merge-ort: record the reason that we want a rename for a directory

[git] / diffcore-rename.c

/*
 *
 * Copyright (C) 2005 Junio C Hamano
 */
#include "cache.h"
#include "diff.h"
#include "diffcore.h"
#include "object-store.h"
#include "hashmap.h"
#include "progress.h"
#include "promisor-remote.h"
#include "strmap.h"

/* Table of rename/copy destinations */

static struct diff_rename_dst {
        struct diff_filepair *p;
        struct diff_filespec *filespec_to_free;
        int is_rename; /* false -> just a create; true -> rename or copy */
} *rename_dst;
static int rename_dst_nr, rename_dst_alloc;
/* Mapping from break source pathname to break destination index */
static struct strintmap *break_idx = NULL;

static struct diff_rename_dst *locate_rename_dst(struct diff_filepair *p)
{
        /* Lookup by p->ONE->path */
        int idx = break_idx ? strintmap_get(break_idx, p->one->path) : -1;
        return (idx == -1) ? NULL : &rename_dst[idx];
}

/*
 * Returns 0 on success, -1 if we found a duplicate.
 */
static int add_rename_dst(struct diff_filepair *p)
{
        ALLOC_GROW(rename_dst, rename_dst_nr + 1, rename_dst_alloc);
        rename_dst[rename_dst_nr].p = p;
        rename_dst[rename_dst_nr].filespec_to_free = NULL;
        rename_dst[rename_dst_nr].is_rename = 0;
        rename_dst_nr++;
        return 0;
}

/* Table of rename/copy src files */
static struct diff_rename_src {
        struct diff_filepair *p;
        unsigned short score; /* to remember the break score */
} *rename_src;
static int rename_src_nr, rename_src_alloc;

static void register_rename_src(struct diff_filepair *p)
{
        if (p->broken_pair) {
                if (!break_idx) {
                        break_idx = xmalloc(sizeof(*break_idx));
                        strintmap_init(break_idx, -1);
                }
                strintmap_set(break_idx, p->one->path, rename_dst_nr);
        }

        ALLOC_GROW(rename_src, rename_src_nr + 1, rename_src_alloc);
        rename_src[rename_src_nr].p = p;
        rename_src[rename_src_nr].score = p->score;
        rename_src_nr++;
}

static int basename_same(struct diff_filespec *src, struct diff_filespec *dst)
{
        int src_len = strlen(src->path), dst_len = strlen(dst->path);
        while (src_len && dst_len) {
                char c1 = src->path[--src_len];
                char c2 = dst->path[--dst_len];
                if (c1 != c2)
                        return 0;
                if (c1 == '/')
                        return 1;
        }
        return (!src_len || src->path[src_len - 1] == '/') &&
                (!dst_len || dst->path[dst_len - 1] == '/');
}

struct diff_score {
        int src; /* index in rename_src */
        int dst; /* index in rename_dst */
        unsigned short score;
        short name_score;
};

struct prefetch_options {
        struct repository *repo;
        int skip_unmodified;
};
static void prefetch(void *prefetch_options)
{
        struct prefetch_options *options = prefetch_options;
        int i;
        struct oid_array to_fetch = OID_ARRAY_INIT;

        for (i = 0; i < rename_dst_nr; i++) {
                if (rename_dst[i].p->renamed_pair)
                        /*
                         * The loop in diffcore_rename() will not need these
                         * blobs, so skip prefetching.
                         */
                        continue; /* already found exact match */
                diff_add_if_missing(options->repo, &to_fetch,
                                    rename_dst[i].p->two);
        }
        for (i = 0; i < rename_src_nr; i++) {
                if (options->skip_unmodified &&
                    diff_unmodified_pair(rename_src[i].p))
                        /*
                         * The loop in diffcore_rename() will not need these
                         * blobs, so skip prefetching.
                         */
                        continue;
                diff_add_if_missing(options->repo, &to_fetch,
                                    rename_src[i].p->one);
        }
        promisor_remote_get_direct(options->repo, to_fetch.oid, to_fetch.nr);
        oid_array_clear(&to_fetch);
}

static int estimate_similarity(struct repository *r,
                               struct diff_filespec *src,
                               struct diff_filespec *dst,
                               int minimum_score,
                               int skip_unmodified)
{
        /* src points at a file that existed in the original tree (or
         * optionally a file in the destination tree) and dst points
         * at a newly created file.  They may be quite similar, in which
         * case we want to say src is renamed to dst or src is copied into
         * dst, and then some edit has been applied to dst.
         *
         * Compare them and return how similar they are, representing
         * the score as an integer between 0 and MAX_SCORE.
         *
         * When there is an exact match, it is considered a better
         * match than anything else; the destination does not even
         * call into this function in that case.
         */
        unsigned long max_size, delta_size, base_size, src_copied, literal_added;
        int score;
        struct diff_populate_filespec_options dpf_options = {
                .check_size_only = 1
        };
        struct prefetch_options prefetch_options = {r, skip_unmodified};

        if (r == the_repository && has_promisor_remote()) {
                dpf_options.missing_object_cb = prefetch;
                dpf_options.missing_object_data = &prefetch_options;
        }

        /* We deal only with regular files.  Symlink renames are handled
         * only when they are exact matches --- in other words, no edits
         * after renaming.
         */
        if (!S_ISREG(src->mode) || !S_ISREG(dst->mode))
                return 0;

        /*
         * Need to check that source and destination sizes are
         * filled in before comparing them.
         *
         * If we already have "cnt_data" filled in, we know it's
         * all good (avoid checking the size for zero, as that
         * is a possible size - we really should have a flag to
         * say whether the size is valid or not!)
         */
        if (!src->cnt_data &&
            diff_populate_filespec(r, src, &dpf_options))
                return 0;
        if (!dst->cnt_data &&
            diff_populate_filespec(r, dst, &dpf_options))
                return 0;

        max_size = ((src->size > dst->size) ? src->size : dst->size);
        base_size = ((src->size < dst->size) ? src->size : dst->size);
        delta_size = max_size - base_size;

        /* We would not consider edits that change the file size so
         * drastically.  delta_size must be smaller than
         * (MAX_SCORE-minimum_score)/MAX_SCORE * min(src->size, dst->size).
         *
         * Note that base_size == 0 case is handled here already
         * and the final score computation below would not have a
         * divide-by-zero issue.
         */
        if (max_size * (MAX_SCORE-minimum_score) < delta_size * MAX_SCORE)
                return 0;

        dpf_options.check_size_only = 0;

        if (!src->cnt_data && diff_populate_filespec(r, src, &dpf_options))
                return 0;
        if (!dst->cnt_data && diff_populate_filespec(r, dst, &dpf_options))
                return 0;

        if (diffcore_count_changes(r, src, dst,
                                   &src->cnt_data, &dst->cnt_data,
                                   &src_copied, &literal_added))
                return 0;

        /* How similar are they?
         * what percentage of material in dst are from source?
         */
        if (!dst->size)
                score = 0; /* should not happen */
        else
                score = (int)(src_copied * MAX_SCORE / max_size);
        return score;
}

static void record_rename_pair(int dst_index, int src_index, int score)
{
        struct diff_filepair *src = rename_src[src_index].p;
        struct diff_filepair *dst = rename_dst[dst_index].p;

        if (dst->renamed_pair)
                die("internal error: dst already matched.");

        src->one->rename_used++;
        src->one->count++;

        rename_dst[dst_index].filespec_to_free = dst->one;
        rename_dst[dst_index].is_rename = 1;

        dst->one = src->one;
        dst->renamed_pair = 1;
        if (!strcmp(dst->one->path, dst->two->path))
                dst->score = rename_src[src_index].score;
        else
                dst->score = score;
}

/*
 * We sort the rename similarity matrix with the score, in descending
 * order (the most similar first).
 */
static int score_compare(const void *a_, const void *b_)
{
        const struct diff_score *a = a_, *b = b_;

        /* sink the unused ones to the bottom */
        if (a->dst < 0)
                return (0 <= b->dst);
        else if (b->dst < 0)
                return -1;

        if (a->score == b->score)
                return b->name_score - a->name_score;

        return b->score - a->score;
}

struct file_similarity {
        struct hashmap_entry entry;
        int index;
        struct diff_filespec *filespec;
};

static unsigned int hash_filespec(struct repository *r,
                                  struct diff_filespec *filespec)
{
        if (!filespec->oid_valid) {
                if (diff_populate_filespec(r, filespec, NULL))
                        return 0;
                hash_object_file(r->hash_algo, filespec->data, filespec->size,
                                 "blob", &filespec->oid);
        }
        return oidhash(&filespec->oid);
}

static int find_identical_files(struct hashmap *srcs,
                                int dst_index,
                                struct diff_options *options)
{
        int renames = 0;
        struct diff_filespec *target = rename_dst[dst_index].p->two;
        struct file_similarity *p, *best = NULL;
        int i = 100, best_score = -1;
        unsigned int hash = hash_filespec(options->repo, target);

        /*
         * Find the best source match for specified destination.
         */
        p = hashmap_get_entry_from_hash(srcs, hash, NULL,
                                        struct file_similarity, entry);
        hashmap_for_each_entry_from(srcs, p, entry) {
                int score;
                struct diff_filespec *source = p->filespec;

                /* False hash collision? */
                if (!oideq(&source->oid, &target->oid))
                        continue;
                /* Non-regular files? If so, the modes must match! */
                if (!S_ISREG(source->mode) || !S_ISREG(target->mode)) {
                        if (source->mode != target->mode)
                                continue;
                }
                /* Give higher scores to sources that haven't been used already */
                score = !source->rename_used;
                if (source->rename_used && options->detect_rename != DIFF_DETECT_COPY)
                        continue;
                score += basename_same(source, target);
                if (score > best_score) {
                        best = p;
                        best_score = score;
                        if (score == 2)
                                break;
                }

                /* Too many identical alternatives? Pick one */
                if (!--i)
                        break;
        }
        if (best) {
                record_rename_pair(dst_index, best->index, MAX_SCORE);
                renames++;
        }
        return renames;
}

static void insert_file_table(struct repository *r,
                              struct hashmap *table, int index,
                              struct diff_filespec *filespec)
{
        struct file_similarity *entry = xmalloc(sizeof(*entry));

        entry->index = index;
        entry->filespec = filespec;

        hashmap_entry_init(&entry->entry, hash_filespec(r, filespec));
        hashmap_add(table, &entry->entry);
}

/*
 * Find exact renames first.
 *
 * The first round matches up the up-to-date entries,
 * and then during the second round we try to match
 * cache-dirty entries as well.
 */
static int find_exact_renames(struct diff_options *options)
{
        int i, renames = 0;
        struct hashmap file_table;

        /* Add all sources to the hash table in reverse order, because
         * later on they will be retrieved in LIFO order.
         */
        hashmap_init(&file_table, NULL, NULL, rename_src_nr);
        for (i = rename_src_nr-1; i >= 0; i--)
                insert_file_table(options->repo,
                                  &file_table, i,
                                  rename_src[i].p->one);

        /* Walk the destinations and find best source match */
        for (i = 0; i < rename_dst_nr; i++)
                renames += find_identical_files(&file_table, i, options);

        /* Free the hash data structure and entries */
        hashmap_clear_and_free(&file_table, struct file_similarity, entry);

        return renames;
}

struct dir_rename_info {
        struct strintmap idx_map;
        struct strmap dir_rename_guess;
        struct strmap *dir_rename_count;
        struct strintmap *relevant_source_dirs;
        unsigned setup;
};

static char *get_dirname(const char *filename)
{
        char *slash = strrchr(filename, '/');
        return slash ? xstrndup(filename, slash - filename) : xstrdup("");
}

static void dirname_munge(char *filename)
{
        char *slash = strrchr(filename, '/');
        if (!slash)
                slash = filename;
        *slash = '\0';
}

static const char *get_highest_rename_path(struct strintmap *counts)
{
        int highest_count = 0;
        const char *highest_destination_dir = NULL;
        struct hashmap_iter iter;
        struct strmap_entry *entry;

        strintmap_for_each_entry(counts, &iter, entry) {
                const char *destination_dir = entry->key;
                intptr_t count = (intptr_t)entry->value;
                if (count > highest_count) {
                        highest_count = count;
                        highest_destination_dir = destination_dir;
                }
        }
        return highest_destination_dir;
}

static void increment_count(struct dir_rename_info *info,
                            char *old_dir,
                            char *new_dir)
{
        struct strintmap *counts;
        struct strmap_entry *e;

        /* Get the {new_dirs -> counts} mapping using old_dir */
        e = strmap_get_entry(info->dir_rename_count, old_dir);
        if (e) {
                counts = e->value;
        } else {
                counts = xmalloc(sizeof(*counts));
                strintmap_init_with_options(counts, 0, NULL, 1);
                strmap_put(info->dir_rename_count, old_dir, counts);
        }

        /* Increment the count for new_dir */
        strintmap_incr(counts, new_dir, 1);
}

static void update_dir_rename_counts(struct dir_rename_info *info,
                                     struct strintmap *dirs_removed,
                                     const char *oldname,
                                     const char *newname)
{
        char *old_dir = xstrdup(oldname);
        char *new_dir = xstrdup(newname);
        char new_dir_first_char = new_dir[0];
        int first_time_in_loop = 1;

        if (!info->setup)
                /*
                 * info->setup is 0 here in two cases: (1) all auxiliary
                 * vars (like dirs_removed) were NULL so
                 * initialize_dir_rename_info() returned early, or (2)
                 * either break detection or copy detection are active so
                 * that we never called initialize_dir_rename_info().  In
                 * the former case, we don't have enough info to know if
                 * directories were renamed (because dirs_removed lets us
                 * know about a necessary prerequisite, namely if they were
                 * removed), and in the latter, we don't care about
                 * directory renames or find_basename_matches.
                 *
                 * This matters because both basename and inexact matching
                 * will also call update_dir_rename_counts().  In either of
                 * the above two cases info->dir_rename_counts will not
                 * have been properly initialized which prevents us from
                 * updating it, but in these two cases we don't care about
                 * dir_rename_counts anyway, so we can just exit early.
                 */
                return;

        while (1) {
                /* Get old_dir, skip if its directory isn't relevant. */
                dirname_munge(old_dir);
                if (info->relevant_source_dirs &&
                    !strintmap_contains(info->relevant_source_dirs, old_dir))
                        break;

                /* Get new_dir */
                dirname_munge(new_dir);

                /*
                 * When renaming
                 *   "a/b/c/d/e/foo.c" -> "a/b/some/thing/else/e/foo.c"
                 * then this suggests that both
                 *   a/b/c/d/e/ => a/b/some/thing/else/e/
                 *   a/b/c/d/   => a/b/some/thing/else/
                 * so we want to increment counters for both.  We do NOT,
                 * however, also want to suggest that there was the following
                 * rename:
                 *   a/b/c/ => a/b/some/thing/
                 * so we need to quit at that point.
                 *
                 * Note the when first_time_in_loop, we only strip off the
                 * basename, and we don't care if that's different.
                 */
                if (!first_time_in_loop) {
                        char *old_sub_dir = strchr(old_dir, '\0')+1;
                        char *new_sub_dir = strchr(new_dir, '\0')+1;
                        if (!*new_dir) {
                                /*
                                 * Special case when renaming to root directory,
                                 * i.e. when new_dir == "".  In this case, we had
                                 * something like
                                 *    a/b/subdir => subdir
                                 * and so dirname_munge() sets things up so that
                                 *    old_dir = "a/b\0subdir\0"
                                 *    new_dir = "\0ubdir\0"
                                 * We didn't have a '/' to overwrite a '\0' onto
                                 * in new_dir, so we have to compare differently.
                                 */
                                if (new_dir_first_char != old_sub_dir[0] ||
                                    strcmp(old_sub_dir+1, new_sub_dir))
                                        break;
                        } else {
                                if (strcmp(old_sub_dir, new_sub_dir))
                                        break;
                        }
                }

                if (strintmap_contains(dirs_removed, old_dir))
                        increment_count(info, old_dir, new_dir);
                else
                        break;

                /* If we hit toplevel directory ("") for old or new dir, quit */
                if (!*old_dir || !*new_dir)
                        break;

                first_time_in_loop = 0;
        }

        /* Free resources we don't need anymore */
        free(old_dir);
        free(new_dir);
}

static void initialize_dir_rename_info(struct dir_rename_info *info,
                                       struct strintmap *relevant_sources,
                                       struct strintmap *dirs_removed,
                                       struct strmap *dir_rename_count)
{
        struct hashmap_iter iter;
        struct strmap_entry *entry;
        int i;

        if (!dirs_removed && !relevant_sources) {
                info->setup = 0;
                return;
        }
        info->setup = 1;

        info->dir_rename_count = dir_rename_count;
        if (!info->dir_rename_count) {
                info->dir_rename_count = xmalloc(sizeof(*dir_rename_count));
                strmap_init(info->dir_rename_count);
        }
        strintmap_init_with_options(&info->idx_map, -1, NULL, 0);
        strmap_init_with_options(&info->dir_rename_guess, NULL, 0);

        /* Setup info->relevant_source_dirs */
        info->relevant_source_dirs = NULL;
        if (dirs_removed || !relevant_sources) {
                info->relevant_source_dirs = dirs_removed; /* might be NULL */
        } else {
                info->relevant_source_dirs = xmalloc(sizeof(struct strintmap));
                strintmap_init(info->relevant_source_dirs, 0 /* unused */);
                strintmap_for_each_entry(relevant_sources, &iter, entry) {
                        char *dirname = get_dirname(entry->key);
                        if (!dirs_removed ||
                            strintmap_contains(dirs_removed, dirname))
                                strintmap_set(info->relevant_source_dirs,
                                              dirname, 0 /* value irrelevant */);
                        free(dirname);
                }
        }

        /*
         * Loop setting up both info->idx_map, and doing setup of
         * info->dir_rename_count.
         */
        for (i = 0; i < rename_dst_nr; ++i) {
                /*
                 * For non-renamed files, make idx_map contain mapping of
                 *   filename -> index (index within rename_dst, that is)
                 */
                if (!rename_dst[i].is_rename) {
                        char *filename = rename_dst[i].p->two->path;
                        strintmap_set(&info->idx_map, filename, i);
                        continue;
                }

                /*
                 * For everything else (i.e. renamed files), make
                 * dir_rename_count contain a map of a map:
                 *   old_directory -> {new_directory -> count}
                 * In other words, for every pair look at the directories for
                 * the old filename and the new filename and count how many
                 * times that pairing occurs.
                 */
                update_dir_rename_counts(info, dirs_removed,
                                         rename_dst[i].p->one->path,
                                         rename_dst[i].p->two->path);
        }

        /*
         * Now we collapse
         *    dir_rename_count: old_directory -> {new_directory -> count}
         * down to
         *    dir_rename_guess: old_directory -> best_new_directory
         * where best_new_directory is the one with the highest count.
         */
        strmap_for_each_entry(info->dir_rename_count, &iter, entry) {
                /* entry->key is source_dir */
                struct strintmap *counts = entry->value;
                char *best_newdir;

                best_newdir = xstrdup(get_highest_rename_path(counts));
                strmap_put(&info->dir_rename_guess, entry->key,
                           best_newdir);
        }
}

void partial_clear_dir_rename_count(struct strmap *dir_rename_count)
{
        struct hashmap_iter iter;
        struct strmap_entry *entry;

        strmap_for_each_entry(dir_rename_count, &iter, entry) {
                struct strintmap *counts = entry->value;
                strintmap_clear(counts);
        }
        strmap_partial_clear(dir_rename_count, 1);
}

static void cleanup_dir_rename_info(struct dir_rename_info *info,
                                    struct strintmap *dirs_removed,
                                    int keep_dir_rename_count)
{
        struct hashmap_iter iter;
        struct strmap_entry *entry;
        struct string_list to_remove = STRING_LIST_INIT_NODUP;
        int i;

        if (!info->setup)
                return;

        /* idx_map */
        strintmap_clear(&info->idx_map);

        /* dir_rename_guess */
        strmap_clear(&info->dir_rename_guess, 1);

        /* relevant_source_dirs */
        if (info->relevant_source_dirs &&
            info->relevant_source_dirs != dirs_removed) {
                strintmap_clear(info->relevant_source_dirs);
                FREE_AND_NULL(info->relevant_source_dirs);
        }

        /* dir_rename_count */
        if (!keep_dir_rename_count) {
                partial_clear_dir_rename_count(info->dir_rename_count);
                strmap_clear(info->dir_rename_count, 1);
                FREE_AND_NULL(info->dir_rename_count);
                return;
        }

        /*
         * Although dir_rename_count was passed in
         * diffcore_rename_extended() and we want to keep it around and
         * return it to that caller, we first want to remove any data
         * associated with directories that weren't renamed.
         */
        strmap_for_each_entry(info->dir_rename_count, &iter, entry) {
                const char *source_dir = entry->key;
                struct strintmap *counts = entry->value;

                if (!strintmap_get(dirs_removed, source_dir)) {
                        string_list_append(&to_remove, source_dir);
                        strintmap_clear(counts);
                        continue;
                }
        }
        for (i = 0; i < to_remove.nr; ++i)
                strmap_remove(info->dir_rename_count,
                              to_remove.items[i].string, 1);
        string_list_clear(&to_remove, 0);
}

static const char *get_basename(const char *filename)
{
        /*
         * gitbasename() has to worry about special drives, multiple
         * directory separator characters, trailing slashes, NULL or
         * empty strings, etc.  We only work on filenames as stored in
         * git, and thus get to ignore all those complications.
         */
        const char *base = strrchr(filename, '/');
        return base ? base + 1 : filename;
}

static int idx_possible_rename(char *filename, struct dir_rename_info *info)
{
        /*
         * Our comparison of files with the same basename (see
         * find_basename_matches() below), is only helpful when after exact
         * rename detection we have exactly one file with a given basename
         * among the rename sources and also only exactly one file with
         * that basename among the rename destinations.  When we have
         * multiple files with the same basename in either set, we do not
         * know which to compare against.  However, there are some
         * filenames that occur in large numbers (particularly
         * build-related filenames such as 'Makefile', '.gitignore', or
         * 'build.gradle' that potentially exist within every single
         * subdirectory), and for performance we want to be able to quickly
         * find renames for these files too.
         *
         * The reason basename comparisons are a useful heuristic was that it
         * is common for people to move files across directories while keeping
         * their filename the same.  If we had a way of determining or even
         * making a good educated guess about which directory these non-unique
         * basename files had moved the file to, we could check it.
         * Luckily...
         *
         * When an entire directory is in fact renamed, we have two factors
         * helping us out:
         *   (a) the original directory disappeared giving us a hint
         *       about when we can apply an extra heuristic.
         *   (a) we often have several files within that directory and
         *       subdirectories that are renamed without changes
         * So, rules for a heuristic:
         *   (0) If there basename matches are non-unique (the condition under
         *       which this function is called) AND
         *   (1) the directory in which the file was found has disappeared
         *       (i.e. dirs_removed is non-NULL and has a relevant entry) THEN
         *   (2) use exact renames of files within the directory to determine
         *       where the directory is likely to have been renamed to.  IF
         *       there is at least one exact rename from within that
         *       directory, we can proceed.
         *   (3) If there are multiple places the directory could have been
         *       renamed to based on exact renames, ignore all but one of them.
         *       Just use the destination with the most renames going to it.
         *   (4) Check if applying that directory rename to the original file
         *       would result in a destination filename that is in the
         *       potential rename set.  If so, return the index of the
         *       destination file (the index within rename_dst).
         *   (5) Compare the original file and returned destination for
         *       similarity, and if they are sufficiently similar, record the
         *       rename.
         *
         * This function, idx_possible_rename(), is only responsible for (4).
         * The conditions/steps in (1)-(3) are handled via setting up
         * dir_rename_count and dir_rename_guess in
         * initialize_dir_rename_info().  Steps (0) and (5) are handled by
         * the caller of this function.
         */
        char *old_dir, *new_dir;
        struct strbuf new_path = STRBUF_INIT;
        int idx;

        if (!info->setup)
                return -1;

        old_dir = get_dirname(filename);
        new_dir = strmap_get(&info->dir_rename_guess, old_dir);
        free(old_dir);
        if (!new_dir)
                return -1;

        strbuf_addstr(&new_path, new_dir);
        strbuf_addch(&new_path, '/');
        strbuf_addstr(&new_path, get_basename(filename));

        idx = strintmap_get(&info->idx_map, new_path.buf);
        strbuf_release(&new_path);
        return idx;
}

static int find_basename_matches(struct diff_options *options,
                                 int minimum_score,
                                 struct dir_rename_info *info,
                                 struct strintmap *relevant_sources,
                                 struct strintmap *dirs_removed)
{
        /*
         * When I checked in early 2020, over 76% of file renames in linux
         * just moved files to a different directory but kept the same
         * basename.  gcc did that with over 64% of renames, gecko did it
         * with over 79%, and WebKit did it with over 89%.
         *
         * Therefore we can bypass the normal exhaustive NxM matrix
         * comparison of similarities between all potential rename sources
         * and destinations by instead using file basename as a hint (i.e.
         * the portion of the filename after the last '/'), checking for
         * similarity between files with the same basename, and if we find
         * a pair that are sufficiently similar, record the rename pair and
         * exclude those two from the NxM matrix.
         *
         * This *might* cause us to find a less than optimal pairing (if
         * there is another file that we are even more similar to but has a
         * different basename).  Given the huge performance advantage
         * basename matching provides, and given the frequency with which
         * people use the same basename in real world projects, that's a
         * trade-off we are willing to accept when doing just rename
         * detection.
         *
         * If someone wants copy detection that implies they are willing to
         * spend more cycles to find similarities between files, so it may
         * be less likely that this heuristic is wanted.  If someone is
         * doing break detection, that means they do not want filename
         * similarity to imply any form of content similiarity, and thus
         * this heuristic would definitely be incompatible.
         */

        int i, renames = 0;
        struct strintmap sources;
        struct strintmap dests;

        /*
         * The prefeteching stuff wants to know if it can skip prefetching
         * blobs that are unmodified...and will then do a little extra work
         * to verify that the oids are indeed different before prefetching.
         * Unmodified blobs are only relevant when doing copy detection;
         * when limiting to rename detection, diffcore_rename[_extended]()
         * will never be called with unmodified source paths fed to us, so
         * the extra work necessary to check if rename_src entries are
         * unmodified would be a small waste.
         */
        int skip_unmodified = 0;

        /*
         * Create maps of basename -> fullname(s) for remaining sources and
         * dests.
         */
        strintmap_init_with_options(&sources, -1, NULL, 0);
        strintmap_init_with_options(&dests, -1, NULL, 0);
        for (i = 0; i < rename_src_nr; ++i) {
                char *filename = rename_src[i].p->one->path;
                const char *base;

                /* exact renames removed in remove_unneeded_paths_from_src() */
                assert(!rename_src[i].p->one->rename_used);

                /* Record index within rename_src (i) if basename is unique */
                base = get_basename(filename);
                if (strintmap_contains(&sources, base))
                        strintmap_set(&sources, base, -1);
                else
                        strintmap_set(&sources, base, i);
        }
        for (i = 0; i < rename_dst_nr; ++i) {
                char *filename = rename_dst[i].p->two->path;
                const char *base;

                if (rename_dst[i].is_rename)
                        continue; /* involved in exact match already. */

                /* Record index within rename_dst (i) if basename is unique */
                base = get_basename(filename);
                if (strintmap_contains(&dests, base))
                        strintmap_set(&dests, base, -1);
                else
                        strintmap_set(&dests, base, i);
        }

        /* Now look for basename matchups and do similarity estimation */
        for (i = 0; i < rename_src_nr; ++i) {
                char *filename = rename_src[i].p->one->path;
                const char *base = NULL;
                intptr_t src_index;
                intptr_t dst_index;

                /* Skip irrelevant sources */
                if (relevant_sources &&
                    !strintmap_contains(relevant_sources, filename))
                        continue;

                /*
                 * If the basename is unique among remaining sources, then
                 * src_index will equal 'i' and we can attempt to match it
                 * to a unique basename in the destinations.  Otherwise,
                 * use directory rename heuristics, if possible.
                 */
                base = get_basename(filename);
                src_index = strintmap_get(&sources, base);
                assert(src_index == -1 || src_index == i);

                if (strintmap_contains(&dests, base)) {
                        struct diff_filespec *one, *two;
                        int score;

                        /* Find a matching destination, if possible */
                        dst_index = strintmap_get(&dests, base);
                        if (src_index == -1 || dst_index == -1) {
                                src_index = i;
                                dst_index = idx_possible_rename(filename, info);
                        }
                        if (dst_index == -1)
                                continue;

                        /* Ignore this dest if already used in a rename */
                        if (rename_dst[dst_index].is_rename)
                                continue; /* already used previously */

                        /* Estimate the similarity */
                        one = rename_src[src_index].p->one;
                        two = rename_dst[dst_index].p->two;
                        score = estimate_similarity(options->repo, one, two,
                                                    minimum_score, skip_unmodified);

                        /* If sufficiently similar, record as rename pair */
                        if (score < minimum_score)
                                continue;
                        record_rename_pair(dst_index, src_index, score);
                        renames++;
                        update_dir_rename_counts(info, dirs_removed,
                                                 one->path, two->path);

                        /*
                         * Found a rename so don't need text anymore; if we
                         * didn't find a rename, the filespec_blob would get
                         * re-used when doing the matrix of comparisons.
                         */
                        diff_free_filespec_blob(one);
                        diff_free_filespec_blob(two);
                }
        }

        strintmap_clear(&sources);
        strintmap_clear(&dests);

        return renames;
}

#define NUM_CANDIDATE_PER_DST 4
static void record_if_better(struct diff_score m[], struct diff_score *o)
{
        int i, worst;

        /* find the worst one */
        worst = 0;
        for (i = 1; i < NUM_CANDIDATE_PER_DST; i++)
                if (score_compare(&m[i], &m[worst]) > 0)
                        worst = i;

        /* is it better than the worst one? */
        if (score_compare(&m[worst], o) > 0)
                m[worst] = *o;
}

/*
 * Returns:
 * 0 if we are under the limit;
 * 1 if we need to disable inexact rename detection;
 * 2 if we would be under the limit if we were given -C instead of -C -C.
 */
static int too_many_rename_candidates(int num_destinations, int num_sources,
                                      struct diff_options *options)
{
        int rename_limit = options->rename_limit;
        int i, limited_sources;

        options->needed_rename_limit = 0;

        /*
         * This basically does a test for the rename matrix not
         * growing larger than a "rename_limit" square matrix, ie:
         *
         *    num_destinations * num_sources > rename_limit * rename_limit
         *
         * We use st_mult() to check overflow conditions; in the
         * exceptional circumstance that size_t isn't large enough to hold
         * the multiplication, the system won't be able to allocate enough
         * memory for the matrix anyway.
         */
        if (rename_limit <= 0)
                rename_limit = 32767;
        if (st_mult(num_destinations, num_sources)
            <= st_mult(rename_limit, rename_limit))
                return 0;

        options->needed_rename_limit =
                num_sources > num_destinations ? num_sources : num_destinations;

        /* Are we running under -C -C? */
        if (!options->flags.find_copies_harder)
                return 1;

        /* Would we bust the limit if we were running under -C? */
        for (limited_sources = i = 0; i < num_sources; i++) {
                if (diff_unmodified_pair(rename_src[i].p))
                        continue;
                limited_sources++;
        }
        if (st_mult(num_destinations, limited_sources)
            <= st_mult(rename_limit, rename_limit))
                return 2;
        return 1;
}

static int find_renames(struct diff_score *mx,
                        int dst_cnt,
                        int minimum_score,
                        int copies,
                        struct dir_rename_info *info,
                        struct strintmap *dirs_removed)
{
        int count = 0, i;

        for (i = 0; i < dst_cnt * NUM_CANDIDATE_PER_DST; i++) {
                struct diff_rename_dst *dst;

                if ((mx[i].dst < 0) ||
                    (mx[i].score < minimum_score))
                        break; /* there is no more usable pair. */
                dst = &rename_dst[mx[i].dst];
                if (dst->is_rename)
                        continue; /* already done, either exact or fuzzy. */
                if (!copies && rename_src[mx[i].src].p->one->rename_used)
                        continue;
                record_rename_pair(mx[i].dst, mx[i].src, mx[i].score);
                count++;
                update_dir_rename_counts(info, dirs_removed,
                                         rename_src[mx[i].src].p->one->path,
                                         rename_dst[mx[i].dst].p->two->path);
        }
        return count;
}

static void remove_unneeded_paths_from_src(int detecting_copies,
                                           struct strintmap *interesting)
{
        int i, new_num_src;

        if (detecting_copies && !interesting)
                return; /* nothing to remove */
        if (break_idx)
                return; /* culling incompatible with break detection */

        /*
         * Note on reasons why we cull unneeded sources but not destinations:
         *   1) Pairings are stored in rename_dst (not rename_src), which we
         *      need to keep around.  So, we just can't cull rename_dst even
         *      if we wanted to.  But doing so wouldn't help because...
         *
         *   2) There is a matrix pairwise comparison that follows the
         *      "Performing inexact rename detection" progress message.
         *      Iterating over the destinations is done in the outer loop,
         *      hence we only iterate over each of those once and we can
         *      easily skip the outer loop early if the destination isn't
         *      relevant.  That's only one check per destination path to
         *      skip.
         *
         *      By contrast, the sources are iterated in the inner loop; if
         *      we check whether a source can be skipped, then we'll be
         *      checking it N separate times, once for each destination.
         *      We don't want to have to iterate over known-not-needed
         *      sources N times each, so avoid that by removing the sources
         *      from rename_src here.
         */
        for (i = 0, new_num_src = 0; i < rename_src_nr; i++) {
                struct diff_filespec *one = rename_src[i].p->one;

                /*
                 * renames are stored in rename_dst, so if a rename has
                 * already been detected using this source, we can just
                 * remove the source knowing rename_dst has its info.
                 */
                if (!detecting_copies && one->rename_used)
                        continue;

                /* If we don't care about the source path, skip it */
                if (interesting && !strintmap_contains(interesting, one->path))
                        continue;

                if (new_num_src < i)
                        memcpy(&rename_src[new_num_src], &rename_src[i],
                               sizeof(struct diff_rename_src));
                new_num_src++;
        }

        rename_src_nr = new_num_src;
}

static void handle_early_known_dir_renames(struct dir_rename_info *info,
                                           struct strintmap *relevant_sources,
                                           struct strintmap *dirs_removed)
{
        /*
         * Not yet implemented; directory renames are determined via an
         * aggregate of all renames under them and using a "majority wins"
         * rule.  The fact that "majority wins", though, means we don't need
         * all the renames under the given directory, we only need enough to
         * ensure we have a majority.
         *
         * For now, we don't have enough information to know if we have a
         * majority after exact renames and basename-guided rename detection,
         * so just return early without doing any extra filtering.
         */
        return;
}

void diffcore_rename_extended(struct diff_options *options,
                              struct strintmap *relevant_sources,
                              struct strintmap *dirs_removed,
                              struct strmap *dir_rename_count)
{
        int detect_rename = options->detect_rename;
        int minimum_score = options->rename_score;
        struct diff_queue_struct *q = &diff_queued_diff;
        struct diff_queue_struct outq;
        struct diff_score *mx;
        int i, j, rename_count, skip_unmodified = 0;
        int num_destinations, dst_cnt;
        int num_sources, want_copies;
        struct progress *progress = NULL;
        struct dir_rename_info info;

        trace2_region_enter("diff", "setup", options->repo);
        info.setup = 0;
        assert(!dir_rename_count || strmap_empty(dir_rename_count));
        want_copies = (detect_rename == DIFF_DETECT_COPY);
        if (dirs_removed && (break_idx || want_copies))
                BUG("dirs_removed incompatible with break/copy detection");
        if (break_idx && relevant_sources)
                BUG("break detection incompatible with source specification");
        if (!minimum_score)
                minimum_score = DEFAULT_RENAME_SCORE;

        for (i = 0; i < q->nr; i++) {
                struct diff_filepair *p = q->queue[i];
                if (!DIFF_FILE_VALID(p->one)) {
                        if (!DIFF_FILE_VALID(p->two))
                                continue; /* unmerged */
                        else if (options->single_follow &&
                                 strcmp(options->single_follow, p->two->path))
                                continue; /* not interested */
                        else if (!options->flags.rename_empty &&
                                 is_empty_blob_oid(&p->two->oid))
                                continue;
                        else if (add_rename_dst(p) < 0) {
                                warning("skipping rename detection, detected"
                                        " duplicate destination '%s'",
                                        p->two->path);
                                goto cleanup;
                        }
                }
                else if (!options->flags.rename_empty &&
                         is_empty_blob_oid(&p->one->oid))
                        continue;
                else if (!DIFF_PAIR_UNMERGED(p) && !DIFF_FILE_VALID(p->two)) {
                        /*
                         * If the source is a broken "delete", and
                         * they did not really want to get broken,
                         * that means the source actually stays.
                         * So we increment the "rename_used" score
                         * by one, to indicate ourselves as a user
                         */
                        if (p->broken_pair && !p->score)
                                p->one->rename_used++;
                        register_rename_src(p);
                }
                else if (want_copies) {
                        /*
                         * Increment the "rename_used" score by
                         * one, to indicate ourselves as a user.
                         */
                        p->one->rename_used++;
                        register_rename_src(p);
                }
        }
        trace2_region_leave("diff", "setup", options->repo);
        if (rename_dst_nr == 0 || rename_src_nr == 0)
                goto cleanup; /* nothing to do */

        trace2_region_enter("diff", "exact renames", options->repo);
        /*
         * We really want to cull the candidates list early
         * with cheap tests in order to avoid doing deltas.
         */
        rename_count = find_exact_renames(options);
        trace2_region_leave("diff", "exact renames", options->repo);

        /* Did we only want exact renames? */
        if (minimum_score == MAX_SCORE)
                goto cleanup;

        num_sources = rename_src_nr;

        if (want_copies || break_idx) {
                /*
                 * Cull sources:
                 *   - remove ones corresponding to exact renames
                 *   - remove ones not found in relevant_sources
                 */
                trace2_region_enter("diff", "cull after exact", options->repo);
                remove_unneeded_paths_from_src(want_copies, relevant_sources);
                trace2_region_leave("diff", "cull after exact", options->repo);
        } else {
                /* Determine minimum score to match basenames */
                double factor = 0.5;
                char *basename_factor = getenv("GIT_BASENAME_FACTOR");
                int min_basename_score;

                if (basename_factor)
                        factor = strtol(basename_factor, NULL, 10)/100.0;
                assert(factor >= 0.0 && factor <= 1.0);
                min_basename_score = minimum_score +
                        (int)(factor * (MAX_SCORE - minimum_score));

                /*
                 * Cull sources:
                 *   - remove ones involved in renames (found via exact match)
                 */
                trace2_region_enter("diff", "cull after exact", options->repo);
                remove_unneeded_paths_from_src(want_copies, NULL);
                trace2_region_leave("diff", "cull after exact", options->repo);

                /* Preparation for basename-driven matching. */
                trace2_region_enter("diff", "dir rename setup", options->repo);
                initialize_dir_rename_info(&info, relevant_sources,
                                           dirs_removed, dir_rename_count);
                trace2_region_leave("diff", "dir rename setup", options->repo);

                /* Utilize file basenames to quickly find renames. */
                trace2_region_enter("diff", "basename matches", options->repo);
                rename_count += find_basename_matches(options,
                                                      min_basename_score,
                                                      &info,
                                                      relevant_sources,
                                                      dirs_removed);
                trace2_region_leave("diff", "basename matches", options->repo);

                /*
                 * Cull sources, again:
                 *   - remove ones involved in renames (found via basenames)
                 *   - remove ones not found in relevant_sources
                 * and
                 *   - remove ones in relevant_sources which are needed only
                 *     for directory renames IF no ancestory directory
                 *     actually needs to know any more individual path
                 *     renames under them
                 */
                trace2_region_enter("diff", "cull basename", options->repo);
                remove_unneeded_paths_from_src(want_copies, relevant_sources);
                handle_early_known_dir_renames(&info, relevant_sources,
                                               dirs_removed);
                trace2_region_leave("diff", "cull basename", options->repo);
        }

        /* Calculate how many rename destinations are left */
        num_destinations = (rename_dst_nr - rename_count);
        num_sources = rename_src_nr; /* rename_src_nr reflects lower number */

        /* All done? */
        if (!num_destinations || !num_sources)
                goto cleanup;

        switch (too_many_rename_candidates(num_destinations, num_sources,
                                           options)) {
        case 1:
                goto cleanup;
        case 2:
                options->degraded_cc_to_c = 1;
                skip_unmodified = 1;
                break;
        default:
                break;
        }

        trace2_region_enter("diff", "inexact renames", options->repo);
        if (options->show_rename_progress) {
                progress = start_delayed_progress(
                                _("Performing inexact rename detection"),
                                (uint64_t)num_destinations * (uint64_t)num_sources);
        }

        mx = xcalloc(st_mult(NUM_CANDIDATE_PER_DST, num_destinations),
                     sizeof(*mx));
        for (dst_cnt = i = 0; i < rename_dst_nr; i++) {
                struct diff_filespec *two = rename_dst[i].p->two;
                struct diff_score *m;

                if (rename_dst[i].is_rename)
                        continue; /* exact or basename match already handled */

                m = &mx[dst_cnt * NUM_CANDIDATE_PER_DST];
                for (j = 0; j < NUM_CANDIDATE_PER_DST; j++)
                        m[j].dst = -1;

                for (j = 0; j < rename_src_nr; j++) {
                        struct diff_filespec *one = rename_src[j].p->one;
                        struct diff_score this_src;

                        assert(!one->rename_used || want_copies || break_idx);

                        if (skip_unmodified &&
                            diff_unmodified_pair(rename_src[j].p))
                                continue;

                        this_src.score = estimate_similarity(options->repo,
                                                             one, two,
                                                             minimum_score,
                                                             skip_unmodified);
                        this_src.name_score = basename_same(one, two);
                        this_src.dst = i;
                        this_src.src = j;
                        record_if_better(m, &this_src);
                        /*
                         * Once we run estimate_similarity,
                         * We do not need the text anymore.
                         */
                        diff_free_filespec_blob(one);
                        diff_free_filespec_blob(two);
                }
                dst_cnt++;
                display_progress(progress,
                                 (uint64_t)dst_cnt * (uint64_t)num_sources);
        }
        stop_progress(&progress);

        /* cost matrix sorted by most to least similar pair */
        STABLE_QSORT(mx, dst_cnt * NUM_CANDIDATE_PER_DST, score_compare);

        rename_count += find_renames(mx, dst_cnt, minimum_score, 0,
                                     &info, dirs_removed);
        if (want_copies)
                rename_count += find_renames(mx, dst_cnt, minimum_score, 1,
                                             &info, dirs_removed);
        free(mx);
        trace2_region_leave("diff", "inexact renames", options->repo);

 cleanup:
        /* At this point, we have found some renames and copies and they
         * are recorded in rename_dst.  The original list is still in *q.
         */
        trace2_region_enter("diff", "write back to queue", options->repo);
        DIFF_QUEUE_CLEAR(&outq);
        for (i = 0; i < q->nr; i++) {
                struct diff_filepair *p = q->queue[i];
                struct diff_filepair *pair_to_free = NULL;

                if (DIFF_PAIR_UNMERGED(p)) {
                        diff_q(&outq, p);
                }
                else if (!DIFF_FILE_VALID(p->one) && DIFF_FILE_VALID(p->two)) {
                        /* Creation */
                        diff_q(&outq, p);
                }
                else if (DIFF_FILE_VALID(p->one) && !DIFF_FILE_VALID(p->two)) {
                        /*
                         * Deletion
                         *
                         * We would output this delete record if:
                         *
                         * (1) this is a broken delete and the counterpart
                         *     broken create remains in the output; or
                         * (2) this is not a broken delete, and rename_dst
                         *     does not have a rename/copy to move p->one->path
                         *     out of existence.
                         *
                         * Otherwise, the counterpart broken create
                         * has been turned into a rename-edit; or
                         * delete did not have a matching create to
                         * begin with.
                         */
                        if (DIFF_PAIR_BROKEN(p)) {
                                /* broken delete */
                                struct diff_rename_dst *dst = locate_rename_dst(p);
                                if (!dst)
                                        BUG("tracking failed somehow; failed to find associated dst for broken pair");
                                if (dst->is_rename)
                                        /* counterpart is now rename/copy */
                                        pair_to_free = p;
                        }
                        else {
                                if (p->one->rename_used)
                                        /* this path remains */
                                        pair_to_free = p;
                        }

                        if (!pair_to_free)
                                diff_q(&outq, p);
                }
                else if (!diff_unmodified_pair(p))
                        /* all the usual ones need to be kept */
                        diff_q(&outq, p);
                else
                        /* no need to keep unmodified pairs; FIXME: remove earlier? */
                        pair_to_free = p;

                if (pair_to_free)
                        diff_free_filepair(pair_to_free);
        }
        diff_debug_queue("done copying original", &outq);

        free(q->queue);
        *q = outq;
        diff_debug_queue("done collapsing", q);

        for (i = 0; i < rename_dst_nr; i++)
                if (rename_dst[i].filespec_to_free)
                        free_filespec(rename_dst[i].filespec_to_free);

        cleanup_dir_rename_info(&info, dirs_removed, dir_rename_count != NULL);
        FREE_AND_NULL(rename_dst);
        rename_dst_nr = rename_dst_alloc = 0;
        FREE_AND_NULL(rename_src);
        rename_src_nr = rename_src_alloc = 0;
        if (break_idx) {
                strintmap_clear(break_idx);
                FREE_AND_NULL(break_idx);
        }
        trace2_region_leave("diff", "write back to queue", options->repo);
        return;
}

void diffcore_rename(struct diff_options *options)
{
        diffcore_rename_extended(options, NULL, NULL, NULL);
}