/*
 * Copyright (C) 2005 Junio C Hamano
 */
#include "cache.h"
#include "diff.h"
#include "diffcore.h"

static int should_break(struct diff_filespec *src,
                        struct diff_filespec *dst,
                        int break_score,
                        int *merge_score_p)
{
        /* dst is recorded as a modification of src.  Are they so
         * different that we are better off recording this as a pair
         * of delete and create?
         *
         * There are two criteria used in this algorithm.  For the
         * purposes of helping later rename/copy, we take both delete
         * and insert into account and estimate the amount of "edit".
         * If the edit is very large, we break this pair so that
         * rename/copy can pick the pieces up to match with other
         * files.
         *
         * On the other hand, we would want to ignore inserts for the
         * pure "complete rewrite" detection.  As long as most of the
         * existing contents were removed from the file, it is a
         * complete rewrite, and if sizable chunk from the original
         * still remains in the result, it is not a rewrite.  It does
         * not matter how much or how little new material is added to
         * the file.
         *
         * The score we leave for such a broken filepair uses the
         * latter definition so that later clean-up stage can find the
         * pieces that should not have been broken according to the
         * latter definition after rename/copy runs, and merge the
         * broken pair that have a score lower than given criteria
         * back together.  The break operation itself happens
         * according to the former definition.
         *
         * The minimum_edit parameter tells us when to break (the
         * amount of "edit" required for us to consider breaking the
         * pair).  We leave the amount of deletion in *merge_score_p
         * when we return.
         *
         * The value we return is 1 if we want the pair to be broken,
         * or 0 if we do not.
         */
        unsigned long delta_size, base_size, src_copied, literal_added;
        int to_break = 0;

        *merge_score_p = 0; /* assume no deletion --- "do not break"
                             * is the default.
                             */

        if (!S_ISREG(src->mode) || !S_ISREG(dst->mode))
                return 0; /* leave symlink rename alone */

        if (src->sha1_valid && dst->sha1_valid &&
            !memcmp(src->sha1, dst->sha1, 20))
                return 0; /* they are the same */

        if (diff_populate_filespec(src, 0) || diff_populate_filespec(dst, 0))
                return 0; /* error but caught downstream */

        base_size = ((src->size < dst->size) ? src->size : dst->size);
        if (base_size < MINIMUM_BREAK_SIZE)
                return 0; /* we do not break too small filepair */

        if (diffcore_count_changes(src->data, src->size,
                                   dst->data, dst->size,
                                   0,
                                   &src_copied, &literal_added))
                return 0;

        /* Compute merge-score, which is "how much is removed
         * from the source material".  The clean-up stage will
         * merge the surviving pair together if the score is
         * less than the minimum, after rename/copy runs.
         */
        if (src->size <= src_copied)
                ; /* all copied, nothing removed */
        else {
                delta_size = src->size - src_copied;
                *merge_score_p = delta_size * MAX_SCORE / src->size;
        }
        
        /* Extent of damage, which counts both inserts and
         * deletes.
         */
        if (src->size + literal_added <= src_copied)
                delta_size = 0; /* avoid wrapping around */
        else
                delta_size = (src->size - src_copied) + literal_added;
        
        /* We break if the edit exceeds the minimum.
         * i.e. (break_score / MAX_SCORE < delta_size / base_size)
         */
        if (break_score * base_size < delta_size * MAX_SCORE)
                to_break = 1;

        return to_break;
}

void diffcore_break(int break_score)
{
        struct diff_queue_struct *q = &diff_queued_diff;
        struct diff_queue_struct outq;

        /* When the filepair has this much edit (insert and delete),
         * it is first considered to be a rewrite and broken into a
         * create and delete filepair.  This is to help breaking a
         * file that had too much new stuff added, possibly from
         * moving contents from another file, so that rename/copy can
         * match it with the other file.
         *
         * int break_score; we reuse incoming parameter for this.
         */

        /* After a pair is broken according to break_score and
         * subjected to rename/copy, both of them may survive intact,
         * due to lack of suitable rename/copy peer.  Or, the caller
         * may be calling us without using rename/copy.  When that
         * happens, we merge the broken pieces back into one
         * modification together if the pair did not have more than
         * this much delete.  For this computation, we do not take
         * insert into account at all.  If you start from a 100-line
         * file and delete 97 lines of it, it does not matter if you
         * add 27 lines to it to make a new 30-line file or if you add
         * 997 lines to it to make a 1000-line file.  Either way what
         * you did was a rewrite of 97%.  On the other hand, if you
         * delete 3 lines, keeping 97 lines intact, it does not matter
         * if you add 3 lines to it to make a new 100-line file or if
         * you add 903 lines to it to make a new 1000-line file.
         * Either way you did a lot of additions and not a rewrite.
         * This merge happens to catch the latter case.  A merge_score
         * of 80% would be a good default value (a broken pair that
         * has score lower than merge_score will be merged back
         * together).
         */
        int merge_score;
        int i;

        /* See comment on DEFAULT_BREAK_SCORE and
         * DEFAULT_MERGE_SCORE in diffcore.h
         */
        merge_score = (break_score >> 16) & 0xFFFF;
        break_score = (break_score & 0xFFFF);

        if (!break_score)
                break_score = DEFAULT_BREAK_SCORE;
        if (!merge_score)
                merge_score = DEFAULT_MERGE_SCORE;

        outq.nr = outq.alloc = 0;
        outq.queue = NULL;

        for (i = 0; i < q->nr; i++) {
                struct diff_filepair *p = q->queue[i];
                int score;

                /* We deal only with in-place edit of non directory.
                 * We do not break anything else.
                 */
                if (DIFF_FILE_VALID(p->one) && DIFF_FILE_VALID(p->two) &&
                    !S_ISDIR(p->one->mode) && !S_ISDIR(p->two->mode) &&
                    !strcmp(p->one->path, p->two->path)) {
                        if (should_break(p->one, p->two,
                                         break_score, &score)) {
                                /* Split this into delete and create */
                                struct diff_filespec *null_one, *null_two;
                                struct diff_filepair *dp;

                                /* Set score to 0 for the pair that
                                 * needs to be merged back together
                                 * should they survive rename/copy.
                                 * Also we do not want to break very
                                 * small files.
                                 */
                                if (score < merge_score)
                                        score = 0;

                                /* deletion of one */
                                null_one = alloc_filespec(p->one->path);
                                dp = diff_queue(&outq, p->one, null_one);
                                dp->score = score;
                                dp->broken_pair = 1;

                                /* creation of two */
                                null_two = alloc_filespec(p->two->path);
                                dp = diff_queue(&outq, null_two, p->two);
                                dp->score = score;
                                dp->broken_pair = 1;

                                free(p); /* not diff_free_filepair(), we are
                                          * reusing one and two here.
                                          */
                                continue;
                        }
                }
                diff_q(&outq, p);
        }
        free(q->queue);
        *q = outq;

        return;
}

static void merge_broken(struct diff_filepair *p,
                         struct diff_filepair *pp,
                         struct diff_queue_struct *outq)
{
        /* p and pp are broken pairs we want to merge */
        struct diff_filepair *c = p, *d = pp, *dp;
        if (DIFF_FILE_VALID(p->one)) {
                /* this must be a delete half */
                d = p; c = pp;
        }
        /* Sanity check */
        if (!DIFF_FILE_VALID(d->one))
                die("internal error in merge #1");
        if (DIFF_FILE_VALID(d->two))
                die("internal error in merge #2");
        if (DIFF_FILE_VALID(c->one))
                die("internal error in merge #3");
        if (!DIFF_FILE_VALID(c->two))
                die("internal error in merge #4");

        dp = diff_queue(outq, d->one, c->two);
        dp->score = p->score;
        diff_free_filespec_data(d->two);
        diff_free_filespec_data(c->one);
        free(d);
        free(c);
}

void diffcore_merge_broken(void)
{
        struct diff_queue_struct *q = &diff_queued_diff;
        struct diff_queue_struct outq;
        int i, j;

        outq.nr = outq.alloc = 0;
        outq.queue = NULL;

        for (i = 0; i < q->nr; i++) {
                struct diff_filepair *p = q->queue[i];
                if (!p)
                        /* we already merged this with its peer */
                        continue;
                else if (p->broken_pair &&
                         !strcmp(p->one->path, p->two->path)) {
                        /* If the peer also survived rename/copy, then
                         * we merge them back together.
                         */
                        for (j = i + 1; j < q->nr; j++) {
                                struct diff_filepair *pp = q->queue[j];
                                if (pp->broken_pair &&
                                    !strcmp(pp->one->path, pp->two->path) &&
                                    !strcmp(p->one->path, pp->two->path)) {
                                        /* Peer survived.  Merge them */
                                        merge_broken(p, pp, &outq);
                                        q->queue[j] = NULL;
                                        break;
                                }
                        }
                        if (q->nr <= j)
                                /* The peer did not survive, so we keep
                                 * it in the output.
                                 */
                                diff_q(&outq, p);
                }
                else
                        diff_q(&outq, p);
        }
        free(q->queue);
        *q = outq;

        return;
}