/*
 * 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_internal(struct cache_entry *merge, struct cache_entry *old, struct cache_entry **dst, int allow_dirty)
{
        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 if (!allow_dirty) {
                        verify_uptodate(old);
                }
        }
        merge->ce_flags &= ~htons(CE_STAGEMASK);
        *dst++ = merge;
        return 1;
}

static int merged_entry_allow_dirty(struct cache_entry *merge, struct cache_entry *old, struct cache_entry **dst)
{
        return merged_entry_internal(merge, old, dst, 1);
}

static int merged_entry(struct cache_entry *merge, struct cache_entry *old, struct cache_entry **dst)
{
        return merged_entry_internal(merge, old, dst, 0);
}

static int deleted_entry(struct cache_entry *ce, struct cache_entry *old, struct cache_entry **dst)
{
        if (old)
                verify_uptodate(old);
        ce->ce_mode = 0;
        *dst++ = ce;
        return 1;
}

static int causes_df_conflict(struct cache_entry *ce, int stage,
                              struct cache_entry **dst_,
                              struct cache_entry **next_,
                              int tail)
{
        /* This is called during the merge operation and walking
         * the active_cache[] array is messy, because it is in the
         * middle of overlapping copy operation.  The invariants
         * are:
         * (1) active_cache points at the first (zeroth) entry.
         * (2) up to dst pointer are resolved entries.
         * (3) from the next pointer (head-inclusive) to the tail
         *     of the active_cache array have the remaining paths
         *     to be processed.  There can be a gap between dst
         *     and next.  Note that next is called "src" in the
         *     merge_cache() function, and tail is the original
         *     end of active_cache array when merge_cache() started.
         * (4) the path corresponding to *ce is not found in (2)
         *     or (3).  It is in the gap.
         *
         *  active_cache -----......+++++++++++++.
         *                    ^dst  ^next        ^tail
         */
        int i, next, dst;
        const char *path = ce->name;
        int namelen = ce_namelen(ce);

        next = next_ - active_cache;
        dst = dst_ - active_cache;

        for (i = 0; i < tail; i++) {
                int entlen, len;
                const char *one, *two;
                if (dst <= i && i < next)
                        continue;
                ce = active_cache[i];
                if (ce_stage(ce) != stage)
                        continue;
                /* If ce->name is a prefix of path, then path is a file
                 * that hangs underneath ce->name, which is bad.
                 * If path is a prefix of ce->name, then it is the
                 * other way around which also is bad.
                 */
                entlen = ce_namelen(ce);
                if (namelen == entlen)
                        continue;
                if (namelen < entlen) {
                        len = namelen;
                        one = path;
                        two = ce->name;
                } else {
                        len = entlen;
                        one = ce->name;
                        two = path;
                }
                if (memcmp(one, two, len))
                        continue;
                if (two[len] == '/')
                        return 1;
        }
        return 0;
}

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

        /* #5ALT */
        if (!a && b && c && same(b, c)) {
                if (old && !same(b, old))
                        return -1;
                return merged_entry_allow_dirty(b, old, dst);
        }
        /* #2ALT and #3ALT */
        if (!a && (!!b != !!c)) {
                /*
                 * The reason we need to worry about directory/file
                 * conflicts only in #2ALT and #3ALT case is this:
                 *
                 * (1) For all other cases that read-tree internally
                 *     resolves a path, we always have such a path in
                 *     *both* stage2 and stage3 when we begin.
                 *     Traditionally, the behaviour has been even
                 *     stricter and we did not resolve a path without
                 *     initially being in all of stage1, 2, and 3.
                 *
                 * (2) When read-tree finishes, all resolved paths (i.e.
                 *     the paths that are in stage0) must have come from
                 *     either stage2 or stage3.  It is not possible to
                 *     have a stage0 path as a result of a merge if
                 *     neither stage2 nor stage3 had that path.
                 *
                 * (3) It is guaranteed that just after reading the
                 *     stages, each stage cannot have directory/file
                 *     conflicts on its own, because they are populated
                 *     by reading hierarchy of a tree.  Combined with
                 *     (1) and (2) above, this means that no matter what
                 *     combination of paths we take from stage2 and
                 *     stage3 as a result of a merge, they cannot cause
                 *     a directory/file conflict situation (otherwise
                 *     the "guilty" path would have already had such a
                 *     conflict in the original stage, either stage2
                 *     or stage3).  Although its stage2 is synthesized
                 *     by overlaying the current index on top of "our
                 *     head" tree, --emu23 case also has this guarantee,
                 *     by calling add_cache_entry() to create such stage2
                 *     entries.
                 *
                 * (4) Only #2ALT and #3ALT lack the guarantee (1).
                 *     They resolve paths that exist only in stage2
                 *     or stage3.  The stage2 tree may have a file DF
                 *     while stage3 tree may have a file DF/DF.  If
                 *     #2ALT and #3ALT rules happen to apply to both
                 *     of them, we would end up having DF (coming from
                 *     stage2) and DF/DF (from stage3) in the result.
                 *     When we attempt to resolve a path that exists
                 *     only in stage2, we need to make sure there is
                 *     no path that would conflict with it in stage3
                 *     and vice versa.
                 */
                if (c) { /* #2ALT */
                        if (!causes_df_conflict(c, 2, dst, next, tail) &&
                            (!old || same(c, old)))
                                return merged_entry_allow_dirty(c, old, dst);
                }
                else { /* #3ALT */
                        if (!causes_df_conflict(b, 3, dst, next, tail) &&
                            (!old || same(b, old)))
                                return merged_entry_allow_dirty(b, old, dst);
                }
                /* otherwise we will apply the original rule */
        }
        /*
         * 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 **next, int tail)
{
        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 */
                        return deleted_entry(oldtree, current, dst);
                }
                else if (oldtree && newtree &&
                         same(current, oldtree) && !same(current, newtree)) {
                        /* 20 or 21 */
                        return merged_entry(newtree, current, dst);
                }
                else
                        /* all other failures */
                        return -1;
        }
        else if (newtree)
                return merged_entry(newtree, current, dst);
        else
                return deleted_entry(oldtree, current, dst);
}

/*
 * Two-way merge emulated with three-way merge.
 *
 * This treats "read-tree -m H M" by transforming it internally
 * into "read-tree -m H I+H M", where I+H is a tree that would
 * contain the contents of the current index file, overlayed on
 * top of H.  Unlike the traditional two-way merge, this leaves
 * the stages in the resulting index file and lets the user resolve
 * the merge conflicts using standard tools for three-way merge.
 *
 * This function is just to set-up such an arrangement, and the
 * actual merge uses threeway_merge() function.
 */
static void setup_emu23(void)
{
        /* stage0 contains I, stage1 H, stage2 M.
         * move stage2 to stage3, and create stage2 entries
         * by scanning stage0 and stage1 entries.
         */
        int i, namelen, size;
        struct cache_entry *ce, *stage2;

        for (i = 0; i < active_nr; i++) {
                ce = active_cache[i];
                if (ce_stage(ce) != 2)
                        continue;
                /* hoist them up to stage 3 */
                namelen = ce_namelen(ce);
                ce->ce_flags = create_ce_flags(namelen, 3);
        }

        for (i = 0; i < active_nr; i++) {
                ce = active_cache[i];
                if (ce_stage(ce) > 1)
                        continue;
                namelen = ce_namelen(ce);
                size = cache_entry_size(namelen);
                stage2 = xmalloc(size);
                memcpy(stage2, ce, size);
                stage2->ce_flags = create_ce_flags(namelen, 2);
                if (add_cache_entry(stage2, ADD_CACHE_OK_TO_ADD) < 0)
                        die("cannot merge index and our head tree");

                /* We are done with this name, so skip to next name */
                while (i < active_nr &&
                       ce_namelen(active_cache[i]) == namelen &&
                       !memcmp(active_cache[i]->name, ce->name, namelen))
                        i++;
                i--; /* compensate for the loop control */
        }
}

/*
 * 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 **next, int tail)
{
        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_mode) {
                        if (update)
                                unlink(ce->name);
                        continue;
                }
                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 **, struct cache_entry **, int);

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

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

static int read_cache_unmerged(void)
{
        int i, deleted;
        struct cache_entry **dst;

        read_cache();
        dst = active_cache;
        deleted = 0;
        for (i = 0; i < active_nr; i++) {
                struct cache_entry *ce = active_cache[i];
                if (ce_stage(ce)) {
                        deleted++;
                        continue;
                }
                if (deleted)
                        *dst = ce;
                dst++;
        }
        active_nr -= deleted;
        return deleted;
}

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, reset, emu23;
        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;
        reset = 0;
        emu23 = 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;
                }

                /* This differs from "-m" in that we'll silently ignore unmerged entries */
                if (!strcmp(arg, "--reset")) {
                        if (stage || merge || emu23)
                                usage(read_tree_usage);
                        reset = 1;
                        merge = 1;
                        stage = 1;
                        read_cache_unmerged();
                }

                /* "-m" stands for "merge", meaning we start in stage 1 */
                if (!strcmp(arg, "-m")) {
                        if (stage || merge || emu23)
                                usage(read_tree_usage);
                        if (read_cache_unmerged())
                                die("you need to resolve your current index first");
                        stage = 1;
                        merge = 1;
                        continue;
                }

                /* "-emu23" uses 3-way merge logic to perform fast-forward */
                if (!strcmp(arg, "--emu23")) {
                        if (stage || merge || emu23)
                                usage(read_tree_usage);
                        if (read_cache_unmerged())
                                die("you need to resolve your current index first");
                        merge = emu23 = stage = 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,
                };
                merge_fn_t fn;

                if (stage < 2 || stage > 4)
                        die("just how do you expect me to merge %d trees?", stage-1);
                if (emu23 && stage != 3)
                        die("--emu23 takes only two trees");
                fn = merge_function[stage-1];
                if (stage == 3 && emu23) { 
                        setup_emu23();
                        fn = merge_function[3];
                }
                merge_cache(active_cache, active_nr, fn);
        }
        if (write_cache(newfd, active_cache, active_nr) ||
            commit_index_file(&cache_file))
                die("unable to write new index file");
        return 0;
}