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

static int stage = 0;
static int update = 0;

static int unpack_tree(unsigned char *sha1)
{
        void *buffer;
        unsigned long size;
        int ret;

        buffer = read_object_with_reference(sha1, "tree", &size, NULL);
        if (!buffer)
                return -1;
        ret = read_tree(buffer, size, stage);
        free(buffer);
        return ret;
}

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.
 */
static struct cache_entry *merge_entries(struct cache_entry *a,
                                         struct cache_entry *b,
                                         struct cache_entry *c)
{
        /*
         * 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 (a && b && c) {
                if (same(b,c))
                        return c;
                if (same(a,b))
                        return c;
                if (same(a,c))
                        return b;
        }
        return NULL;
}

/*
 * When a CE gets turned into an unmerged entry, we
 * want it to be up-to-date
 */
static void verify_uptodate(struct cache_entry *ce)
{
        struct stat st;

        if (!lstat(ce->name, &st)) {
                unsigned changed = ce_match_stat(ce, &st);
                if (!changed)
                        return;
                errno = 0;
        }
        if (errno == ENOENT)
                return;
        die("Entry '%s' not uptodate. Cannot merge.", ce->name);
}

/*
 * If the old tree contained a CE that isn't even in the
 * result, that's always a problem, regardless of whether
 * it's up-to-date or not (ie it can be a file that we
 * have updated but not committed yet).
 */
static void reject_merge(struct cache_entry *ce)
{
        die("Entry '%s' would be overwritten by merge. Cannot merge.", ce->name);
}

static int merged_entry(struct cache_entry *merge, struct cache_entry *old, struct cache_entry **dst)
{
        merge->ce_flags |= htons(CE_UPDATE);
        if (old) {
                /*
                 * See if we can re-use the old CE directly?
                 * That way we get the uptodate stat info.
                 *
                 * This also removes the UPDATE flag on
                 * a match.
                 */
                if (same(old, merge)) {
                        *merge = *old;
                } else {
                        verify_uptodate(old);
                }
        }
        merge->ce_flags &= ~htons(CE_STAGEMASK);
        *dst++ = merge;
        return 1;
}

static int threeway_merge(struct cache_entry *stages[4], struct cache_entry **dst)
{
        struct cache_entry *old = stages[0];
        struct cache_entry *a = stages[1], *b = stages[2], *c = stages[3];
        struct cache_entry *merge;
        int count;

        /*
         * If we have an entry in the index cache ("old"), then we want
         * to make sure that it matches any entries in stage 2 ("first
         * branch", aka "b").
         */
        if (old) {
                if (!b || !same(old, b))
                        return -1;
        }
        merge = merge_entries(a, b, c);
        if (merge)
                return merged_entry(merge, old, dst);
        if (old)
                verify_uptodate(old);
        count = 0;
        if (a) { *dst++ = a; count++; }
        if (b) { *dst++ = b; count++; }
        if (c) { *dst++ = c; count++; }
        return count;
}

/*
 * Two-way merge.
 *
 * The rule is to "carry forward" what is in the index without losing
 * information across a "fast forward", favoring a successful merge
 * over a merge failure when it makes sense.  For details of the
 * "carry forward" rule, please see <Documentation/git-read-tree.txt>.
 *
 */
static int twoway_merge(struct cache_entry **src, struct cache_entry **dst)
{
        struct cache_entry *current = src[0];
        struct cache_entry *oldtree = src[1], *newtree = src[2];

        if (src[3])
                return -1;

        if (current) {
                if ((!oldtree && !newtree) || /* 4 and 5 */
                    (!oldtree && newtree &&
                     same(current, newtree)) || /* 6 and 7 */
                    (oldtree && newtree &&
                     same(oldtree, newtree)) || /* 14 and 15 */
                    (oldtree && newtree &&
                     !same(oldtree, newtree) && /* 18 and 19*/
                     same(current, newtree))) {
                        *dst++ = current;
                        return 1;
                }
                else if (oldtree && !newtree && same(current, oldtree)) {
                        /* 10 or 11 */
                        verify_uptodate(current);
                        return 0;
                }
                else if (oldtree && newtree &&
                         same(current, oldtree) && !same(current, newtree)) {
                        /* 20 or 21 */
                        verify_uptodate(current);
                        return merged_entry(newtree, NULL, dst);
                }
                else
                        /* all other failures */
                        return -1;
        }
        else if (newtree)
                return merged_entry(newtree, NULL, dst);
        else
                return 0;
}

/*
 * One-way merge.
 *
 * The rule is:
 * - take the stat information from stage0, take the data from stage1
 */
static int oneway_merge(struct cache_entry **src, struct cache_entry **dst)
{
        struct cache_entry *old = src[0];
        struct cache_entry *a = src[1];

        if (src[2] || src[3])
                return -1;

        if (!a)
                return 0;
        if (old && same(old, a)) {
                *dst++ = old;
                return 1;
        }
        return merged_entry(a, NULL, dst);
}

static void check_updates(struct cache_entry **src, int nr)
{
        static struct checkout state = {
                .base_dir = "",
                .force = 1,
                .quiet = 1,
                .refresh_cache = 1,
        };
        unsigned short mask = htons(CE_UPDATE);
        while (nr--) {
                struct cache_entry *ce = *src++;
                if (ce->ce_flags & mask) {
                        ce->ce_flags &= ~mask;
                        if (update)
                                checkout_entry(ce, &state);
                }
        }
}

typedef int (*merge_fn_t)(struct cache_entry **, struct cache_entry **);

static void merge_cache(struct cache_entry **src, int nr, merge_fn_t fn)
{
        struct cache_entry **dst = src;

        while (nr) {
                int entries;
                struct cache_entry *name, *ce, *stages[4] = { NULL, };

                name = ce = *src;
                for (;;) {
                        int stage = ce_stage(ce);
                        stages[stage] = ce;
                        ce = *++src;
                        active_nr--;
                        if (!--nr)
                                break;
                        if (!path_matches(ce, name))
                                break;
                }

                entries = fn(stages, dst);
                if (entries < 0)
                        reject_merge(name);
                dst += entries;
                active_nr += entries;
        }
        check_updates(active_cache, active_nr);
}

static char *read_tree_usage = "git-read-tree (<sha> | -m [-u] <sha1> [<sha2> [<sha3>]])";

static struct cache_file cache_file;

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

        newfd = hold_index_file_for_update(&cache_file, get_index_file());
        if (newfd < 0)
                die("unable to create new cachefile");

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

                /* "-u" means "update", meaning that a merge will update the working directory */
                if (!strcmp(arg, "-u")) {
                        update = 1;
                        continue;
                }

                /* "-m" stands for "merge", meaning we start in stage 1 */
                if (!strcmp(arg, "-m")) {
                        int i;
                        if (stage)
                                die("-m needs to come first");
                        read_cache();
                        for (i = 0; i < active_nr; i++) {
                                if (ce_stage(active_cache[i]))
                                        die("you need to resolve your current index first");
                        }
                        stage = 1;
                        merge = 1;
                        continue;
                }
                if (get_sha1(arg, sha1) < 0)
                        usage(read_tree_usage);
                if (stage > 3)
                        usage(read_tree_usage);
                if (unpack_tree(sha1) < 0)
                        die("failed to unpack tree object %s", arg);
                stage++;
        }
        if (update && !merge)
                usage(read_tree_usage);
        if (merge) {
                static const merge_fn_t merge_function[] = {
                        [1] = oneway_merge,
                        [2] = twoway_merge,
                        [3] = threeway_merge,
                };
                if (stage < 2 || stage > 4)
                        die("just how do you expect me to merge %d trees?", stage-1);
                merge_cache(active_cache, active_nr, merge_function[stage-1]);
        }
        if (write_cache(newfd, active_cache, active_nr) ||
            commit_index_file(&cache_file))
                die("unable to write new index file");
        return 0;
}