merge-ort: basic outline for merge_switch_to_result()

[git] / merge-ort.c

/*
 * "Ostensibly Recursive's Twin" merge strategy, or "ort" for short.  Meant
 * as a drop-in replacement for the "recursive" merge strategy, allowing one
 * to replace
 *
 *   git merge [-s recursive]
 *
 * with
 *
 *   git merge -s ort
 *
 * Note: git's parser allows the space between '-s' and its argument to be
 * missing.  (Should I have backronymed "ham", "alsa", "kip", "nap, "alvo",
 * "cale", "peedy", or "ins" instead of "ort"?)
 */

#include "cache.h"
#include "merge-ort.h"

#include "diff.h"
#include "diffcore.h"
#include "object-store.h"
#include "strmap.h"
#include "tree.h"
#include "xdiff-interface.h"

/*
 * We have many arrays of size 3.  Whenever we have such an array, the
 * indices refer to one of the sides of the three-way merge.  This is so
 * pervasive that the constants 0, 1, and 2 are used in many places in the
 * code (especially in arithmetic operations to find the other side's index
 * or to compute a relevant mask), but sometimes these enum names are used
 * to aid code clarity.
 *
 * See also 'filemask' and 'dirmask' in struct conflict_info; the "ith side"
 * referred to there is one of these three sides.
 */
enum merge_side {
        MERGE_BASE = 0,
        MERGE_SIDE1 = 1,
        MERGE_SIDE2 = 2
};

struct merge_options_internal {
        /*
         * paths: primary data structure in all of merge ort.
         *
         * The keys of paths:
         *   * are full relative paths from the toplevel of the repository
         *     (e.g. "drivers/firmware/raspberrypi.c").
         *   * store all relevant paths in the repo, both directories and
         *     files (e.g. drivers, drivers/firmware would also be included)
         *   * these keys serve to intern all the path strings, which allows
         *     us to do pointer comparison on directory names instead of
         *     strcmp; we just have to be careful to use the interned strings.
         *
         * The values of paths:
         *   * either a pointer to a merged_info, or a conflict_info struct
         *   * merged_info contains all relevant information for a
         *     non-conflicted entry.
         *   * conflict_info contains a merged_info, plus any additional
         *     information about a conflict such as the higher orders stages
         *     involved and the names of the paths those came from (handy
         *     once renames get involved).
         *   * a path may start "conflicted" (i.e. point to a conflict_info)
         *     and then a later step (e.g. three-way content merge) determines
         *     it can be cleanly merged, at which point it'll be marked clean
         *     and the algorithm will ignore any data outside the contained
         *     merged_info for that entry
         *   * If an entry remains conflicted, the merged_info portion of a
         *     conflict_info will later be filled with whatever version of
         *     the file should be placed in the working directory (e.g. an
         *     as-merged-as-possible variation that contains conflict markers).
         */
        struct strmap paths;

        /*
         * conflicted: a subset of keys->values from "paths"
         *
         * conflicted is basically an optimization between process_entries()
         * and record_conflicted_index_entries(); the latter could loop over
         * ALL the entries in paths AGAIN and look for the ones that are
         * still conflicted, but since process_entries() has to loop over
         * all of them, it saves the ones it couldn't resolve in this strmap
         * so that record_conflicted_index_entries() can iterate just the
         * relevant entries.
         */
        struct strmap conflicted;

        /*
         * current_dir_name: temporary var used in collect_merge_info_callback()
         *
         * Used to set merged_info.directory_name; see documentation for that
         * variable and the requirements placed on that field.
         */
        const char *current_dir_name;

        /* call_depth: recursion level counter for merging merge bases */
        int call_depth;
};

struct version_info {
        struct object_id oid;
        unsigned short mode;
};

struct merged_info {
        /* if is_null, ignore result.  otherwise result has oid & mode */
        struct version_info result;
        unsigned is_null:1;

        /*
         * clean: whether the path in question is cleanly merged.
         *
         * see conflict_info.merged for more details.
         */
        unsigned clean:1;

        /*
         * basename_offset: offset of basename of path.
         *
         * perf optimization to avoid recomputing offset of final '/'
         * character in pathname (0 if no '/' in pathname).
         */
        size_t basename_offset;

         /*
          * directory_name: containing directory name.
          *
          * Note that we assume directory_name is constructed such that
          *    strcmp(dir1_name, dir2_name) == 0 iff dir1_name == dir2_name,
          * i.e. string equality is equivalent to pointer equality.  For this
          * to hold, we have to be careful setting directory_name.
          */
        const char *directory_name;
};

struct conflict_info {
        /*
         * merged: the version of the path that will be written to working tree
         *
         * WARNING: It is critical to check merged.clean and ensure it is 0
         * before reading any conflict_info fields outside of merged.
         * Allocated merge_info structs will always have clean set to 1.
         * Allocated conflict_info structs will have merged.clean set to 0
         * initially.  The merged.clean field is how we know if it is safe
         * to access other parts of conflict_info besides merged; if a
         * conflict_info's merged.clean is changed to 1, the rest of the
         * algorithm is not allowed to look at anything outside of the
         * merged member anymore.
         */
        struct merged_info merged;

        /* oids & modes from each of the three trees for this path */
        struct version_info stages[3];

        /* pathnames for each stage; may differ due to rename detection */
        const char *pathnames[3];

        /* Whether this path is/was involved in a directory/file conflict */
        unsigned df_conflict:1;

        /*
         * For filemask and dirmask, the ith bit corresponds to whether the
         * ith entry is a file (filemask) or a directory (dirmask).  Thus,
         * filemask & dirmask is always zero, and filemask | dirmask is at
         * most 7 but can be less when a path does not appear as either a
         * file or a directory on at least one side of history.
         *
         * Note that these masks are related to enum merge_side, as the ith
         * entry corresponds to side i.
         *
         * These values come from a traverse_trees() call; more info may be
         * found looking at tree-walk.h's struct traverse_info,
         * particularly the documentation above the "fn" member (note that
         * filemask = mask & ~dirmask from that documentation).
         */
        unsigned filemask:3;
        unsigned dirmask:3;

        /*
         * Optimization to track which stages match, to avoid the need to
         * recompute it in multiple steps. Either 0 or at least 2 bits are
         * set; if at least 2 bits are set, their corresponding stages match.
         */
        unsigned match_mask:3;
};

/*
 * For the next three macros, see warning for conflict_info.merged.
 *
 * In each of the below, mi is a struct merged_info*, and ci was defined
 * as a struct conflict_info* (but we need to verify ci isn't actually
 * pointed at a struct merged_info*).
 *
 * INITIALIZE_CI: Assign ci to mi but only if it's safe; set to NULL otherwise.
 * VERIFY_CI: Ensure that something we assigned to a conflict_info* is one.
 * ASSIGN_AND_VERIFY_CI: Similar to VERIFY_CI but do assignment first.
 */
#define INITIALIZE_CI(ci, mi) do {                                           \
        (ci) = (!(mi) || (mi)->clean) ? NULL : (struct conflict_info *)(mi); \
} while (0)
#define VERIFY_CI(ci) assert(ci && !ci->merged.clean);
#define ASSIGN_AND_VERIFY_CI(ci, mi) do {    \
        (ci) = (struct conflict_info *)(mi);  \
        assert((ci) && !(mi)->clean);        \
} while (0)

static int err(struct merge_options *opt, const char *err, ...)
{
        va_list params;
        struct strbuf sb = STRBUF_INIT;

        strbuf_addstr(&sb, "error: ");
        va_start(params, err);
        strbuf_vaddf(&sb, err, params);
        va_end(params);

        error("%s", sb.buf);
        strbuf_release(&sb);

        return -1;
}

static void setup_path_info(struct merge_options *opt,
                            struct string_list_item *result,
                            const char *current_dir_name,
                            int current_dir_name_len,
                            char *fullpath, /* we'll take over ownership */
                            struct name_entry *names,
                            struct name_entry *merged_version,
                            unsigned is_null,     /* boolean */
                            unsigned df_conflict, /* boolean */
                            unsigned filemask,
                            unsigned dirmask,
                            int resolved          /* boolean */)
{
        /* result->util is void*, so mi is a convenience typed variable */
        struct merged_info *mi;

        assert(!is_null || resolved);
        assert(!df_conflict || !resolved); /* df_conflict implies !resolved */
        assert(resolved == (merged_version != NULL));

        mi = xcalloc(1, resolved ? sizeof(struct merged_info) :
                                   sizeof(struct conflict_info));
        mi->directory_name = current_dir_name;
        mi->basename_offset = current_dir_name_len;
        mi->clean = !!resolved;
        if (resolved) {
                mi->result.mode = merged_version->mode;
                oidcpy(&mi->result.oid, &merged_version->oid);
                mi->is_null = !!is_null;
        } else {
                int i;
                struct conflict_info *ci;

                ASSIGN_AND_VERIFY_CI(ci, mi);
                for (i = MERGE_BASE; i <= MERGE_SIDE2; i++) {
                        ci->pathnames[i] = fullpath;
                        ci->stages[i].mode = names[i].mode;
                        oidcpy(&ci->stages[i].oid, &names[i].oid);
                }
                ci->filemask = filemask;
                ci->dirmask = dirmask;
                ci->df_conflict = !!df_conflict;
                if (dirmask)
                        /*
                         * Assume is_null for now, but if we have entries
                         * under the directory then when it is complete in
                         * write_completed_directory() it'll update this.
                         * Also, for D/F conflicts, we have to handle the
                         * directory first, then clear this bit and process
                         * the file to see how it is handled -- that occurs
                         * near the top of process_entry().
                         */
                        mi->is_null = 1;
        }
        strmap_put(&opt->priv->paths, fullpath, mi);
        result->string = fullpath;
        result->util = mi;
}

static int collect_merge_info_callback(int n,
                                       unsigned long mask,
                                       unsigned long dirmask,
                                       struct name_entry *names,
                                       struct traverse_info *info)
{
        /*
         * n is 3.  Always.
         * common ancestor (mbase) has mask 1, and stored in index 0 of names
         * head of side 1  (side1) has mask 2, and stored in index 1 of names
         * head of side 2  (side2) has mask 4, and stored in index 2 of names
         */
        struct merge_options *opt = info->data;
        struct merge_options_internal *opti = opt->priv;
        struct string_list_item pi;  /* Path Info */
        struct conflict_info *ci; /* typed alias to pi.util (which is void*) */
        struct name_entry *p;
        size_t len;
        char *fullpath;
        const char *dirname = opti->current_dir_name;
        unsigned filemask = mask & ~dirmask;
        unsigned match_mask = 0; /* will be updated below */
        unsigned mbase_null = !(mask & 1);
        unsigned side1_null = !(mask & 2);
        unsigned side2_null = !(mask & 4);
        unsigned side1_matches_mbase = (!side1_null && !mbase_null &&
                                        names[0].mode == names[1].mode &&
                                        oideq(&names[0].oid, &names[1].oid));
        unsigned side2_matches_mbase = (!side2_null && !mbase_null &&
                                        names[0].mode == names[2].mode &&
                                        oideq(&names[0].oid, &names[2].oid));
        unsigned sides_match = (!side1_null && !side2_null &&
                                names[1].mode == names[2].mode &&
                                oideq(&names[1].oid, &names[2].oid));

        /*
         * Note: When a path is a file on one side of history and a directory
         * in another, we have a directory/file conflict.  In such cases, if
         * the conflict doesn't resolve from renames and deletions, then we
         * always leave directories where they are and move files out of the
         * way.  Thus, while struct conflict_info has a df_conflict field to
         * track such conflicts, we ignore that field for any directories at
         * a path and only pay attention to it for files at the given path.
         * The fact that we leave directories were they are also means that
         * we do not need to worry about getting additional df_conflict
         * information propagated from parent directories down to children
         * (unlike, say traverse_trees_recursive() in unpack-trees.c, which
         * sets a newinfo.df_conflicts field specifically to propagate it).
         */
        unsigned df_conflict = (filemask != 0) && (dirmask != 0);

        /* n = 3 is a fundamental assumption. */
        if (n != 3)
                BUG("Called collect_merge_info_callback wrong");

        /*
         * A bunch of sanity checks verifying that traverse_trees() calls
         * us the way I expect.  Could just remove these at some point,
         * though maybe they are helpful to future code readers.
         */
        assert(mbase_null == is_null_oid(&names[0].oid));
        assert(side1_null == is_null_oid(&names[1].oid));
        assert(side2_null == is_null_oid(&names[2].oid));
        assert(!mbase_null || !side1_null || !side2_null);
        assert(mask > 0 && mask < 8);

        /* Determine match_mask */
        if (side1_matches_mbase)
                match_mask = (side2_matches_mbase ? 7 : 3);
        else if (side2_matches_mbase)
                match_mask = 5;
        else if (sides_match)
                match_mask = 6;

        /*
         * Get the name of the relevant filepath, which we'll pass to
         * setup_path_info() for tracking.
         */
        p = names;
        while (!p->mode)
                p++;
        len = traverse_path_len(info, p->pathlen);

        /* +1 in both of the following lines to include the NUL byte */
        fullpath = xmalloc(len + 1);
        make_traverse_path(fullpath, len + 1, info, p->path, p->pathlen);

        /*
         * If mbase, side1, and side2 all match, we can resolve early.  Even
         * if these are trees, there will be no renames or anything
         * underneath.
         */
        if (side1_matches_mbase && side2_matches_mbase) {
                /* mbase, side1, & side2 all match; use mbase as resolution */
                setup_path_info(opt, &pi, dirname, info->pathlen, fullpath,
                                names, names+0, mbase_null, 0,
                                filemask, dirmask, 1);
                return mask;
        }

        /*
         * Record information about the path so we can resolve later in
         * process_entries.
         */
        setup_path_info(opt, &pi, dirname, info->pathlen, fullpath,
                        names, NULL, 0, df_conflict, filemask, dirmask, 0);

        ci = pi.util;
        VERIFY_CI(ci);
        ci->match_mask = match_mask;

        /* If dirmask, recurse into subdirectories */
        if (dirmask) {
                struct traverse_info newinfo;
                struct tree_desc t[3];
                void *buf[3] = {NULL, NULL, NULL};
                const char *original_dir_name;
                int i, ret;

                ci->match_mask &= filemask;
                newinfo = *info;
                newinfo.prev = info;
                newinfo.name = p->path;
                newinfo.namelen = p->pathlen;
                newinfo.pathlen = st_add3(newinfo.pathlen, p->pathlen, 1);
                /*
                 * If this directory we are about to recurse into cared about
                 * its parent directory (the current directory) having a D/F
                 * conflict, then we'd propagate the masks in this way:
                 *    newinfo.df_conflicts |= (mask & ~dirmask);
                 * But we don't worry about propagating D/F conflicts.  (See
                 * comment near setting of local df_conflict variable near
                 * the beginning of this function).
                 */

                for (i = MERGE_BASE; i <= MERGE_SIDE2; i++) {
                        if (i == 1 && side1_matches_mbase)
                                t[1] = t[0];
                        else if (i == 2 && side2_matches_mbase)
                                t[2] = t[0];
                        else if (i == 2 && sides_match)
                                t[2] = t[1];
                        else {
                                const struct object_id *oid = NULL;
                                if (dirmask & 1)
                                        oid = &names[i].oid;
                                buf[i] = fill_tree_descriptor(opt->repo,
                                                              t + i, oid);
                        }
                        dirmask >>= 1;
                }

                original_dir_name = opti->current_dir_name;
                opti->current_dir_name = pi.string;
                ret = traverse_trees(NULL, 3, t, &newinfo);
                opti->current_dir_name = original_dir_name;

                for (i = MERGE_BASE; i <= MERGE_SIDE2; i++)
                        free(buf[i]);

                if (ret < 0)
                        return -1;
        }

        return mask;
}

static int collect_merge_info(struct merge_options *opt,
                              struct tree *merge_base,
                              struct tree *side1,
                              struct tree *side2)
{
        int ret;
        struct tree_desc t[3];
        struct traverse_info info;
        const char *toplevel_dir_placeholder = "";

        opt->priv->current_dir_name = toplevel_dir_placeholder;
        setup_traverse_info(&info, toplevel_dir_placeholder);
        info.fn = collect_merge_info_callback;
        info.data = opt;
        info.show_all_errors = 1;

        parse_tree(merge_base);
        parse_tree(side1);
        parse_tree(side2);
        init_tree_desc(t + 0, merge_base->buffer, merge_base->size);
        init_tree_desc(t + 1, side1->buffer, side1->size);
        init_tree_desc(t + 2, side2->buffer, side2->size);

        ret = traverse_trees(NULL, 3, t, &info);

        return ret;
}

static int detect_and_process_renames(struct merge_options *opt,
                                      struct tree *merge_base,
                                      struct tree *side1,
                                      struct tree *side2)
{
        int clean = 1;

        /*
         * Rename detection works by detecting file similarity.  Here we use
         * a really easy-to-implement scheme: files are similar IFF they have
         * the same filename.  Therefore, by this scheme, there are no renames.
         *
         * TODO: Actually implement a real rename detection scheme.
         */
        return clean;
}

static int string_list_df_name_compare(const char *one, const char *two)
{
        int onelen = strlen(one);
        int twolen = strlen(two);
        /*
         * Here we only care that entries for D/F conflicts are
         * adjacent, in particular with the file of the D/F conflict
         * appearing before files below the corresponding directory.
         * The order of the rest of the list is irrelevant for us.
         *
         * To achieve this, we sort with df_name_compare and provide
         * the mode S_IFDIR so that D/F conflicts will sort correctly.
         * We use the mode S_IFDIR for everything else for simplicity,
         * since in other cases any changes in their order due to
         * sorting cause no problems for us.
         */
        int cmp = df_name_compare(one, onelen, S_IFDIR,
                                  two, twolen, S_IFDIR);
        /*
         * Now that 'foo' and 'foo/bar' compare equal, we have to make sure
         * that 'foo' comes before 'foo/bar'.
         */
        if (cmp)
                return cmp;
        return onelen - twolen;
}

struct directory_versions {
        /*
         * versions: list of (basename -> version_info)
         *
         * The basenames are in reverse lexicographic order of full pathnames,
         * as processed in process_entries().  This puts all entries within
         * a directory together, and covers the directory itself after
         * everything within it, allowing us to write subtrees before needing
         * to record information for the tree itself.
         */
        struct string_list versions;

        /*
         * offsets: list of (full relative path directories -> integer offsets)
         *
         * Since versions contains basenames from files in multiple different
         * directories, we need to know which entries in versions correspond
         * to which directories.  Values of e.g.
         *     ""             0
         *     src            2
         *     src/moduleA    5
         * Would mean that entries 0-1 of versions are files in the toplevel
         * directory, entries 2-4 are files under src/, and the remaining
         * entries starting at index 5 are files under src/moduleA/.
         */
        struct string_list offsets;

        /*
         * last_directory: directory that previously processed file found in
         *
         * last_directory starts NULL, but records the directory in which the
         * previous file was found within.  As soon as
         *    directory(current_file) != last_directory
         * then we need to start updating accounting in versions & offsets.
         * Note that last_directory is always the last path in "offsets" (or
         * NULL if "offsets" is empty) so this exists just for quick access.
         */
        const char *last_directory;

        /* last_directory_len: cached computation of strlen(last_directory) */
        unsigned last_directory_len;
};

static int tree_entry_order(const void *a_, const void *b_)
{
        const struct string_list_item *a = a_;
        const struct string_list_item *b = b_;

        const struct merged_info *ami = a->util;
        const struct merged_info *bmi = b->util;
        return base_name_compare(a->string, strlen(a->string), ami->result.mode,
                                 b->string, strlen(b->string), bmi->result.mode);
}

static void write_tree(struct object_id *result_oid,
                       struct string_list *versions,
                       unsigned int offset,
                       size_t hash_size)
{
        size_t maxlen = 0, extra;
        unsigned int nr = versions->nr - offset;
        struct strbuf buf = STRBUF_INIT;
        struct string_list relevant_entries = STRING_LIST_INIT_NODUP;
        int i;

        /*
         * We want to sort the last (versions->nr-offset) entries in versions.
         * Do so by abusing the string_list API a bit: make another string_list
         * that contains just those entries and then sort them.
         *
         * We won't use relevant_entries again and will let it just pop off the
         * stack, so there won't be allocation worries or anything.
         */
        relevant_entries.items = versions->items + offset;
        relevant_entries.nr = versions->nr - offset;
        QSORT(relevant_entries.items, relevant_entries.nr, tree_entry_order);

        /* Pre-allocate some space in buf */
        extra = hash_size + 8; /* 8: 6 for mode, 1 for space, 1 for NUL char */
        for (i = 0; i < nr; i++) {
                maxlen += strlen(versions->items[offset+i].string) + extra;
        }
        strbuf_grow(&buf, maxlen);

        /* Write each entry out to buf */
        for (i = 0; i < nr; i++) {
                struct merged_info *mi = versions->items[offset+i].util;
                struct version_info *ri = &mi->result;
                strbuf_addf(&buf, "%o %s%c",
                            ri->mode,
                            versions->items[offset+i].string, '\0');
                strbuf_add(&buf, ri->oid.hash, hash_size);
        }

        /* Write this object file out, and record in result_oid */
        write_object_file(buf.buf, buf.len, tree_type, result_oid);
        strbuf_release(&buf);
}

static void record_entry_for_tree(struct directory_versions *dir_metadata,
                                  const char *path,
                                  struct merged_info *mi)
{
        const char *basename;

        if (mi->is_null)
                /* nothing to record */
                return;

        basename = path + mi->basename_offset;
        assert(strchr(basename, '/') == NULL);
        string_list_append(&dir_metadata->versions,
                           basename)->util = &mi->result;
}

static void write_completed_directory(struct merge_options *opt,
                                      const char *new_directory_name,
                                      struct directory_versions *info)
{
        const char *prev_dir;
        struct merged_info *dir_info = NULL;
        unsigned int offset;

        /*
         * Some explanation of info->versions and info->offsets...
         *
         * process_entries() iterates over all relevant files AND
         * directories in reverse lexicographic order, and calls this
         * function.  Thus, an example of the paths that process_entries()
         * could operate on (along with the directories for those paths
         * being shown) is:
         *
         *     xtract.c             ""
         *     tokens.txt           ""
         *     src/moduleB/umm.c    src/moduleB
         *     src/moduleB/stuff.h  src/moduleB
         *     src/moduleB/baz.c    src/moduleB
         *     src/moduleB          src
         *     src/moduleA/foo.c    src/moduleA
         *     src/moduleA/bar.c    src/moduleA
         *     src/moduleA          src
         *     src                  ""
         *     Makefile             ""
         *
         * info->versions:
         *
         *     always contains the unprocessed entries and their
         *     version_info information.  For example, after the first five
         *     entries above, info->versions would be:
         *
         *         xtract.c     <xtract.c's version_info>
         *         token.txt    <token.txt's version_info>
         *         umm.c        <src/moduleB/umm.c's version_info>
         *         stuff.h      <src/moduleB/stuff.h's version_info>
         *         baz.c        <src/moduleB/baz.c's version_info>
         *
         *     Once a subdirectory is completed we remove the entries in
         *     that subdirectory from info->versions, writing it as a tree
         *     (write_tree()).  Thus, as soon as we get to src/moduleB,
         *     info->versions would be updated to
         *
         *         xtract.c     <xtract.c's version_info>
         *         token.txt    <token.txt's version_info>
         *         moduleB      <src/moduleB's version_info>
         *
         * info->offsets:
         *
         *     helps us track which entries in info->versions correspond to
         *     which directories.  When we are N directories deep (e.g. 4
         *     for src/modA/submod/subdir/), we have up to N+1 unprocessed
         *     directories (+1 because of toplevel dir).  Corresponding to
         *     the info->versions example above, after processing five entries
         *     info->offsets will be:
         *
         *         ""           0
         *         src/moduleB  2
         *
         *     which is used to know that xtract.c & token.txt are from the
         *     toplevel dirctory, while umm.c & stuff.h & baz.c are from the
         *     src/moduleB directory.  Again, following the example above,
         *     once we need to process src/moduleB, then info->offsets is
         *     updated to
         *
         *         ""           0
         *         src          2
         *
         *     which says that moduleB (and only moduleB so far) is in the
         *     src directory.
         *
         *     One unique thing to note about info->offsets here is that
         *     "src" was not added to info->offsets until there was a path
         *     (a file OR directory) immediately below src/ that got
         *     processed.
         *
         * Since process_entry() just appends new entries to info->versions,
         * write_completed_directory() only needs to do work if the next path
         * is in a directory that is different than the last directory found
         * in info->offsets.
         */

        /*
         * If we are working with the same directory as the last entry, there
         * is no work to do.  (See comments above the directory_name member of
         * struct merged_info for why we can use pointer comparison instead of
         * strcmp here.)
         */
        if (new_directory_name == info->last_directory)
                return;

        /*
         * If we are just starting (last_directory is NULL), or last_directory
         * is a prefix of the current directory, then we can just update
         * info->offsets to record the offset where we started this directory
         * and update last_directory to have quick access to it.
         */
        if (info->last_directory == NULL ||
            !strncmp(new_directory_name, info->last_directory,
                     info->last_directory_len)) {
                uintptr_t offset = info->versions.nr;

                info->last_directory = new_directory_name;
                info->last_directory_len = strlen(info->last_directory);
                /*
                 * Record the offset into info->versions where we will
                 * start recording basenames of paths found within
                 * new_directory_name.
                 */
                string_list_append(&info->offsets,
                                   info->last_directory)->util = (void*)offset;
                return;
        }

        /*
         * The next entry that will be processed will be within
         * new_directory_name.  Since at this point we know that
         * new_directory_name is within a different directory than
         * info->last_directory, we have all entries for info->last_directory
         * in info->versions and we need to create a tree object for them.
         */
        dir_info = strmap_get(&opt->priv->paths, info->last_directory);
        assert(dir_info);
        offset = (uintptr_t)info->offsets.items[info->offsets.nr-1].util;
        if (offset == info->versions.nr) {
                /*
                 * Actually, we don't need to create a tree object in this
                 * case.  Whenever all files within a directory disappear
                 * during the merge (e.g. unmodified on one side and
                 * deleted on the other, or files were renamed elsewhere),
                 * then we get here and the directory itself needs to be
                 * omitted from its parent tree as well.
                 */
                dir_info->is_null = 1;
        } else {
                /*
                 * Write out the tree to the git object directory, and also
                 * record the mode and oid in dir_info->result.
                 */
                dir_info->is_null = 0;
                dir_info->result.mode = S_IFDIR;
                write_tree(&dir_info->result.oid, &info->versions, offset,
                           opt->repo->hash_algo->rawsz);
        }

        /*
         * We've now used several entries from info->versions and one entry
         * from info->offsets, so we get rid of those values.
         */
        info->offsets.nr--;
        info->versions.nr = offset;

        /*
         * Now we've taken care of the completed directory, but we need to
         * prepare things since future entries will be in
         * new_directory_name.  (In particular, process_entry() will be
         * appending new entries to info->versions.)  So, we need to make
         * sure new_directory_name is the last entry in info->offsets.
         */
        prev_dir = info->offsets.nr == 0 ? NULL :
                   info->offsets.items[info->offsets.nr-1].string;
        if (new_directory_name != prev_dir) {
                uintptr_t c = info->versions.nr;
                string_list_append(&info->offsets,
                                   new_directory_name)->util = (void*)c;
        }

        /* And, of course, we need to update last_directory to match. */
        info->last_directory = new_directory_name;
        info->last_directory_len = strlen(info->last_directory);
}

/* Per entry merge function */
static void process_entry(struct merge_options *opt,
                          const char *path,
                          struct conflict_info *ci,
                          struct directory_versions *dir_metadata)
{
        VERIFY_CI(ci);
        assert(ci->filemask >= 0 && ci->filemask <= 7);
        /* ci->match_mask == 7 was handled in collect_merge_info_callback() */
        assert(ci->match_mask == 0 || ci->match_mask == 3 ||
               ci->match_mask == 5 || ci->match_mask == 6);

        if (ci->dirmask) {
                record_entry_for_tree(dir_metadata, path, &ci->merged);
                if (ci->filemask == 0)
                        /* nothing else to handle */
                        return;
                assert(ci->df_conflict);
        }

        if (ci->df_conflict) {
                die("Not yet implemented.");
        }

        /*
         * NOTE: Below there is a long switch-like if-elseif-elseif... block
         *       which the code goes through even for the df_conflict cases
         *       above.  Well, it will once we don't die-not-implemented above.
         */
        if (ci->match_mask) {
                ci->merged.clean = 1;
                if (ci->match_mask == 6) {
                        /* stages[1] == stages[2] */
                        ci->merged.result.mode = ci->stages[1].mode;
                        oidcpy(&ci->merged.result.oid, &ci->stages[1].oid);
                } else {
                        /* determine the mask of the side that didn't match */
                        unsigned int othermask = 7 & ~ci->match_mask;
                        int side = (othermask == 4) ? 2 : 1;

                        ci->merged.result.mode = ci->stages[side].mode;
                        ci->merged.is_null = !ci->merged.result.mode;
                        oidcpy(&ci->merged.result.oid, &ci->stages[side].oid);

                        assert(othermask == 2 || othermask == 4);
                        assert(ci->merged.is_null ==
                               (ci->filemask == ci->match_mask));
                }
        } else if (ci->filemask >= 6 &&
                   (S_IFMT & ci->stages[1].mode) !=
                   (S_IFMT & ci->stages[2].mode)) {
                /*
                 * Two different items from (file/submodule/symlink)
                 */
                die("Not yet implemented.");
        } else if (ci->filemask >= 6) {
                /*
                 * TODO: Needs a two-way or three-way content merge, but we're
                 * just being lazy and copying the version from HEAD and
                 * leaving it as conflicted.
                 */
                ci->merged.clean = 0;
                ci->merged.result.mode = ci->stages[1].mode;
                oidcpy(&ci->merged.result.oid, &ci->stages[1].oid);
        } else if (ci->filemask == 3 || ci->filemask == 5) {
                /* Modify/delete */
                die("Not yet implemented.");
        } else if (ci->filemask == 2 || ci->filemask == 4) {
                /* Added on one side */
                int side = (ci->filemask == 4) ? 2 : 1;
                ci->merged.result.mode = ci->stages[side].mode;
                oidcpy(&ci->merged.result.oid, &ci->stages[side].oid);
                ci->merged.clean = !ci->df_conflict;
        } else if (ci->filemask == 1) {
                /* Deleted on both sides */
                ci->merged.is_null = 1;
                ci->merged.result.mode = 0;
                oidcpy(&ci->merged.result.oid, &null_oid);
                ci->merged.clean = 1;
        }

        /*
         * If still conflicted, record it separately.  This allows us to later
         * iterate over just conflicted entries when updating the index instead
         * of iterating over all entries.
         */
        if (!ci->merged.clean)
                strmap_put(&opt->priv->conflicted, path, ci);
        record_entry_for_tree(dir_metadata, path, &ci->merged);
}

static void process_entries(struct merge_options *opt,
                            struct object_id *result_oid)
{
        struct hashmap_iter iter;
        struct strmap_entry *e;
        struct string_list plist = STRING_LIST_INIT_NODUP;
        struct string_list_item *entry;
        struct directory_versions dir_metadata = { STRING_LIST_INIT_NODUP,
                                                   STRING_LIST_INIT_NODUP,
                                                   NULL, 0 };

        if (strmap_empty(&opt->priv->paths)) {
                oidcpy(result_oid, opt->repo->hash_algo->empty_tree);
                return;
        }

        /* Hack to pre-allocate plist to the desired size */
        ALLOC_GROW(plist.items, strmap_get_size(&opt->priv->paths), plist.alloc);

        /* Put every entry from paths into plist, then sort */
        strmap_for_each_entry(&opt->priv->paths, &iter, e) {
                string_list_append(&plist, e->key)->util = e->value;
        }
        plist.cmp = string_list_df_name_compare;
        string_list_sort(&plist);

        /*
         * Iterate over the items in reverse order, so we can handle paths
         * below a directory before needing to handle the directory itself.
         *
         * This allows us to write subtrees before we need to write trees,
         * and it also enables sane handling of directory/file conflicts
         * (because it allows us to know whether the directory is still in
         * the way when it is time to process the file at the same path).
         */
        for (entry = &plist.items[plist.nr-1]; entry >= plist.items; --entry) {
                char *path = entry->string;
                /*
                 * NOTE: mi may actually be a pointer to a conflict_info, but
                 * we have to check mi->clean first to see if it's safe to
                 * reassign to such a pointer type.
                 */
                struct merged_info *mi = entry->util;

                write_completed_directory(opt, mi->directory_name,
                                          &dir_metadata);
                if (mi->clean)
                        record_entry_for_tree(&dir_metadata, path, mi);
                else {
                        struct conflict_info *ci = (struct conflict_info *)mi;
                        process_entry(opt, path, ci, &dir_metadata);
                }
        }

        if (dir_metadata.offsets.nr != 1 ||
            (uintptr_t)dir_metadata.offsets.items[0].util != 0) {
                printf("dir_metadata.offsets.nr = %d (should be 1)\n",
                       dir_metadata.offsets.nr);
                printf("dir_metadata.offsets.items[0].util = %u (should be 0)\n",
                       (unsigned)(uintptr_t)dir_metadata.offsets.items[0].util);
                fflush(stdout);
                BUG("dir_metadata accounting completely off; shouldn't happen");
        }
        write_tree(result_oid, &dir_metadata.versions, 0,
                   opt->repo->hash_algo->rawsz);
        string_list_clear(&plist, 0);
        string_list_clear(&dir_metadata.versions, 0);
        string_list_clear(&dir_metadata.offsets, 0);
}

static int checkout(struct merge_options *opt,
                    struct tree *prev,
                    struct tree *next)
{
        die("Not yet implemented.");
}

static int record_conflicted_index_entries(struct merge_options *opt,
                                           struct index_state *index,
                                           struct strmap *paths,
                                           struct strmap *conflicted)
{
        if (strmap_empty(conflicted))
                return 0;

        die("Not yet implemented.");
}

void merge_switch_to_result(struct merge_options *opt,
                            struct tree *head,
                            struct merge_result *result,
                            int update_worktree_and_index,
                            int display_update_msgs)
{
        assert(opt->priv == NULL);
        if (result->clean >= 0 && update_worktree_and_index) {
                struct merge_options_internal *opti = result->priv;

                if (checkout(opt, head, result->tree)) {
                        /* failure to function */
                        result->clean = -1;
                        return;
                }

                if (record_conflicted_index_entries(opt, opt->repo->index,
                                                    &opti->paths,
                                                    &opti->conflicted)) {
                        /* failure to function */
                        result->clean = -1;
                        return;
                }
        }

        if (display_update_msgs) {
                /* TODO: print out CONFLICT and other informational messages. */
        }

        merge_finalize(opt, result);
}

void merge_finalize(struct merge_options *opt,
                    struct merge_result *result)
{
        die("Not yet implemented");
}

static void merge_start(struct merge_options *opt, struct merge_result *result)
{
        /* Sanity checks on opt */
        assert(opt->repo);

        assert(opt->branch1 && opt->branch2);

        assert(opt->detect_directory_renames >= MERGE_DIRECTORY_RENAMES_NONE &&
               opt->detect_directory_renames <= MERGE_DIRECTORY_RENAMES_TRUE);
        assert(opt->rename_limit >= -1);
        assert(opt->rename_score >= 0 && opt->rename_score <= MAX_SCORE);
        assert(opt->show_rename_progress >= 0 && opt->show_rename_progress <= 1);

        assert(opt->xdl_opts >= 0);
        assert(opt->recursive_variant >= MERGE_VARIANT_NORMAL &&
               opt->recursive_variant <= MERGE_VARIANT_THEIRS);

        /*
         * detect_renames, verbosity, buffer_output, and obuf are ignored
         * fields that were used by "recursive" rather than "ort" -- but
         * sanity check them anyway.
         */
        assert(opt->detect_renames >= -1 &&
               opt->detect_renames <= DIFF_DETECT_COPY);
        assert(opt->verbosity >= 0 && opt->verbosity <= 5);
        assert(opt->buffer_output <= 2);
        assert(opt->obuf.len == 0);

        assert(opt->priv == NULL);

        /* Default to histogram diff.  Actually, just hardcode it...for now. */
        opt->xdl_opts = DIFF_WITH_ALG(opt, HISTOGRAM_DIFF);

        /* Initialization of opt->priv, our internal merge data */
        opt->priv = xcalloc(1, sizeof(*opt->priv));

        /*
         * Although we initialize opt->priv->paths with strdup_strings=0,
         * that's just to avoid making yet another copy of an allocated
         * string.  Putting the entry into paths means we are taking
         * ownership, so we will later free it.
         *
         * In contrast, conflicted just has a subset of keys from paths, so
         * we don't want to free those (it'd be a duplicate free).
         */
        strmap_init_with_options(&opt->priv->paths, NULL, 0);
        strmap_init_with_options(&opt->priv->conflicted, NULL, 0);
}

/*
 * Originally from merge_trees_internal(); heavily adapted, though.
 */
static void merge_ort_nonrecursive_internal(struct merge_options *opt,
                                            struct tree *merge_base,
                                            struct tree *side1,
                                            struct tree *side2,
                                            struct merge_result *result)
{
        struct object_id working_tree_oid;

        if (collect_merge_info(opt, merge_base, side1, side2) != 0) {
                /*
                 * TRANSLATORS: The %s arguments are: 1) tree hash of a merge
                 * base, and 2-3) the trees for the two trees we're merging.
                 */
                err(opt, _("collecting merge info failed for trees %s, %s, %s"),
                    oid_to_hex(&merge_base->object.oid),
                    oid_to_hex(&side1->object.oid),
                    oid_to_hex(&side2->object.oid));
                result->clean = -1;
                return;
        }

        result->clean = detect_and_process_renames(opt, merge_base,
                                                   side1, side2);
        process_entries(opt, &working_tree_oid);

        /* Set return values */
        result->tree = parse_tree_indirect(&working_tree_oid);
        /* existence of conflicted entries implies unclean */
        result->clean &= strmap_empty(&opt->priv->conflicted);
        if (!opt->priv->call_depth) {
                result->priv = opt->priv;
                opt->priv = NULL;
        }
}

void merge_incore_nonrecursive(struct merge_options *opt,
                               struct tree *merge_base,
                               struct tree *side1,
                               struct tree *side2,
                               struct merge_result *result)
{
        assert(opt->ancestor != NULL);
        merge_start(opt, result);
        merge_ort_nonrecursive_internal(opt, merge_base, side1, side2, result);
}

void merge_incore_recursive(struct merge_options *opt,
                            struct commit_list *merge_bases,
                            struct commit *side1,
                            struct commit *side2,
                            struct merge_result *result)
{
        die("Not yet implemented");
}