index-pack: make quantum of work smaller

[git] / xdiff / xpatience.c

/*
 *  LibXDiff by Davide Libenzi ( File Differential Library )
 *  Copyright (C) 2003-2016 Davide Libenzi, Johannes E. Schindelin
 *
 *  This library is free software; you can redistribute it and/or
 *  modify it under the terms of the GNU Lesser General Public
 *  License as published by the Free Software Foundation; either
 *  version 2.1 of the License, or (at your option) any later version.
 *
 *  This library is distributed in the hope that it will be useful,
 *  but WITHOUT ANY WARRANTY; without even the implied warranty of
 *  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
 *  Lesser General Public License for more details.
 *
 *  You should have received a copy of the GNU Lesser General Public
 *  License along with this library; if not, see
 *  <http://www.gnu.org/licenses/>.
 *
 *  Davide Libenzi <davidel@xmailserver.org>
 *
 */
#include "xinclude.h"

/*
 * The basic idea of patience diff is to find lines that are unique in
 * both files.  These are intuitively the ones that we want to see as
 * common lines.
 *
 * The maximal ordered sequence of such line pairs (where ordered means
 * that the order in the sequence agrees with the order of the lines in
 * both files) naturally defines an initial set of common lines.
 *
 * Now, the algorithm tries to extend the set of common lines by growing
 * the line ranges where the files have identical lines.
 *
 * Between those common lines, the patience diff algorithm is applied
 * recursively, until no unique line pairs can be found; these line ranges
 * are handled by the well-known Myers algorithm.
 */

#define NON_UNIQUE ULONG_MAX

/*
 * This is a hash mapping from line hash to line numbers in the first and
 * second file.
 */
struct hashmap {
        int nr, alloc;
        struct entry {
                unsigned long hash;
                /*
                 * 0 = unused entry, 1 = first line, 2 = second, etc.
                 * line2 is NON_UNIQUE if the line is not unique
                 * in either the first or the second file.
                 */
                unsigned long line1, line2;
                /*
                 * "next" & "previous" are used for the longest common
                 * sequence;
                 * initially, "next" reflects only the order in file1.
                 */
                struct entry *next, *previous;

                /*
                 * If 1, this entry can serve as an anchor. See
                 * Documentation/diff-options.txt for more information.
                 */
                unsigned anchor : 1;
        } *entries, *first, *last;
        /* were common records found? */
        unsigned long has_matches;
        mmfile_t *file1, *file2;
        xdfenv_t *env;
        xpparam_t const *xpp;
};

static int is_anchor(xpparam_t const *xpp, const char *line)
{
        int i;
        for (i = 0; i < xpp->anchors_nr; i++) {
                if (!strncmp(line, xpp->anchors[i], strlen(xpp->anchors[i])))
                        return 1;
        }
        return 0;
}

/* The argument "pass" is 1 for the first file, 2 for the second. */
static void insert_record(xpparam_t const *xpp, int line, struct hashmap *map,
                          int pass)
{
        xrecord_t **records = pass == 1 ?
                map->env->xdf1.recs : map->env->xdf2.recs;
        xrecord_t *record = records[line - 1], *other;
        /*
         * After xdl_prepare_env() (or more precisely, due to
         * xdl_classify_record()), the "ha" member of the records (AKA lines)
         * is _not_ the hash anymore, but a linearized version of it.  In
         * other words, the "ha" member is guaranteed to start with 0 and
         * the second record's ha can only be 0 or 1, etc.
         *
         * So we multiply ha by 2 in the hope that the hashing was
         * "unique enough".
         */
        int index = (int)((record->ha << 1) % map->alloc);

        while (map->entries[index].line1) {
                other = map->env->xdf1.recs[map->entries[index].line1 - 1];
                if (map->entries[index].hash != record->ha ||
                                !xdl_recmatch(record->ptr, record->size,
                                        other->ptr, other->size,
                                        map->xpp->flags)) {
                        if (++index >= map->alloc)
                                index = 0;
                        continue;
                }
                if (pass == 2)
                        map->has_matches = 1;
                if (pass == 1 || map->entries[index].line2)
                        map->entries[index].line2 = NON_UNIQUE;
                else
                        map->entries[index].line2 = line;
                return;
        }
        if (pass == 2)
                return;
        map->entries[index].line1 = line;
        map->entries[index].hash = record->ha;
        map->entries[index].anchor = is_anchor(xpp, map->env->xdf1.recs[line - 1]->ptr);
        if (!map->first)
                map->first = map->entries + index;
        if (map->last) {
                map->last->next = map->entries + index;
                map->entries[index].previous = map->last;
        }
        map->last = map->entries + index;
        map->nr++;
}

/*
 * This function has to be called for each recursion into the inter-hunk
 * parts, as previously non-unique lines can become unique when being
 * restricted to a smaller part of the files.
 *
 * It is assumed that env has been prepared using xdl_prepare().
 */
static int fill_hashmap(mmfile_t *file1, mmfile_t *file2,
                xpparam_t const *xpp, xdfenv_t *env,
                struct hashmap *result,
                int line1, int count1, int line2, int count2)
{
        result->file1 = file1;
        result->file2 = file2;
        result->xpp = xpp;
        result->env = env;

        /* We know exactly how large we want the hash map */
        result->alloc = count1 * 2;
        result->entries = (struct entry *)
                xdl_malloc(result->alloc * sizeof(struct entry));
        if (!result->entries)
                return -1;
        memset(result->entries, 0, result->alloc * sizeof(struct entry));

        /* First, fill with entries from the first file */
        while (count1--)
                insert_record(xpp, line1++, result, 1);

        /* Then search for matches in the second file */
        while (count2--)
                insert_record(xpp, line2++, result, 2);

        return 0;
}

/*
 * Find the longest sequence with a smaller last element (meaning a smaller
 * line2, as we construct the sequence with entries ordered by line1).
 */
static int binary_search(struct entry **sequence, int longest,
                struct entry *entry)
{
        int left = -1, right = longest;

        while (left + 1 < right) {
                int middle = left + (right - left) / 2;
                /* by construction, no two entries can be equal */
                if (sequence[middle]->line2 > entry->line2)
                        right = middle;
                else
                        left = middle;
        }
        /* return the index in "sequence", _not_ the sequence length */
        return left;
}

/*
 * The idea is to start with the list of common unique lines sorted by
 * the order in file1.  For each of these pairs, the longest (partial)
 * sequence whose last element's line2 is smaller is determined.
 *
 * For efficiency, the sequences are kept in a list containing exactly one
 * item per sequence length: the sequence with the smallest last
 * element (in terms of line2).
 */
static struct entry *find_longest_common_sequence(struct hashmap *map)
{
        struct entry **sequence = xdl_malloc(map->nr * sizeof(struct entry *));
        int longest = 0, i;
        struct entry *entry;

        /*
         * If not -1, this entry in sequence must never be overridden.
         * Therefore, overriding entries before this has no effect, so
         * do not do that either.
         */
        int anchor_i = -1;

        for (entry = map->first; entry; entry = entry->next) {
                if (!entry->line2 || entry->line2 == NON_UNIQUE)
                        continue;
                i = binary_search(sequence, longest, entry);
                entry->previous = i < 0 ? NULL : sequence[i];
                ++i;
                if (i <= anchor_i)
                        continue;
                sequence[i] = entry;
                if (entry->anchor) {
                        anchor_i = i;
                        longest = anchor_i + 1;
                } else if (i == longest) {
                        longest++;
                }
        }

        /* No common unique lines were found */
        if (!longest) {
                xdl_free(sequence);
                return NULL;
        }

        /* Iterate starting at the last element, adjusting the "next" members */
        entry = sequence[longest - 1];
        entry->next = NULL;
        while (entry->previous) {
                entry->previous->next = entry;
                entry = entry->previous;
        }
        xdl_free(sequence);
        return entry;
}

static int match(struct hashmap *map, int line1, int line2)
{
        xrecord_t *record1 = map->env->xdf1.recs[line1 - 1];
        xrecord_t *record2 = map->env->xdf2.recs[line2 - 1];
        return xdl_recmatch(record1->ptr, record1->size,
                record2->ptr, record2->size, map->xpp->flags);
}

static int patience_diff(mmfile_t *file1, mmfile_t *file2,
                xpparam_t const *xpp, xdfenv_t *env,
                int line1, int count1, int line2, int count2);

static int walk_common_sequence(struct hashmap *map, struct entry *first,
                int line1, int count1, int line2, int count2)
{
        int end1 = line1 + count1, end2 = line2 + count2;
        int next1, next2;

        for (;;) {
                /* Try to grow the line ranges of common lines */
                if (first) {
                        next1 = first->line1;
                        next2 = first->line2;
                        while (next1 > line1 && next2 > line2 &&
                                        match(map, next1 - 1, next2 - 1)) {
                                next1--;
                                next2--;
                        }
                } else {
                        next1 = end1;
                        next2 = end2;
                }
                while (line1 < next1 && line2 < next2 &&
                                match(map, line1, line2)) {
                        line1++;
                        line2++;
                }

                /* Recurse */
                if (next1 > line1 || next2 > line2) {
                        struct hashmap submap;

                        memset(&submap, 0, sizeof(submap));
                        if (patience_diff(map->file1, map->file2,
                                        map->xpp, map->env,
                                        line1, next1 - line1,
                                        line2, next2 - line2))
                                return -1;
                }

                if (!first)
                        return 0;

                while (first->next &&
                                first->next->line1 == first->line1 + 1 &&
                                first->next->line2 == first->line2 + 1)
                        first = first->next;

                line1 = first->line1 + 1;
                line2 = first->line2 + 1;

                first = first->next;
        }
}

static int fall_back_to_classic_diff(struct hashmap *map,
                int line1, int count1, int line2, int count2)
{
        xpparam_t xpp;
        xpp.flags = map->xpp->flags & ~XDF_DIFF_ALGORITHM_MASK;

        return xdl_fall_back_diff(map->env, &xpp,
                                  line1, count1, line2, count2);
}

/*
 * Recursively find the longest common sequence of unique lines,
 * and if none was found, ask xdl_do_diff() to do the job.
 *
 * This function assumes that env was prepared with xdl_prepare_env().
 */
static int patience_diff(mmfile_t *file1, mmfile_t *file2,
                xpparam_t const *xpp, xdfenv_t *env,
                int line1, int count1, int line2, int count2)
{
        struct hashmap map;
        struct entry *first;
        int result = 0;

        /* trivial case: one side is empty */
        if (!count1) {
                while(count2--)
                        env->xdf2.rchg[line2++ - 1] = 1;
                return 0;
        } else if (!count2) {
                while(count1--)
                        env->xdf1.rchg[line1++ - 1] = 1;
                return 0;
        }

        memset(&map, 0, sizeof(map));
        if (fill_hashmap(file1, file2, xpp, env, &map,
                        line1, count1, line2, count2))
                return -1;

        /* are there any matching lines at all? */
        if (!map.has_matches) {
                while(count1--)
                        env->xdf1.rchg[line1++ - 1] = 1;
                while(count2--)
                        env->xdf2.rchg[line2++ - 1] = 1;
                xdl_free(map.entries);
                return 0;
        }

        first = find_longest_common_sequence(&map);
        if (first)
                result = walk_common_sequence(&map, first,
                        line1, count1, line2, count2);
        else
                result = fall_back_to_classic_diff(&map,
                        line1, count1, line2, count2);

        xdl_free(map.entries);
        return result;
}

int xdl_do_patience_diff(mmfile_t *file1, mmfile_t *file2,
                xpparam_t const *xpp, xdfenv_t *env)
{
        if (xdl_prepare_env(file1, file2, xpp, env) < 0)
                return -1;

        /* environment is cleaned up in xdl_diff() */
        return patience_diff(file1, file2, xpp, env,
                        1, env->xdf1.nrec, 1, env->xdf2.nrec);
}