta_size-1;
        src_size = src_entry->size;
        sizediff = src_size < size ? size - src_size : 0;
        if (sizediff >= max_size)
                return 0;

        delta_buf = create_delta(src_index, trg->data, size, &delta_size, max_size);
        if (!delta_buf)
                return 0;

        trg_entry->delta = src_entry;
        trg_entry->delta_size = delta_size;
        trg_entry->depth = src_entry->depth + 1;
        free(delta_buf);
        return 1;
}

static void progress_interval(int signum)
{
        progress_update = 1;
}

static void find_deltas(struct object_entry **list, int window, int depth)
{
        int i, idx;
        unsigned int array_size = window * sizeof(struct unpacked);
        struct unpacked *array = xmalloc(array_size);
        unsigned processed = 0;
        unsigned last_percent = 999;

        memset(array, 0, array_size);
        i = nr_objects;
        idx = 0;
        if (progress)
                fprintf(stderr, "Deltifying %d objects.\n", nr_result);

        while (--i >= 0) {
                struct object_entry *entry = list[i];
                struct unpacked *n = array + idx;
                unsigned long size;
                char type[10];
                int j;

                if (!entry->preferred_base)
                        processed++;

                if (progress) {
                        unsigned percent = processed * 100 / nr_result;
                        if (percent != last_percent || progress_update) {
                                fprintf(stderr, "%4u%% (%u/%u) done\r",
                                        percent, processed, nr_result);
                                progress_update = 0;
                                last_percent = percent;
                        }
                }

                if (entry->delta)
                        /* This happens if we decided to reuse existing
                         * delta from a pack.  "!no_reuse_delta &&" is implied.
                         */
                        continue;

                if (entry->size < 50)
                        continue;
                if (n->index)
                        free_delta_index(n->index);
                free(n->data);
                n->entry = entry;
                n->data = read_sha1_file(entry->sha1, type, &size);
                if (size != entry->size)
                        die("object %s inconsistent object length (%lu vs %lu)",
                            sha1_to_hex(entry->sha1), size, entry->size);
                n->index = create_delta_index(n->data, size);
                if (!n->index)
                        die("out of memory");

                j = window;
                while (--j > 0) {
                        unsigned int other_idx = idx + j;
                        struct unpacked *m;
                        if (other_idx >= window)
                                other_idx -= window;
                        m = array + other_idx;
                        if (!m->entry)
                                break;
                        if (try_delta(n, m, m->index, depth) < 0)
                                break;
                }
                /* if we made n a delta, and if n is already at max
                 * depth, leaving it in the window is pointless.  we
                 * should evict it first.
                 */
                if (entry->delta && depth <= entry->depth)
                        continue;
                idx++;
                if (idx >= window)
                        idx = 0;
        }

        if (progress)
                fputc('\n', stderr);

        for (i = 0; i < window; ++i) {
                if (array[i].index)
                        free_delta_index(array[i].index);
                free(array[i].data);
        }
        free(array);
}

static void prepare_pack(int window, int depth)
{
        get_object_details();
        sorted_by_type = create_sorted_list(type_size_sort);
        if (window && depth)
                find_deltas(sorted_by_type, window+1, depth);
}

static int reuse_cached_pack(unsigned char *sha1, int pack_to_stdout)
{
        static const char cache[] = "pack-cache/pack-%s.%s";
        char *cached_pack, *cached_idx;
        int ifd, ofd, ifd_ix = -1;

        cached_pack = git_path(cache, sha1_to_hex(sha1), "pack");
        ifd = open(cached_pack, O_RDONLY);
        if (ifd < 0)
                return 0;

        if (!pack_to_stdout) {
                cached_idx = git_path(cache, sha1_to_hex(sha1), "idx");
                ifd_ix = open(cached_idx, O_RDONLY);
                if (ifd_ix < 0) {
                        close(ifd);
                        return 0;
                }
        }

        if (progress)
                fprintf(stderr, "Reusing %d objects pack %s\n", nr_objects,
                        sha1_to_hex(sha1));

        if (pack_to_stdout) {
                if (copy_fd(ifd, 1))
                        exit(1);
                close(ifd);
        }
        else {
                char name[PATH_MAX];
                snprintf(name, sizeof(name),
                         "%s-%s.%s", base_name, sha1_to_hex(sha1), "pack");
                ofd = open(name, O_CREAT | O_EXCL | O_WRONLY, 0666);
                if (ofd < 0)
                        die("unable to open %s (%s)", name, strerror(errno));
                if (copy_fd(ifd, ofd))
                        exit(1);
                close(ifd);

                snprintf(name, sizeof(name),
                         "%s-%s.%s", base_name, sha1_to_hex(sha1), "idx");
                ofd = open(name, O_CREAT | O_EXCL | O_WRONLY, 0666);
                if (ofd < 0)
                        die("unable to open %s (%s)", name, strerror(errno));
                if (copy_fd(ifd_ix, ofd))
                        exit(1);
                close(ifd_ix);
                puts(sha1_to_hex(sha1));
        }

        return 1;
}

static void setup_progress_signal(void)
{
        struct sigaction sa;
        struct itimerval v;

        memset(&sa, 0, sizeof(sa));
        sa.sa_handler = progress_interval;
        sigemptyset(&sa.sa_mask);
        sa.sa_flags = SA_RESTART;
        sigaction(SIGALRM, &sa, NULL);

        v.it_interval.tv_sec = 1;
        v.it_interval.tv_usec = 0;
        v.it_value = v.it_interval;
        setitimer(ITIMER_REAL, &v, NULL);
}

int main(int argc, char **argv)
{
        SHA_CTX ctx;
        char line[40 + 1 + PATH_MAX + 2];
        int window = 10, depth = 10, pack_to_stdout = 0;
        struct object_entry **list;
        int num_preferred_base = 0;
        int i;

        setup_git_directory();

        progress = isatty(2);
        for (i = 1; i < argc; i++) {
                const char *arg = argv[i];

                if (*arg == '-') {
                        if (!strcmp("--non-empty", arg)) {
                                non_empty = 1;
                                continue;
                        }
                        if (!strcmp("--local", arg)) {
                                local = 1;
                                continue;
                        }
                        if (!strcmp("--incremental", arg)) {
                                incremental = 1;
                                continue;
                        }
                        if (!strncmp("--window=", arg, 9)) {
                                char *end;
                                window = strtoul(arg+9, &end, 0);
                                if (!arg[9] || *end)
                                        usage(pack_usage);
                                continue;
                        }
                        if (!strncmp("--depth=", arg, 8)) {
                                char *end;
                                depth = strtoul(arg+8, &end, 0);
                                if (!arg[8] || *end)
                                        usage(pack_usage);
                                continue;
                        }
                        if (!strcmp("--progress", arg)) {
                                progress = 1;
                                continue;
                        }
                        if (!strcmp("-q", arg)) {
                                progress = 0;
                                continue;
                        }
                        if (!strcmp("--no-reuse-delta", arg)) {
                                no_reuse_delta = 1;
                                continue;
                        }
                        if (!strcmp("--stdout", arg)) {
                                pack_to_stdout = 1;
                                continue;
                        }
                        usage(pack_usage);
                }
                if (base_name)
                        usage(pack_usage);
                base_name = arg;
        }

        if (pack_to_stdout != !base_name)
                usage(pack_usage);

        prepare_packed_git();

        if (progress) {
                fprintf(stderr, "Generating pack...\n");
                setup_progress_signal();
        }

        for (;;) {
                unsigned char sha1[20];
                unsigned hash;

                if (!fgets(line, sizeof(line), stdin)) {
                        if (feof(stdin))
                                break;
                        if (!ferror(stdin))
                                die("fgets returned NULL, not EOF, not error!");
                        if (errno != EINTR)
                                die("fgets: %s", strerror(errno));
                        clearerr(stdin);
                        continue;
                }

                if (line[0] == '-') {
                        if (get_sha1_hex(line+1, sha1))
                                die("expected edge sha1, got garbage:\n %s",
                                    line+1);
                        if (num_preferred_base++ < window)
                                add_preferred_base(sha1);
                        continue;
                }
                if (get_sha1_hex(line, sha1))
                        die("expected sha1, got garbage:\n %s", line);
                hash = name_hash(NULL, line+41);
                add_preferred_base_object(line+41, hash);
                add_object_entry(sha1, hash, 0);
        }
        if (progress)
                fprintf(stderr, "Done counting %d objects.\n", nr_objects);
        sorted_by_sha = create_final_object_list();
        if (non_empty && !nr_result)
                return 0;

        SHA1_Init(&ctx);
        list = sorted_by_sha;
        for (i = 0; i < nr_result; i++) {
                struct object_entry *entry = *list++;
                SHA1_Update(&ctx, entry->sha1, 20);
        }
        SHA1_Final(object_list_sha1, &ctx);
        if (progress && (nr_objects != nr_result))
                fprintf(stderr, "Result has %d objects.\n", nr_result);

        if (reuse_cached_pack(object_list_sha1, pack_to_stdout))
                ;
        else {
                if (nr_result)
                        prepare_pack(window, depth);
                if (progress && pack_to_stdout) {
                        /* the other end usually displays progress itself */
                        struct itimerval v = {{0,},};
                        setitimer(ITIMER_REAL, &v, NULL);
                        signal(SIGALRM, SIG_IGN );
                        progress_update = 0;
                }
                write_pack_file();
                if (!pack_to_stdout) {
                        write_index_file();
                        puts(sha1_to_hex(object_list_sha1));
                }
        }
        if (progress)
                fprintf(stderr, "Total %d, written %d (delta %d), reused %d (delta %d)\n",
                        nr_result, written, written_delta, reused, reused_delta);
        return 0;
}