text: /*
   * 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, max_size;
  	unsigned long src_copied, literal_added, src_removed;
  
  	*merge_score_p = 0; /* assume no deletion --- "do not break"
  			     * is the default.
  			     */
  
  	if (S_ISREG(src->mode) != S_ISREG(dst->mode)) {
  		*merge_score_p = (int)MAX_SCORE;
  		return 1; /* even their types are different */
  	}
  
  	if (src->sha1_valid && dst->sha1_valid &&
  	    !hashcmp(src->sha1, dst->sha1))
  		return 0; /* they are the same */
  
  	if (diff_populate_filespec(src, 0) || diff_populate_filespec(dst, 0))
  		return 0; /* error but caught downstream */
  
  	max_size = ((src->size > dst->size) ? src->size : dst->size);
  	if (max_size < MINIMUM_BREAK_SIZE)
  		return 0; /* we do not break too small filepair */
  
  	if (diffcore_count_changes(src, dst,
  				   &src->cnt_data, &dst->cnt_data,
  				   0,
  				   &src_copied, &literal_added))
  		return 0;
  
  	/* sanity */
  	if (src->size < src_copied)
  		src_copied = src->size;
  	if (dst->size < literal_added + src_copied) {
  		if (src_copied < dst->size)
  			literal_added = dst->size - src_copied;
  		else
  			literal_added = 0;
  	}
  	src_removed = src->size - src_copied;
  
  	/* 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.
  	 */
  	*merge_score_p = (int)(src_removed * MAX_SCORE / src->size);
  	if (*merge_score_p > break_score)
  		return 1;
  
  	/* Extent of damage, which counts both inserts and
  	 * deletes.
  	 */
  	delta_size = src_removed + literal_added;
  	if (delta_size * MAX_SCORE / max_size < break_score)
  		return 0;
  
  	/* If you removed a lot without adding new material, that is
  	 * not really a rewrite.
  	 */
  	if ((src->size * break_score < src_removed * MAX_SCORE) &&
  	    (literal_added * 20 < src_removed) &&
  	    (literal_added * 20 < src_copied))
  		return 0;
  
  	return 1;
  }
  
  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;
  
  	DIFF_QUEUE_CLEAR(&outq);
  
  	for (i = 0; i < q->nr; i++) {
  		struct diff_filepair *p = q->queue[i];
  		int score;
  
  		/*
  		 * We deal only with in-place edit of blobs.
  		 * We do not break anything else.
  		 */
  		if (DIFF_FILE_VALID(p->one) && DIFF_FILE_VALID(p->two) &&
  		    object_type(p->one->mode) == OBJ_BLOB &&
  		    object_type(p->two->mode) == OBJ_BLOB &&
  		    !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;
  
  				diff_free_filespec_blob(p->one);
  				diff_free_filespec_blob(p->two);
  				free(p); /* not diff_free_filepair(), we are
  					  * reusing one and two here.
  					  */
  				continue;
  			}
  		}
  		diff_free_filespec_data(p->one);
  		diff_free_filespec_data(p->two);
  		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;
  
  	DIFF_QUEUE_CLEAR(&outq);
  
  	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;
  }
  