/*
 * 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: 
 *  - 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 int twoway_merge(struct cache_entry **src, struct cache_entry **dst)
{
	struct cache_entry *old = src[0];
	struct cache_entry *a = src[1], *b = src[2];

	if (src[3])
		return -1;

	if (old) {
		if (!a || !same(old, a))
			return -1;
	}
	if (b)
		return merged_entry(b, old, dst);
	if (old)
		verify_uptodate(old);
	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))
		*a = *old;
	a->ce_flags &= ~htons(CE_STAGEMASK);
	*dst++ = a;
	return 1;
}

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 <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) {
		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;
}