/*
 * 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.
 *
 * _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;
}

/*
 * 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);
}

#define CHECK_OLD(ce) if (old && same(old, ce)) { verify_uptodate(old); old = NULL; }

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

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

                ce = *src++;

                /* We throw away original cache entries except for the stat information */
                if (!ce_stage(ce)) {
                        if (old)
                                reject_merge(old);
                        old = ce;
                        active_nr--;
                        continue;
                }
                if (old && !path_matches(old, ce))
                        reject_merge(old);
                if (nr > 1 && (result = merge_entries(ce, src[0], src[1])) != NULL) {
                        result->ce_flags |= htons(CE_UPDATE);
                        /*
                         * 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 (old && same(old, result)) {
                                *result = *old;
                                old = NULL;
                        }
                        CHECK_OLD(ce);
                        CHECK_OLD(src[0]);
                        CHECK_OLD(src[1]);
                        ce = result;
                        ce->ce_flags &= ~htons(CE_STAGEMASK);
                        src += 2;
                        nr -= 2;
                        active_nr -= 2;
                }

                /*
                 * If we had an old entry that we now effectively
                 * overwrite, make sure it wasn't dirty.
                 */
                CHECK_OLD(ce);
                *dst++ = ce;
        }
        if (old)
                reject_merge(old);
}

/*
 * When we find a "stage2" entry in the two-way merge, that's
 * the one that will remain. If we have an exact old match,
 * we don't care whether the file is up-to-date or not, we just
 * re-use the thing directly.
 *
 * If we didn't have an exact match, then we want to make sure
 * that we've seen a stage1 that matched the old, and that the
 * old file was up-to-date. Because it will be gone after this
 * merge..
 */
static void twoway_check(struct cache_entry *old, int seen_stage1, struct cache_entry *ce)
{
        if (path_matches(old, ce)) {
                /*
                 * This also removes the UPDATE flag on
                 * a match
                 */
                if (same(old, ce)) {
                        *ce = *old;
                        return;
                }
                if (!seen_stage1)
                        reject_merge(old);
        }
        verify_uptodate(old);
}

/*
 * Two-way merge.
 *
 * The rule is: 
 *  - every current entry has to match the old tree
 *  - if the current entry matches the new tree, we leave it
 *    as-is. Otherwise we require that it be up-to-date.
 */
static void twoway_merge(struct cache_entry **src, int nr)
{
        int seen_stage1 = 0;
        struct cache_entry *old = NULL;
        struct cache_entry **dst = src;

        while (nr--) {
                struct cache_entry *ce = *src++;
                int stage = ce_stage(ce);

                switch (stage) {
                case 0:
                        if (old)
                                reject_merge(old);
                        old = ce;
                        seen_stage1 = 0;
                        active_nr--;
                        continue;

                case 1:
                        active_nr--;
                        if (!old)
                                continue;
                        if (!path_matches(old, ce) || !same(old, ce))
                                reject_merge(old);
                        seen_stage1 = 1;
                        continue;

                case 2:
                        ce->ce_flags |= htons(CE_UPDATE);
                        if (old) {
                                twoway_check(old, seen_stage1, ce);
                                old = NULL;
                        }
                        ce->ce_flags &= ~htons(CE_STAGEMASK);
                        *dst++ = ce;
                        continue;
                }
                die("impossible two-way stage");
        }

        /*
         * Unmatched with a new entry? Make sure it was
         * at least uptodate in the working directory _and_
         * the original tree..
         */
        if (old) {
                if (!seen_stage1)
                        reject_merge(old);
                verify_uptodate(old);
        }
}

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 *stat = &null_entry;

        while (nr--) {
                struct cache_entry *ce = *src++;

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

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);
                }
        }
}

static char *read_tree_usage = "git-read-tree (<sha> | -m <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 (merge) {
                switch (stage) {
                case 4: /* Three-way merge */
                        trivially_merge_cache(active_cache, active_nr);
                        check_updates(active_cache, active_nr);
                        break;
                case 3: /* Update from one tree to another */
                        twoway_merge(active_cache, active_nr);
                        check_updates(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) ||
            commit_index_file(&cache_file))
                die("unable to write new index file");
        return 0;
}