#include "cache.h"
#include "diff.h"

static const char merge_tree_usage[] = "git-merge-tree <base-tree> <branch1> <branch2>";
static int resolve_directories = 1;

static void merge_trees(struct tree_desc t[3], const char *base);

static void *fill_tree_descriptor(struct tree_desc *desc, const unsigned char *sha1)
{
        unsigned long size = 0;
        void *buf = NULL;

        if (sha1) {
                buf = read_object_with_reference(sha1, "tree", &size, NULL);
                if (!buf)
                        die("unable to read tree %s", sha1_to_hex(sha1));
        }
        desc->size = size;
        desc->buf = buf;
        return buf;
}

struct name_entry {
        const unsigned char *sha1;
        const char *path;
        unsigned int mode;
        int pathlen;
};

static void entry_clear(struct name_entry *a)
{
        memset(a, 0, sizeof(*a));
}

static int entry_compare(struct name_entry *a, struct name_entry *b)
{
        return base_name_compare(
                        a->path, a->pathlen, a->mode,
                        b->path, b->pathlen, b->mode);
}

static void entry_extract(struct tree_desc *t, struct name_entry *a)
{
        a->sha1 = tree_entry_extract(t, &a->path, &a->mode);
        a->pathlen = strlen(a->path);
}

/* An empty entry never compares same, not even to another empty entry */
static int same_entry(struct name_entry *a, struct name_entry *b)
{
        return  a->sha1 &&
                b->sha1 &&
                !memcmp(a->sha1, b->sha1, 20) &&
                a->mode == b->mode;
}

static const char *sha1_to_hex_zero(const unsigned char *sha1)
{
        if (sha1)
                return sha1_to_hex(sha1);
        return "0000000000000000000000000000000000000000";
}

static void resolve(const char *base, struct name_entry *branch1, struct name_entry *result)
{
        char branch1_sha1[50];

        /* If it's already branch1, don't bother showing it */
        if (!branch1)
                return;
        memcpy(branch1_sha1, sha1_to_hex_zero(branch1->sha1), 41);

        printf("0 %06o->%06o %s->%s %s%s\n",
                branch1->mode, result->mode,
                branch1_sha1, sha1_to_hex_zero(result->sha1),
                base, result->path);
}

static int unresolved_directory(const char *base, struct name_entry n[3])
{
        int baselen;
        char *newbase;
        struct name_entry *p;
        struct tree_desc t[3];
        void *buf0, *buf1, *buf2;

        if (!resolve_directories)
                return 0;
        p = n;
        if (!p->mode) {
                p++;
                if (!p->mode)
                        p++;
        }
        if (!S_ISDIR(p->mode))
                return 0;
        baselen = strlen(base);
        newbase = xmalloc(baselen + p->pathlen + 2);
        memcpy(newbase, base, baselen);
        memcpy(newbase + baselen, p->path, p->pathlen);
        memcpy(newbase + baselen + p->pathlen, "/", 2);

        buf0 = fill_tree_descriptor(t+0, n[0].sha1);
        buf1 = fill_tree_descriptor(t+1, n[1].sha1);
        buf2 = fill_tree_descriptor(t+2, n[2].sha1);
        merge_trees(t, newbase);

        free(buf0);
        free(buf1);
        free(buf2);
        free(newbase);
        return 1;
}

static void unresolved(const char *base, struct name_entry n[3])
{
        if (unresolved_directory(base, n))
                return;
        if (n[0].sha1)
                printf("1 %06o %s %s%s\n", n[0].mode, sha1_to_hex(n[0].sha1), base, n[0].path);
        if (n[1].sha1)
                printf("2 %06o %s %s%s\n", n[1].mode, sha1_to_hex(n[1].sha1), base, n[1].path);
        if (n[2].sha1)
                printf("3 %06o %s %s%s\n", n[2].mode, sha1_to_hex(n[2].sha1), base, n[2].path);
}

/*
 * Merge two trees together (t[1] and t[2]), using a common base (t[0])
 * as the origin.
 *
 * This walks the (sorted) trees in lock-step, checking every possible
 * name. Note that directories automatically sort differently from other
 * files (see "base_name_compare"), so you'll never see file/directory
 * conflicts, because they won't ever compare the same.
 *
 * IOW, if a directory changes to a filename, it will automatically be
 * seen as the directory going away, and the filename being created.
 *
 * Think of this as a three-way diff.
 *
 * The output will be either:
 *  - successful merge
 *       "0 mode sha1 filename"
 *    NOTE NOTE NOTE! FIXME! We really really need to walk the index
 *    in parallel with this too!
 * 
 *  - conflict:
 *      "1 mode sha1 filename"
 *      "2 mode sha1 filename"
 *      "3 mode sha1 filename"
 *    where not all of the 1/2/3 lines may exist, of course.
 *
 * The successful merge rules are the same as for the three-way merge
 * in git-read-tree.
 */
static void merge_trees(struct tree_desc t[3], const char *base)
{
        for (;;) {
                struct name_entry entry[3];
                unsigned int mask = 0;
                int i, last;

                last = -1;
                for (i = 0; i < 3; i++) {
                        if (!t[i].size)
                                continue;
                        entry_extract(t+i, entry+i);
                        if (last >= 0) {
                                int cmp = entry_compare(entry+i, entry+last);

                                /*
                                 * Is the new name bigger than the old one?
                                 * Ignore it
                                 */
                                if (cmp > 0)
                                        continue;
                                /*
                                 * Is the new name smaller than the old one?
                                 * Ignore all old ones
                                 */
                                if (cmp < 0)
                                        mask = 0;
                        }
                        mask |= 1u << i;
                        last = i;
                }
                if (!mask)
                        break;

                /*
                 * Update the tree entries we've walked, and clear
                 * all the unused name-entries.
                 */
                for (i = 0; i < 3; i++) {
                        if (mask & (1u << i)) {
                                update_tree_entry(t+i);
                                continue;
                        }
                        entry_clear(entry + i);
                }

                /* Same in both? */
                if (same_entry(entry+1, entry+2)) {
                        if (entry[0].sha1) {
                                resolve(base, NULL, entry+1);
                                continue;
                        }
                }

                if (same_entry(entry+0, entry+1)) {
                        if (entry[2].sha1 && !S_ISDIR(entry[2].mode)) {
                                resolve(base, entry+1, entry+2);
                                continue;
                        }
                }

                if (same_entry(entry+0, entry+2)) {
                        if (entry[1].sha1 && !S_ISDIR(entry[1].mode)) {
                                resolve(base, NULL, entry+1);
                                continue;
                        }
                }

                unresolved(base, entry);
        }
}

static void *get_tree_descriptor(struct tree_desc *desc, const char *rev)
{
        unsigned char sha1[20];
        void *buf;

        if (get_sha1(rev, sha1) < 0)
                die("unknown rev %s", rev);
        buf = fill_tree_descriptor(desc, sha1);
        if (!buf)
                die("%s is not a tree", rev);
        return buf;
}

int main(int argc, char **argv)
{
        struct tree_desc t[3];
        void *buf1, *buf2, *buf3;

        if (argc < 4)
                usage(merge_tree_usage);

        buf1 = get_tree_descriptor(t+0, argv[1]);
        buf2 = get_tree_descriptor(t+1, argv[2]);
        buf3 = get_tree_descriptor(t+2, argv[3]);
        merge_trees(t, "");
        free(buf1);
        free(buf2);
        free(buf3);
        return 0;
}