/*
 * GIT - The information manager from hell
 *
 * Copyright (C) Linus Torvalds, 2005
 */
#include "cache.h"

static int stage = 0;

static int read_one_entry(unsigned char *sha1, const char *base, int baselen, const char *pathname, unsigned mode)
{
        int len = strlen(pathname);
        unsigned int size = cache_entry_size(baselen + len);
        struct cache_entry *ce = malloc(size);

        memset(ce, 0, size);

        ce->ce_mode = create_ce_mode(mode);
        ce->ce_flags = create_ce_flags(baselen + len, stage);
        memcpy(ce->name, base, baselen);
        memcpy(ce->name + baselen, pathname, len+1);
        memcpy(ce->sha1, sha1, 20);
        return add_cache_entry(ce, 1);
}

static int read_tree(unsigned char *sha1, const char *base, int baselen)
{
        void *buffer;
        unsigned long size;
        char type[20];

        buffer = read_sha1_file(sha1, type, &size);
        if (!buffer)
                return -1;
        if (strcmp(type, "tree"))
                return -1;
        while (size) {
                int len = strlen(buffer)+1;
                unsigned char *sha1 = buffer + len;
                char *path = strchr(buffer, ' ')+1;
                unsigned int mode;

                if (size < len + 20 || sscanf(buffer, "%o", &mode) != 1)
                        return -1;

                buffer = sha1 + 20;
                size -= len + 20;

                if (S_ISDIR(mode)) {
                        int retval;
                        int pathlen = strlen(path);
                        char *newbase = malloc(baselen + 1 + pathlen);
                        memcpy(newbase, base, baselen);
                        memcpy(newbase + baselen, path, pathlen);
                        newbase[baselen + pathlen] = '/';
                        retval = read_tree(sha1, newbase, baselen + pathlen + 1);
                        free(newbase);
                        if (retval)
                                return -1;
                        continue;
                }
                if (read_one_entry(sha1, base, baselen, path, mode) < 0)
                        return -1;
        }
        return 0;
}

static int remove_lock = 0;

static void remove_lock_file(void)
{
        if (remove_lock)
                unlink(".git/index.lock");
}

static int path_matches(struct cache_entry *a, struct cache_entry *b)
{
        int len = ce_namelen(a);
        return ce_namelen(b) == len &&
                !memcmp(a->name, b->name, len);
}

static int same(struct cache_entry *a, struct cache_entry *b)
{
        return a->ce_mode == b->ce_mode && 
                !memcmp(a->sha1, b->sha1, 20);
}


/*
 * This removes all trivial merges that don't change the tree
 * and collapses them to state 0.
 *
 * _Any_ other merge is left to user policy.  That includes "both
 * created the same file", and "both removed the same file" - which are
 * trivial, but the user might still want to _note_ it. 
 */
static struct cache_entry *merge_entries(struct cache_entry *a,
                                         struct cache_entry *b,
                                         struct cache_entry *c)
{
        int len = ce_namelen(a);

        /*
         * Are they all the same filename? We won't do
         * any name merging
         */
        if (ce_namelen(b) != len ||
            ce_namelen(c) != len ||
            memcmp(a->name, b->name, len) ||
            memcmp(a->name, c->name, len))
                return NULL;

        /*
         * Ok, all three entries describe the same
         * filename, but maybe the contents or file
         * mode have changed?
         *
         * The trivial cases end up being the ones where two
         * out of three files are the same:
         *  - both destinations the same, trivially take either
         *  - one of the destination versions hasn't changed,
         *    take the other.
         *
         * The "all entries exactly the same" case falls out as
         * a special case of any of the "two same" cases.
         *
         * Here "a" is "original", and "b" and "c" are the two
         * trees we are merging.
         */
        if (same(b,c))
                return c;
        if (same(a,b))
                return c;
        if (same(a,c))
                return b;
        return NULL;
}

static void trivially_merge_cache(struct cache_entry **src, int nr)
{
        static struct cache_entry null_entry;
        struct cache_entry **dst = src;
        struct cache_entry *old = &null_entry;

        while (nr) {
                struct cache_entry *ce, *result;

                ce = src[0];

                /* We throw away original cache entries except for the stat information */
                if (!ce_stage(ce)) {
                        old = ce;
                        src++;
                        nr--;
                        active_nr--;
                        continue;
                }
                if (nr > 2 && (result = merge_entries(ce, src[1], src[2])) != NULL) {
                        /*
                         * See if we can re-use the old CE directly?
                         * That way we get the uptodate stat info.
                         */
                        if (path_matches(result, old) && same(result, old))
                                *result = *old;
                        ce = result;
                        ce->ce_flags &= ~htons(CE_STAGEMASK);
                        src += 2;
                        nr -= 2;
                        active_nr -= 2;
                }
                *dst++ = ce;
                src++;
                nr--;
        }
}

static void merge_stat_info(struct cache_entry **src, int nr)
{
        static struct cache_entry null_entry;
        struct cache_entry **dst = src;
        struct cache_entry *old = &null_entry;

        while (nr) {
                struct cache_entry *ce;

                ce = src[0];

                /* We throw away original cache entries except for the stat information */
                if (!ce_stage(ce)) {
                        old = ce;
                        src++;
                        nr--;
                        active_nr--;
                        continue;
                }
                if (path_matches(ce, old) && same(ce, old))
                        *ce = *old;
                ce->ce_flags &= ~htons(CE_STAGEMASK);
                *dst++ = ce;
                src++;
                nr--;
        }
}

int main(int argc, char **argv)
{
        int i, newfd, merge;
        unsigned char sha1[20];

        newfd = open(".git/index.lock", O_RDWR | O_CREAT | O_EXCL, 0600);
        if (newfd < 0)
                die("unable to create new cachefile");
        atexit(remove_lock_file);
        remove_lock = 1;

        merge = 0;
        for (i = 1; i < argc; i++) {
                const char *arg = argv[i];

                /* "-m" stands for "merge", meaning we start in stage 1 */
                if (!strcmp(arg, "-m")) {
                        int i;
                        if (stage)
                                usage("-m needs to come first");
                        read_cache();
                        for (i = 0; i < active_nr; i++) {
                                if (ce_stage(active_cache[i]))
                                        usage("you need to resolve your current index first");
                        }
                        stage = 1;
                        merge = 1;
                        continue;
                }
                if (get_sha1_hex(arg, sha1) < 0)
                        usage("read-tree [-m] <sha1>");
                if (stage > 3)
                        usage("can't merge more than two trees");
                if (read_tree(sha1, "", 0) < 0)
                        die("failed to unpack tree object %s", arg);
                stage++;
        }
        if (merge) {
                switch (stage) {
                case 4: /* Three-way merge */
                        trivially_merge_cache(active_cache, active_nr);
                        break;
                case 2: /* Just read a tree, merge with old cache contents */
                        merge_stat_info(active_cache, active_nr);
                        break;
                default:
                        die("just how do you expect me to merge %d trees?", stage-1);
                }
        }
        if (write_cache(newfd, active_cache, active_nr) ||
            rename(".git/index.lock", ".git/index"))
                die("unable to write new index file");
        remove_lock = 0;
        return 0;
}