merge-tree: add comments to clarify what these functions are doing

[git] / builtin / merge-tree.c

#include "builtin.h"
#include "tree-walk.h"
#include "xdiff-interface.h"
#include "blob.h"
#include "exec_cmd.h"
#include "merge-blobs.h"

static const char merge_tree_usage[] = "git merge-tree <base-tree> <branch1> <branch2>";

struct merge_list {
        struct merge_list *next;
        struct merge_list *link;        /* other stages for this object */

        unsigned int stage : 2;
        unsigned int mode;
        const char *path;
        struct blob *blob;
};

static struct merge_list *merge_result, **merge_result_end = &merge_result;

static void add_merge_entry(struct merge_list *entry)
{
        *merge_result_end = entry;
        merge_result_end = &entry->next;
}

static void merge_trees(struct tree_desc t[3], const char *base);

static const char *explanation(struct merge_list *entry)
{
        switch (entry->stage) {
        case 0:
                return "merged";
        case 3:
                return "added in remote";
        case 2:
                if (entry->link)
                        return "added in both";
                return "added in local";
        }

        /* Existed in base */
        entry = entry->link;
        if (!entry)
                return "removed in both";

        if (entry->link)
                return "changed in both";

        if (entry->stage == 3)
                return "removed in local";
        return "removed in remote";
}

static void *result(struct merge_list *entry, unsigned long *size)
{
        enum object_type type;
        struct blob *base, *our, *their;
        const char *path = entry->path;

        if (!entry->stage)
                return read_sha1_file(entry->blob->object.sha1, &type, size);
        base = NULL;
        if (entry->stage == 1) {
                base = entry->blob;
                entry = entry->link;
        }
        our = NULL;
        if (entry && entry->stage == 2) {
                our = entry->blob;
                entry = entry->link;
        }
        their = NULL;
        if (entry)
                their = entry->blob;
        return merge_blobs(path, base, our, their, size);
}

static void *origin(struct merge_list *entry, unsigned long *size)
{
        enum object_type type;
        while (entry) {
                if (entry->stage == 2)
                        return read_sha1_file(entry->blob->object.sha1, &type, size);
                entry = entry->link;
        }
        return NULL;
}

static int show_outf(void *priv_, mmbuffer_t *mb, int nbuf)
{
        int i;
        for (i = 0; i < nbuf; i++)
                printf("%.*s", (int) mb[i].size, mb[i].ptr);
        return 0;
}

static void show_diff(struct merge_list *entry)
{
        unsigned long size;
        mmfile_t src, dst;
        xpparam_t xpp;
        xdemitconf_t xecfg;
        xdemitcb_t ecb;

        xpp.flags = 0;
        memset(&xecfg, 0, sizeof(xecfg));
        xecfg.ctxlen = 3;
        ecb.outf = show_outf;
        ecb.priv = NULL;

        src.ptr = origin(entry, &size);
        if (!src.ptr)
                size = 0;
        src.size = size;
        dst.ptr = result(entry, &size);
        if (!dst.ptr)
                size = 0;
        dst.size = size;
        xdi_diff(&src, &dst, &xpp, &xecfg, &ecb);
        free(src.ptr);
        free(dst.ptr);
}

static void show_result_list(struct merge_list *entry)
{
        printf("%s\n", explanation(entry));
        do {
                struct merge_list *link = entry->link;
                static const char *desc[4] = { "result", "base", "our", "their" };
                printf("  %-6s %o %s %s\n", desc[entry->stage], entry->mode, sha1_to_hex(entry->blob->object.sha1), entry->path);
                entry = link;
        } while (entry);
}

static void show_result(void)
{
        struct merge_list *walk;

        walk = merge_result;
        while (walk) {
                show_result_list(walk);
                show_diff(walk);
                walk = walk->next;
        }
}

/* An empty entry never compares same, not even to another empty entry */
static int same_entry(struct name_entry *a, struct name_entry *b)
{
        return  a->sha1 &&
                b->sha1 &&
                !hashcmp(a->sha1, b->sha1) &&
                a->mode == b->mode;
}

static struct merge_list *create_entry(unsigned stage, unsigned mode, const unsigned char *sha1, const char *path)
{
        struct merge_list *res = xcalloc(1, sizeof(*res));

        res->stage = stage;
        res->path = path;
        res->mode = mode;
        res->blob = lookup_blob(sha1);
        return res;
}

static char *traverse_path(const struct traverse_info *info, const struct name_entry *n)
{
        char *path = xmalloc(traverse_path_len(info, n) + 1);
        return make_traverse_path(path, info, n);
}

static void resolve(const struct traverse_info *info, struct name_entry *ours, struct name_entry *result)
{
        struct merge_list *orig, *final;
        const char *path;

        /* If it's already ours, don't bother showing it */
        if (!ours)
                return;

        path = traverse_path(info, result);
        orig = create_entry(2, ours->mode, ours->sha1, path);
        final = create_entry(0, result->mode, result->sha1, path);

        final->link = orig;

        add_merge_entry(final);
}

static int unresolved_directory(const struct traverse_info *info, struct name_entry n[3])
{
        char *newbase;
        struct name_entry *p;
        struct tree_desc t[3];
        void *buf0, *buf1, *buf2;

        p = n;
        if (!p->mode) {
                p++;
                if (!p->mode)
                        p++;
        }
        if (!S_ISDIR(p->mode))
                return 0;
        /*
         * NEEDSWORK: this is broken. The path can originally be a file
         * and then one side may have turned it into a directory, in which
         * case we return and let the three-way merge as if the tree were
         * a regular file.  If the path that was originally a tree is
         * now a file in either branch, fill_tree_descriptor() below will
         * die when fed a blob sha1.
         */

        newbase = traverse_path(info, p);
        buf0 = fill_tree_descriptor(t+0, n[0].sha1);
        buf1 = fill_tree_descriptor(t+1, n[1].sha1);
        buf2 = fill_tree_descriptor(t+2, n[2].sha1);
        merge_trees(t, newbase);

        free(buf0);
        free(buf1);
        free(buf2);
        free(newbase);
        return 1;
}


static struct merge_list *link_entry(unsigned stage, const struct traverse_info *info, struct name_entry *n, struct merge_list *entry)
{
        const char *path;
        struct merge_list *link;

        if (!n->mode)
                return entry;
        if (entry)
                path = entry->path;
        else
                path = traverse_path(info, n);
        link = create_entry(stage, n->mode, n->sha1, path);
        link->link = entry;
        return link;
}

static void unresolved(const struct traverse_info *info, struct name_entry n[3])
{
        struct merge_list *entry = NULL;

        if (unresolved_directory(info, n))
                return;

        /*
         * Do them in reverse order so that the resulting link
         * list has the stages in order - link_entry adds new
         * links at the front.
         */
        entry = link_entry(3, info, n + 2, entry);
        entry = link_entry(2, info, n + 1, entry);
        entry = link_entry(1, info, n + 0, entry);

        add_merge_entry(entry);
}

/*
 * Merge two trees together (t[1] and t[2]), using a common base (t[0])
 * as the origin.
 *
 * This walks the (sorted) trees in lock-step, checking every possible
 * name. Note that directories automatically sort differently from other
 * files (see "base_name_compare"), so you'll never see file/directory
 * conflicts, because they won't ever compare the same.
 *
 * IOW, if a directory changes to a filename, it will automatically be
 * seen as the directory going away, and the filename being created.
 *
 * Think of this as a three-way diff.
 *
 * The output will be either:
 *  - successful merge
 *       "0 mode sha1 filename"
 *    NOTE NOTE NOTE! FIXME! We really really need to walk the index
 *    in parallel with this too!
 *
 *  - conflict:
 *      "1 mode sha1 filename"
 *      "2 mode sha1 filename"
 *      "3 mode sha1 filename"
 *    where not all of the 1/2/3 lines may exist, of course.
 *
 * The successful merge rules are the same as for the three-way merge
 * in git-read-tree.
 */
static int threeway_callback(int n, unsigned long mask, unsigned long dirmask, struct name_entry *entry, struct traverse_info *info)
{
        /* Same in both? */
        if (same_entry(entry+1, entry+2)) {
                if (entry[0].sha1) {
                        /* Modified identically */
                        resolve(info, NULL, entry+1);
                        return mask;
                }
                /* "Both added the same" is left unresolved */
        }

        if (same_entry(entry+0, entry+1)) {
                if (entry[2].sha1 && !S_ISDIR(entry[2].mode)) {
                        /* We did not touch, they modified -- take theirs */
                        resolve(info, entry+1, entry+2);
                        return mask;
                }
                /*
                 * If we did not touch a directory but they made it
                 * into a file, we fall through and unresolved()
                 * recurses down.  Likewise for the opposite case.
                 */
        }

        if (same_entry(entry+0, entry+2)) {
                if (entry[1].sha1 && !S_ISDIR(entry[1].mode)) {
                        /* We modified, they did not touch -- take ours */
                        resolve(info, NULL, entry+1);
                        return mask;
                }
        }

        unresolved(info, entry);
        return mask;
}

static void merge_trees(struct tree_desc t[3], const char *base)
{
        struct traverse_info info;

        setup_traverse_info(&info, base);
        info.fn = threeway_callback;
        traverse_trees(3, t, &info);
}

static void *get_tree_descriptor(struct tree_desc *desc, const char *rev)
{
        unsigned char sha1[20];
        void *buf;

        if (get_sha1(rev, sha1))
                die("unknown rev %s", rev);
        buf = fill_tree_descriptor(desc, sha1);
        if (!buf)
                die("%s is not a tree", rev);
        return buf;
}

int cmd_merge_tree(int argc, const char **argv, const char *prefix)
{
        struct tree_desc t[3];
        void *buf1, *buf2, *buf3;

        if (argc != 4)
                usage(merge_tree_usage);

        buf1 = get_tree_descriptor(t+0, argv[1]);
        buf2 = get_tree_descriptor(t+1, argv[2]);
        buf3 = get_tree_descriptor(t+2, argv[3]);
        merge_trees(t, "");
        free(buf1);
        free(buf2);
        free(buf3);

        show_result();
        return 0;
}