root/match-trees.c

DEFINITIONS

1. score_missing
2. score_differs
3. score_matches
4. fill_tree_desc_strict
5. base_name_entries_compare
6. score_trees
7. match_trees
8. splice_tree
9. shift_tree
10. shift_tree_by

#include "cache.h"
#include "tree.h"
#include "tree-walk.h"

static int score_missing(unsigned mode, const char *path)
{
        int score;

        if (S_ISDIR(mode))
                score = -1000;
        else if (S_ISLNK(mode))
                score = -500;
        else
                score = -50;
        return score;
}

static int score_differs(unsigned mode1, unsigned mode2, const char *path)
{
        int score;

        if (S_ISDIR(mode1) != S_ISDIR(mode2))
                score = -100;
        else if (S_ISLNK(mode1) != S_ISLNK(mode2))
                score = -50;
        else
                score = -5;
        return score;
}

static int score_matches(unsigned mode1, unsigned mode2, const char *path)
{
        int score;

        /* Heh, we found SHA-1 collisions between different kind of objects */
        if (S_ISDIR(mode1) != S_ISDIR(mode2))
                score = -100;
        else if (S_ISLNK(mode1) != S_ISLNK(mode2))
                score = -50;

        else if (S_ISDIR(mode1))
                score = 1000;
        else if (S_ISLNK(mode1))
                score = 500;
        else
                score = 250;
        return score;
}

static void *fill_tree_desc_strict(struct tree_desc *desc,
                                   const unsigned char *hash)
{
        void *buffer;
        enum object_type type;
        unsigned long size;

        buffer = read_sha1_file(hash, &type, &size);
        if (!buffer)
                die("unable to read tree (%s)", sha1_to_hex(hash));
        if (type != OBJ_TREE)
                die("%s is not a tree", sha1_to_hex(hash));
        init_tree_desc(desc, buffer, size);
        return buffer;
}

static int base_name_entries_compare(const struct name_entry *a,
                                     const struct name_entry *b)
{
        return base_name_compare(a->path, tree_entry_len(a), a->mode,
                                 b->path, tree_entry_len(b), b->mode);
}

/*
 * Inspect two trees, and give a score that tells how similar they are.
 */
static int score_trees(const unsigned char *hash1, const unsigned char *hash2)
{
        struct tree_desc one;
        struct tree_desc two;
        void *one_buf = fill_tree_desc_strict(&one, hash1);
        void *two_buf = fill_tree_desc_strict(&two, hash2);
        int score = 0;

        for (;;) {
                struct name_entry e1, e2;
                int got_entry_from_one = tree_entry(&one, &e1);
                int got_entry_from_two = tree_entry(&two, &e2);
                int cmp;

                if (got_entry_from_one && got_entry_from_two)
                        cmp = base_name_entries_compare(&e1, &e2);
                else if (got_entry_from_one)
                        /* two lacks this entry */
                        cmp = -1;
                else if (got_entry_from_two)
                        /* two has more entries */
                        cmp = 1;
                else
                        break;

                if (cmp < 0)
                        /* path1 does not appear in two */
                        score += score_missing(e1.mode, e1.path);
                else if (cmp > 0)
                        /* path2 does not appear in one */
                        score += score_missing(e2.mode, e2.path);
                else if (hashcmp(e1.sha1, e2.sha1))
                        /* they are different */
                        score += score_differs(e1.mode, e2.mode, e1.path);
                else
                        /* same subtree or blob */
                        score += score_matches(e1.mode, e2.mode, e1.path);
        }
        free(one_buf);
        free(two_buf);
        return score;
}

/*
 * Match one itself and its subtrees with two and pick the best match.
 */
static void match_trees(const unsigned char *hash1,
                        const unsigned char *hash2,
                        int *best_score,
                        char **best_match,
                        const char *base,
                        int recurse_limit)
{
        struct tree_desc one;
        void *one_buf = fill_tree_desc_strict(&one, hash1);

        while (one.size) {
                const char *path;
                const unsigned char *elem;
                unsigned mode;
                int score;

                elem = tree_entry_extract(&one, &path, &mode);
                if (!S_ISDIR(mode))
                        goto next;
                score = score_trees(elem, hash2);
                if (*best_score < score) {
                        free(*best_match);
                        *best_match = xstrfmt("%s%s", base, path);
                        *best_score = score;
                }
                if (recurse_limit) {
                        char *newbase = xstrfmt("%s%s/", base, path);
                        match_trees(elem, hash2, best_score, best_match,
                                    newbase, recurse_limit - 1);
                        free(newbase);
                }

        next:
                update_tree_entry(&one);
        }
        free(one_buf);
}

/*
 * A tree "hash1" has a subdirectory at "prefix".  Come up with a
 * tree object by replacing it with another tree "hash2".
 */
static int splice_tree(const unsigned char *hash1,
                       const char *prefix,
                       const unsigned char *hash2,
                       unsigned char *result)
{
        char *subpath;
        int toplen;
        char *buf;
        unsigned long sz;
        struct tree_desc desc;
        unsigned char *rewrite_here;
        const unsigned char *rewrite_with;
        unsigned char subtree[20];
        enum object_type type;
        int status;

        subpath = strchrnul(prefix, '/');
        toplen = subpath - prefix;
        if (*subpath)
                subpath++;

        buf = read_sha1_file(hash1, &type, &sz);
        if (!buf)
                die("cannot read tree %s", sha1_to_hex(hash1));
        init_tree_desc(&desc, buf, sz);

        rewrite_here = NULL;
        while (desc.size) {
                const char *name;
                unsigned mode;
                const unsigned char *sha1;

                sha1 = tree_entry_extract(&desc, &name, &mode);
                if (strlen(name) == toplen &&
                    !memcmp(name, prefix, toplen)) {
                        if (!S_ISDIR(mode))
                                die("entry %s in tree %s is not a tree",
                                    name, sha1_to_hex(hash1));
                        rewrite_here = (unsigned char *) sha1;
                        break;
                }
                update_tree_entry(&desc);
        }
        if (!rewrite_here)
                die("entry %.*s not found in tree %s",
                    toplen, prefix, sha1_to_hex(hash1));
        if (*subpath) {
                status = splice_tree(rewrite_here, subpath, hash2, subtree);
                if (status)
                        return status;
                rewrite_with = subtree;
        }
        else
                rewrite_with = hash2;
        hashcpy(rewrite_here, rewrite_with);
        status = write_sha1_file(buf, sz, tree_type, result);
        free(buf);
        return status;
}

/*
 * We are trying to come up with a merge between one and two that
 * results in a tree shape similar to one.  The tree two might
 * correspond to a subtree of one, in which case it needs to be
 * shifted down by prefixing otherwise empty directories.  On the
 * other hand, it could cover tree one and we might need to pick a
 * subtree of it.
 */
void shift_tree(const unsigned char *hash1,
                const unsigned char *hash2,
                unsigned char *shifted,
                int depth_limit)
{
        char *add_prefix;
        char *del_prefix;
        int add_score, del_score;

        /*
         * NEEDSWORK: this limits the recursion depth to hardcoded
         * value '2' to avoid excessive overhead.
         */
        if (!depth_limit)
                depth_limit = 2;

        add_score = del_score = score_trees(hash1, hash2);
        add_prefix = xcalloc(1, 1);
        del_prefix = xcalloc(1, 1);

        /*
         * See if one's subtree resembles two; if so we need to prefix
         * two with a few fake trees to match the prefix.
         */
        match_trees(hash1, hash2, &add_score, &add_prefix, "", depth_limit);

        /*
         * See if two's subtree resembles one; if so we need to
         * pick only subtree of two.
         */
        match_trees(hash2, hash1, &del_score, &del_prefix, "", depth_limit);

        /* Assume we do not have to do any shifting */
        hashcpy(shifted, hash2);

        if (add_score < del_score) {
                /* We need to pick a subtree of two */
                unsigned mode;

                if (!*del_prefix)
                        return;

                if (get_tree_entry(hash2, del_prefix, shifted, &mode))
                        die("cannot find path %s in tree %s",
                            del_prefix, sha1_to_hex(hash2));
                return;
        }

        if (!*add_prefix)
                return;

        splice_tree(hash1, add_prefix, hash2, shifted);
}

/*
 * The user says the trees will be shifted by this much.
 * Unfortunately we cannot fundamentally tell which one to
 * be prefixed, as recursive merge can work in either direction.
 */
void shift_tree_by(const unsigned char *hash1,
                   const unsigned char *hash2,
                   unsigned char *shifted,
                   const char *shift_prefix)
{
        unsigned char sub1[20], sub2[20];
        unsigned mode1, mode2;
        unsigned candidate = 0;

        /* Can hash2 be a tree at shift_prefix in tree hash1? */
        if (!get_tree_entry(hash1, shift_prefix, sub1, &mode1) &&
            S_ISDIR(mode1))
                candidate |= 1;

        /* Can hash1 be a tree at shift_prefix in tree hash2? */
        if (!get_tree_entry(hash2, shift_prefix, sub2, &mode2) &&
            S_ISDIR(mode2))
                candidate |= 2;

        if (candidate == 3) {
                /* Both are plausible -- we need to evaluate the score */
                int best_score = score_trees(hash1, hash2);
                int score;

                candidate = 0;
                score = score_trees(sub1, hash2);
                if (score > best_score) {
                        candidate = 1;
                        best_score = score;
                }
                score = score_trees(sub2, hash1);
                if (score > best_score)
                        candidate = 2;
        }

        if (!candidate) {
                /* Neither is plausible -- do not shift */
                hashcpy(shifted, hash2);
                return;
        }

        if (candidate == 1)
                /*
                 * shift tree2 down by adding shift_prefix above it
                 * to match tree1.
                 */
                splice_tree(hash1, shift_prefix, hash2, shifted);
        else
                /*
                 * shift tree2 up by removing shift_prefix from it
                 * to match tree1.
                 */
                hashcpy(shifted, sub2);
}