stash -p: (partially) fix bug concerning split hunks

[git/debian.git] / xdiff / xdiffi.c

/*
 *  LibXDiff by Davide Libenzi ( File Differential Library )
 *  Copyright (C) 2003  Davide Libenzi
 *
 *  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"

#define XDL_MAX_COST_MIN 256
#define XDL_HEUR_MIN_COST 256
#define XDL_LINE_MAX (long)((1UL << (CHAR_BIT * sizeof(long) - 1)) - 1)
#define XDL_SNAKE_CNT 20
#define XDL_K_HEUR 4

typedef struct s_xdpsplit {
        long i1, i2;
        int min_lo, min_hi;
} xdpsplit_t;

/*
 * See "An O(ND) Difference Algorithm and its Variations", by Eugene Myers.
 * Basically considers a "box" (off1, off2, lim1, lim2) and scan from both
 * the forward diagonal starting from (off1, off2) and the backward diagonal
 * starting from (lim1, lim2). If the K values on the same diagonal crosses
 * returns the furthest point of reach. We might encounter expensive edge cases
 * using this algorithm, so a little bit of heuristic is needed to cut the
 * search and to return a suboptimal point.
 */
static long xdl_split(unsigned long const *ha1, long off1, long lim1,
                      unsigned long const *ha2, long off2, long lim2,
                      long *kvdf, long *kvdb, int need_min, xdpsplit_t *spl,
                      xdalgoenv_t *xenv) {
        long dmin = off1 - lim2, dmax = lim1 - off2;
        long fmid = off1 - off2, bmid = lim1 - lim2;
        long odd = (fmid - bmid) & 1;
        long fmin = fmid, fmax = fmid;
        long bmin = bmid, bmax = bmid;
        long ec, d, i1, i2, prev1, best, dd, v, k;

        /*
         * Set initial diagonal values for both forward and backward path.
         */
        kvdf[fmid] = off1;
        kvdb[bmid] = lim1;

        for (ec = 1;; ec++) {
                int got_snake = 0;

                /*
                 * We need to extend the diagonal "domain" by one. If the next
                 * values exits the box boundaries we need to change it in the
                 * opposite direction because (max - min) must be a power of
                 * two.
                 *
                 * Also we initialize the external K value to -1 so that we can
                 * avoid extra conditions in the check inside the core loop.
                 */
                if (fmin > dmin)
                        kvdf[--fmin - 1] = -1;
                else
                        ++fmin;
                if (fmax < dmax)
                        kvdf[++fmax + 1] = -1;
                else
                        --fmax;

                for (d = fmax; d >= fmin; d -= 2) {
                        if (kvdf[d - 1] >= kvdf[d + 1])
                                i1 = kvdf[d - 1] + 1;
                        else
                                i1 = kvdf[d + 1];
                        prev1 = i1;
                        i2 = i1 - d;
                        for (; i1 < lim1 && i2 < lim2 && ha1[i1] == ha2[i2]; i1++, i2++);
                        if (i1 - prev1 > xenv->snake_cnt)
                                got_snake = 1;
                        kvdf[d] = i1;
                        if (odd && bmin <= d && d <= bmax && kvdb[d] <= i1) {
                                spl->i1 = i1;
                                spl->i2 = i2;
                                spl->min_lo = spl->min_hi = 1;
                                return ec;
                        }
                }

                /*
                 * We need to extend the diagonal "domain" by one. If the next
                 * values exits the box boundaries we need to change it in the
                 * opposite direction because (max - min) must be a power of
                 * two.
                 *
                 * Also we initialize the external K value to -1 so that we can
                 * avoid extra conditions in the check inside the core loop.
                 */
                if (bmin > dmin)
                        kvdb[--bmin - 1] = XDL_LINE_MAX;
                else
                        ++bmin;
                if (bmax < dmax)
                        kvdb[++bmax + 1] = XDL_LINE_MAX;
                else
                        --bmax;

                for (d = bmax; d >= bmin; d -= 2) {
                        if (kvdb[d - 1] < kvdb[d + 1])
                                i1 = kvdb[d - 1];
                        else
                                i1 = kvdb[d + 1] - 1;
                        prev1 = i1;
                        i2 = i1 - d;
                        for (; i1 > off1 && i2 > off2 && ha1[i1 - 1] == ha2[i2 - 1]; i1--, i2--);
                        if (prev1 - i1 > xenv->snake_cnt)
                                got_snake = 1;
                        kvdb[d] = i1;
                        if (!odd && fmin <= d && d <= fmax && i1 <= kvdf[d]) {
                                spl->i1 = i1;
                                spl->i2 = i2;
                                spl->min_lo = spl->min_hi = 1;
                                return ec;
                        }
                }

                if (need_min)
                        continue;

                /*
                 * If the edit cost is above the heuristic trigger and if
                 * we got a good snake, we sample current diagonals to see
                 * if some of them have reached an "interesting" path. Our
                 * measure is a function of the distance from the diagonal
                 * corner (i1 + i2) penalized with the distance from the
                 * mid diagonal itself. If this value is above the current
                 * edit cost times a magic factor (XDL_K_HEUR) we consider
                 * it interesting.
                 */
                if (got_snake && ec > xenv->heur_min) {
                        for (best = 0, d = fmax; d >= fmin; d -= 2) {
                                dd = d > fmid ? d - fmid: fmid - d;
                                i1 = kvdf[d];
                                i2 = i1 - d;
                                v = (i1 - off1) + (i2 - off2) - dd;

                                if (v > XDL_K_HEUR * ec && v > best &&
                                    off1 + xenv->snake_cnt <= i1 && i1 < lim1 &&
                                    off2 + xenv->snake_cnt <= i2 && i2 < lim2) {
                                        for (k = 1; ha1[i1 - k] == ha2[i2 - k]; k++)
                                                if (k == xenv->snake_cnt) {
                                                        best = v;
                                                        spl->i1 = i1;
                                                        spl->i2 = i2;
                                                        break;
                                                }
                                }
                        }
                        if (best > 0) {
                                spl->min_lo = 1;
                                spl->min_hi = 0;
                                return ec;
                        }

                        for (best = 0, d = bmax; d >= bmin; d -= 2) {
                                dd = d > bmid ? d - bmid: bmid - d;
                                i1 = kvdb[d];
                                i2 = i1 - d;
                                v = (lim1 - i1) + (lim2 - i2) - dd;

                                if (v > XDL_K_HEUR * ec && v > best &&
                                    off1 < i1 && i1 <= lim1 - xenv->snake_cnt &&
                                    off2 < i2 && i2 <= lim2 - xenv->snake_cnt) {
                                        for (k = 0; ha1[i1 + k] == ha2[i2 + k]; k++)
                                                if (k == xenv->snake_cnt - 1) {
                                                        best = v;
                                                        spl->i1 = i1;
                                                        spl->i2 = i2;
                                                        break;
                                                }
                                }
                        }
                        if (best > 0) {
                                spl->min_lo = 0;
                                spl->min_hi = 1;
                                return ec;
                        }
                }

                /*
                 * Enough is enough. We spent too much time here and now we
                 * collect the furthest reaching path using the (i1 + i2)
                 * measure.
                 */
                if (ec >= xenv->mxcost) {
                        long fbest, fbest1, bbest, bbest1;

                        fbest = fbest1 = -1;
                        for (d = fmax; d >= fmin; d -= 2) {
                                i1 = XDL_MIN(kvdf[d], lim1);
                                i2 = i1 - d;
                                if (lim2 < i2)
                                        i1 = lim2 + d, i2 = lim2;
                                if (fbest < i1 + i2) {
                                        fbest = i1 + i2;
                                        fbest1 = i1;
                                }
                        }

                        bbest = bbest1 = XDL_LINE_MAX;
                        for (d = bmax; d >= bmin; d -= 2) {
                                i1 = XDL_MAX(off1, kvdb[d]);
                                i2 = i1 - d;
                                if (i2 < off2)
                                        i1 = off2 + d, i2 = off2;
                                if (i1 + i2 < bbest) {
                                        bbest = i1 + i2;
                                        bbest1 = i1;
                                }
                        }

                        if ((lim1 + lim2) - bbest < fbest - (off1 + off2)) {
                                spl->i1 = fbest1;
                                spl->i2 = fbest - fbest1;
                                spl->min_lo = 1;
                                spl->min_hi = 0;
                        } else {
                                spl->i1 = bbest1;
                                spl->i2 = bbest - bbest1;
                                spl->min_lo = 0;
                                spl->min_hi = 1;
                        }
                        return ec;
                }
        }
}


/*
 * Rule: "Divide et Impera" (divide & conquer). Recursively split the box in
 * sub-boxes by calling the box splitting function. Note that the real job
 * (marking changed lines) is done in the two boundary reaching checks.
 */
int xdl_recs_cmp(diffdata_t *dd1, long off1, long lim1,
                 diffdata_t *dd2, long off2, long lim2,
                 long *kvdf, long *kvdb, int need_min, xdalgoenv_t *xenv) {
        unsigned long const *ha1 = dd1->ha, *ha2 = dd2->ha;

        /*
         * Shrink the box by walking through each diagonal snake (SW and NE).
         */
        for (; off1 < lim1 && off2 < lim2 && ha1[off1] == ha2[off2]; off1++, off2++);
        for (; off1 < lim1 && off2 < lim2 && ha1[lim1 - 1] == ha2[lim2 - 1]; lim1--, lim2--);

        /*
         * If one dimension is empty, then all records on the other one must
         * be obviously changed.
         */
        if (off1 == lim1) {
                char *rchg2 = dd2->rchg;
                long *rindex2 = dd2->rindex;

                for (; off2 < lim2; off2++)
                        rchg2[rindex2[off2]] = 1;
        } else if (off2 == lim2) {
                char *rchg1 = dd1->rchg;
                long *rindex1 = dd1->rindex;

                for (; off1 < lim1; off1++)
                        rchg1[rindex1[off1]] = 1;
        } else {
                xdpsplit_t spl;
                spl.i1 = spl.i2 = 0;

                /*
                 * Divide ...
                 */
                if (xdl_split(ha1, off1, lim1, ha2, off2, lim2, kvdf, kvdb,
                              need_min, &spl, xenv) < 0) {

                        return -1;
                }

                /*
                 * ... et Impera.
                 */
                if (xdl_recs_cmp(dd1, off1, spl.i1, dd2, off2, spl.i2,
                                 kvdf, kvdb, spl.min_lo, xenv) < 0 ||
                    xdl_recs_cmp(dd1, spl.i1, lim1, dd2, spl.i2, lim2,
                                 kvdf, kvdb, spl.min_hi, xenv) < 0) {

                        return -1;
                }
        }

        return 0;
}


int xdl_do_diff(mmfile_t *mf1, mmfile_t *mf2, xpparam_t const *xpp,
                xdfenv_t *xe) {
        long ndiags;
        long *kvd, *kvdf, *kvdb;
        xdalgoenv_t xenv;
        diffdata_t dd1, dd2;

        if (XDF_DIFF_ALG(xpp->flags) == XDF_PATIENCE_DIFF)
                return xdl_do_patience_diff(mf1, mf2, xpp, xe);

        if (XDF_DIFF_ALG(xpp->flags) == XDF_HISTOGRAM_DIFF)
                return xdl_do_histogram_diff(mf1, mf2, xpp, xe);

        if (xdl_prepare_env(mf1, mf2, xpp, xe) < 0) {

                return -1;
        }

        /*
         * Allocate and setup K vectors to be used by the differential
         * algorithm.
         *
         * One is to store the forward path and one to store the backward path.
         */
        ndiags = xe->xdf1.nreff + xe->xdf2.nreff + 3;
        if (!(kvd = (long *) xdl_malloc((2 * ndiags + 2) * sizeof(long)))) {

                xdl_free_env(xe);
                return -1;
        }
        kvdf = kvd;
        kvdb = kvdf + ndiags;
        kvdf += xe->xdf2.nreff + 1;
        kvdb += xe->xdf2.nreff + 1;

        xenv.mxcost = xdl_bogosqrt(ndiags);
        if (xenv.mxcost < XDL_MAX_COST_MIN)
                xenv.mxcost = XDL_MAX_COST_MIN;
        xenv.snake_cnt = XDL_SNAKE_CNT;
        xenv.heur_min = XDL_HEUR_MIN_COST;

        dd1.nrec = xe->xdf1.nreff;
        dd1.ha = xe->xdf1.ha;
        dd1.rchg = xe->xdf1.rchg;
        dd1.rindex = xe->xdf1.rindex;
        dd2.nrec = xe->xdf2.nreff;
        dd2.ha = xe->xdf2.ha;
        dd2.rchg = xe->xdf2.rchg;
        dd2.rindex = xe->xdf2.rindex;

        if (xdl_recs_cmp(&dd1, 0, dd1.nrec, &dd2, 0, dd2.nrec,
                         kvdf, kvdb, (xpp->flags & XDF_NEED_MINIMAL) != 0, &xenv) < 0) {

                xdl_free(kvd);
                xdl_free_env(xe);
                return -1;
        }

        xdl_free(kvd);

        return 0;
}


static xdchange_t *xdl_add_change(xdchange_t *xscr, long i1, long i2, long chg1, long chg2) {
        xdchange_t *xch;

        if (!(xch = (xdchange_t *) xdl_malloc(sizeof(xdchange_t))))
                return NULL;

        xch->next = xscr;
        xch->i1 = i1;
        xch->i2 = i2;
        xch->chg1 = chg1;
        xch->chg2 = chg2;
        xch->ignore = 0;

        return xch;
}


static int recs_match(xrecord_t *rec1, xrecord_t *rec2, long flags)
{
        return (rec1->ha == rec2->ha &&
                xdl_recmatch(rec1->ptr, rec1->size,
                             rec2->ptr, rec2->size,
                             flags));
}

/*
 * If a line is indented more than this, get_indent() just returns this value.
 * This avoids having to do absurd amounts of work for data that are not
 * human-readable text, and also ensures that the output of get_indent fits
 * within an int.
 */
#define MAX_INDENT 200

/*
 * Return the amount of indentation of the specified line, treating TAB as 8
 * columns. Return -1 if line is empty or contains only whitespace. Clamp the
 * output value at MAX_INDENT.
 */
static int get_indent(xrecord_t *rec)
{
        long i;
        int ret = 0;

        for (i = 0; i < rec->size; i++) {
                char c = rec->ptr[i];

                if (!XDL_ISSPACE(c))
                        return ret;
                else if (c == ' ')
                        ret += 1;
                else if (c == '\t')
                        ret += 8 - ret % 8;
                /* ignore other whitespace characters */

                if (ret >= MAX_INDENT)
                        return MAX_INDENT;
        }

        /* The line contains only whitespace. */
        return -1;
}

/*
 * If more than this number of consecutive blank rows are found, just return
 * this value. This avoids requiring O(N^2) work for pathological cases, and
 * also ensures that the output of score_split fits in an int.
 */
#define MAX_BLANKS 20

/* Characteristics measured about a hypothetical split position. */
struct split_measurement {
        /*
         * Is the split at the end of the file (aside from any blank lines)?
         */
        int end_of_file;

        /*
         * How much is the line immediately following the split indented (or -1
         * if the line is blank):
         */
        int indent;

        /*
         * How many consecutive lines above the split are blank?
         */
        int pre_blank;

        /*
         * How much is the nearest non-blank line above the split indented (or
         * -1 if there is no such line)?
         */
        int pre_indent;

        /*
         * How many lines after the line following the split are blank?
         */
        int post_blank;

        /*
         * How much is the nearest non-blank line after the line following the
         * split indented (or -1 if there is no such line)?
         */
        int post_indent;
};

struct split_score {
        /* The effective indent of this split (smaller is preferred). */
        int effective_indent;

        /* Penalty for this split (smaller is preferred). */
        int penalty;
};

/*
 * Fill m with information about a hypothetical split of xdf above line split.
 */
static void measure_split(const xdfile_t *xdf, long split,
                          struct split_measurement *m)
{
        long i;

        if (split >= xdf->nrec) {
                m->end_of_file = 1;
                m->indent = -1;
        } else {
                m->end_of_file = 0;
                m->indent = get_indent(xdf->recs[split]);
        }

        m->pre_blank = 0;
        m->pre_indent = -1;
        for (i = split - 1; i >= 0; i--) {
                m->pre_indent = get_indent(xdf->recs[i]);
                if (m->pre_indent != -1)
                        break;
                m->pre_blank += 1;
                if (m->pre_blank == MAX_BLANKS) {
                        m->pre_indent = 0;
                        break;
                }
        }

        m->post_blank = 0;
        m->post_indent = -1;
        for (i = split + 1; i < xdf->nrec; i++) {
                m->post_indent = get_indent(xdf->recs[i]);
                if (m->post_indent != -1)
                        break;
                m->post_blank += 1;
                if (m->post_blank == MAX_BLANKS) {
                        m->post_indent = 0;
                        break;
                }
        }
}

/*
 * The empirically-determined weight factors used by score_split() below.
 * Larger values means that the position is a less favorable place to split.
 *
 * Note that scores are only ever compared against each other, so multiplying
 * all of these weight/penalty values by the same factor wouldn't change the
 * heuristic's behavior. Still, we need to set that arbitrary scale *somehow*.
 * In practice, these numbers are chosen to be large enough that they can be
 * adjusted relative to each other with sufficient precision despite using
 * integer math.
 */

/* Penalty if there are no non-blank lines before the split */
#define START_OF_FILE_PENALTY 1

/* Penalty if there are no non-blank lines after the split */
#define END_OF_FILE_PENALTY 21

/* Multiplier for the number of blank lines around the split */
#define TOTAL_BLANK_WEIGHT (-30)

/* Multiplier for the number of blank lines after the split */
#define POST_BLANK_WEIGHT 6

/*
 * Penalties applied if the line is indented more than its predecessor
 */
#define RELATIVE_INDENT_PENALTY (-4)
#define RELATIVE_INDENT_WITH_BLANK_PENALTY 10

/*
 * Penalties applied if the line is indented less than both its predecessor and
 * its successor
 */
#define RELATIVE_OUTDENT_PENALTY 24
#define RELATIVE_OUTDENT_WITH_BLANK_PENALTY 17

/*
 * Penalties applied if the line is indented less than its predecessor but not
 * less than its successor
 */
#define RELATIVE_DEDENT_PENALTY 23
#define RELATIVE_DEDENT_WITH_BLANK_PENALTY 17

/*
 * We only consider whether the sum of the effective indents for splits are
 * less than (-1), equal to (0), or greater than (+1) each other. The resulting
 * value is multiplied by the following weight and combined with the penalty to
 * determine the better of two scores.
 */
#define INDENT_WEIGHT 60

/*
 * How far do we slide a hunk at most?
 */
#define INDENT_HEURISTIC_MAX_SLIDING 100

/*
 * Compute a badness score for the hypothetical split whose measurements are
 * stored in m. The weight factors were determined empirically using the tools
 * and corpus described in
 *
 *     https://github.com/mhagger/diff-slider-tools
 *
 * Also see that project if you want to improve the weights based on, for
 * example, a larger or more diverse corpus.
 */
static void score_add_split(const struct split_measurement *m, struct split_score *s)
{
        /*
         * A place to accumulate penalty factors (positive makes this index more
         * favored):
         */
        int post_blank, total_blank, indent, any_blanks;

        if (m->pre_indent == -1 && m->pre_blank == 0)
                s->penalty += START_OF_FILE_PENALTY;

        if (m->end_of_file)
                s->penalty += END_OF_FILE_PENALTY;

        /*
         * Set post_blank to the number of blank lines following the split,
         * including the line immediately after the split:
         */
        post_blank = (m->indent == -1) ? 1 + m->post_blank : 0;
        total_blank = m->pre_blank + post_blank;

        /* Penalties based on nearby blank lines: */
        s->penalty += TOTAL_BLANK_WEIGHT * total_blank;
        s->penalty += POST_BLANK_WEIGHT * post_blank;

        if (m->indent != -1)
                indent = m->indent;
        else
                indent = m->post_indent;

        any_blanks = (total_blank != 0);

        /* Note that the effective indent is -1 at the end of the file: */
        s->effective_indent += indent;

        if (indent == -1) {
                /* No additional adjustments needed. */
        } else if (m->pre_indent == -1) {
                /* No additional adjustments needed. */
        } else if (indent > m->pre_indent) {
                /*
                 * The line is indented more than its predecessor.
                 */
                s->penalty += any_blanks ?
                        RELATIVE_INDENT_WITH_BLANK_PENALTY :
                        RELATIVE_INDENT_PENALTY;
        } else if (indent == m->pre_indent) {
                /*
                 * The line has the same indentation level as its predecessor.
                 * No additional adjustments needed.
                 */
        } else {
                /*
                 * The line is indented less than its predecessor. It could be
                 * the block terminator of the previous block, but it could
                 * also be the start of a new block (e.g., an "else" block, or
                 * maybe the previous block didn't have a block terminator).
                 * Try to distinguish those cases based on what comes next:
                 */
                if (m->post_indent != -1 && m->post_indent > indent) {
                        /*
                         * The following line is indented more. So it is likely
                         * that this line is the start of a block.
                         */
                        s->penalty += any_blanks ?
                                RELATIVE_OUTDENT_WITH_BLANK_PENALTY :
                                RELATIVE_OUTDENT_PENALTY;
                } else {
                        /*
                         * That was probably the end of a block.
                         */
                        s->penalty += any_blanks ?
                                RELATIVE_DEDENT_WITH_BLANK_PENALTY :
                                RELATIVE_DEDENT_PENALTY;
                }
        }
}

static int score_cmp(struct split_score *s1, struct split_score *s2)
{
        /* -1 if s1.effective_indent < s2->effective_indent, etc. */
        int cmp_indents = ((s1->effective_indent > s2->effective_indent) -
                           (s1->effective_indent < s2->effective_indent));

        return INDENT_WEIGHT * cmp_indents + (s1->penalty - s2->penalty);
}

/*
 * Represent a group of changed lines in an xdfile_t (i.e., a contiguous group
 * of lines that was inserted or deleted from the corresponding version of the
 * file). We consider there to be such a group at the beginning of the file, at
 * the end of the file, and between any two unchanged lines, though most such
 * groups will usually be empty.
 *
 * If the first line in a group is equal to the line following the group, then
 * the group can be slid down. Similarly, if the last line in a group is equal
 * to the line preceding the group, then the group can be slid up. See
 * group_slide_down() and group_slide_up().
 *
 * Note that loops that are testing for changed lines in xdf->rchg do not need
 * index bounding since the array is prepared with a zero at position -1 and N.
 */
struct xdlgroup {
        /*
         * The index of the first changed line in the group, or the index of
         * the unchanged line above which the (empty) group is located.
         */
        long start;

        /*
         * The index of the first unchanged line after the group. For an empty
         * group, end is equal to start.
         */
        long end;
};

/*
 * Initialize g to point at the first group in xdf.
 */
static void group_init(xdfile_t *xdf, struct xdlgroup *g)
{
        g->start = g->end = 0;
        while (xdf->rchg[g->end])
                g->end++;
}

/*
 * Move g to describe the next (possibly empty) group in xdf and return 0. If g
 * is already at the end of the file, do nothing and return -1.
 */
static inline int group_next(xdfile_t *xdf, struct xdlgroup *g)
{
        if (g->end == xdf->nrec)
                return -1;

        g->start = g->end + 1;
        for (g->end = g->start; xdf->rchg[g->end]; g->end++)
                ;

        return 0;
}

/*
 * Move g to describe the previous (possibly empty) group in xdf and return 0.
 * If g is already at the beginning of the file, do nothing and return -1.
 */
static inline int group_previous(xdfile_t *xdf, struct xdlgroup *g)
{
        if (g->start == 0)
                return -1;

        g->end = g->start - 1;
        for (g->start = g->end; xdf->rchg[g->start - 1]; g->start--)
                ;

        return 0;
}

/*
 * If g can be slid toward the end of the file, do so, and if it bumps into a
 * following group, expand this group to include it. Return 0 on success or -1
 * if g cannot be slid down.
 */
static int group_slide_down(xdfile_t *xdf, struct xdlgroup *g, long flags)
{
        if (g->end < xdf->nrec &&
            recs_match(xdf->recs[g->start], xdf->recs[g->end], flags)) {
                xdf->rchg[g->start++] = 0;
                xdf->rchg[g->end++] = 1;

                while (xdf->rchg[g->end])
                        g->end++;

                return 0;
        } else {
                return -1;
        }
}

/*
 * If g can be slid toward the beginning of the file, do so, and if it bumps
 * into a previous group, expand this group to include it. Return 0 on success
 * or -1 if g cannot be slid up.
 */
static int group_slide_up(xdfile_t *xdf, struct xdlgroup *g, long flags)
{
        if (g->start > 0 &&
            recs_match(xdf->recs[g->start - 1], xdf->recs[g->end - 1], flags)) {
                xdf->rchg[--g->start] = 1;
                xdf->rchg[--g->end] = 0;

                while (xdf->rchg[g->start - 1])
                        g->start--;

                return 0;
        } else {
                return -1;
        }
}

static void xdl_bug(const char *msg)
{
        fprintf(stderr, "BUG: %s\n", msg);
        exit(1);
}

/*
 * Move back and forward change groups for a consistent and pretty diff output.
 * This also helps in finding joinable change groups and reducing the diff
 * size.
 */
int xdl_change_compact(xdfile_t *xdf, xdfile_t *xdfo, long flags) {
        struct xdlgroup g, go;
        long earliest_end, end_matching_other;
        long groupsize;

        group_init(xdf, &g);
        group_init(xdfo, &go);

        while (1) {
                /*
                 * If the group is empty in the to-be-compacted file, skip it:
                 */
                if (g.end == g.start)
                        goto next;

                /*
                 * Now shift the change up and then down as far as possible in
                 * each direction. If it bumps into any other changes, merge
                 * them.
                 */
                do {
                        groupsize = g.end - g.start;

                        /*
                         * Keep track of the last "end" index that causes this
                         * group to align with a group of changed lines in the
                         * other file. -1 indicates that we haven't found such
                         * a match yet:
                         */
                        end_matching_other = -1;

                        /* Shift the group backward as much as possible: */
                        while (!group_slide_up(xdf, &g, flags))
                                if (group_previous(xdfo, &go))
                                        xdl_bug("group sync broken sliding up");

                        /*
                         * This is this highest that this group can be shifted.
                         * Record its end index:
                         */
                        earliest_end = g.end;

                        if (go.end > go.start)
                                end_matching_other = g.end;

                        /* Now shift the group forward as far as possible: */
                        while (1) {
                                if (group_slide_down(xdf, &g, flags))
                                        break;
                                if (group_next(xdfo, &go))
                                        xdl_bug("group sync broken sliding down");

                                if (go.end > go.start)
                                        end_matching_other = g.end;
                        }
                } while (groupsize != g.end - g.start);

                /*
                 * If the group can be shifted, then we can possibly use this
                 * freedom to produce a more intuitive diff.
                 *
                 * The group is currently shifted as far down as possible, so
                 * the heuristics below only have to handle upwards shifts.
                 */

                if (g.end == earliest_end) {
                        /* no shifting was possible */
                } else if (end_matching_other != -1) {
                        /*
                         * Move the possibly merged group of changes back to
                         * line up with the last group of changes from the
                         * other file that it can align with.
                         */
                        while (go.end == go.start) {
                                if (group_slide_up(xdf, &g, flags))
                                        xdl_bug("match disappeared");
                                if (group_previous(xdfo, &go))
                                        xdl_bug("group sync broken sliding to match");
                        }
                } else if (flags & XDF_INDENT_HEURISTIC) {
                        /*
                         * Indent heuristic: a group of pure add/delete lines
                         * implies two splits, one between the end of the
                         * "before" context and the start of the group, and
                         * another between the end of the group and the
                         * beginning of the "after" context. Some splits are
                         * aesthetically better and some are worse. We compute
                         * a badness "score" for each split, and add the scores
                         * for the two splits to define a "score" for each
                         * position that the group can be shifted to. Then we
                         * pick the shift with the lowest score.
                         */
                        long shift, best_shift = -1;
                        struct split_score best_score;

                        shift = earliest_end;
                        if (g.end - groupsize - 1 > shift)
                                shift = g.end - groupsize - 1;
                        if (g.end - INDENT_HEURISTIC_MAX_SLIDING > shift)
                                shift = g.end - INDENT_HEURISTIC_MAX_SLIDING;
                        for (; shift <= g.end; shift++) {
                                struct split_measurement m;
                                struct split_score score = {0, 0};

                                measure_split(xdf, shift, &m);
                                score_add_split(&m, &score);
                                measure_split(xdf, shift - groupsize, &m);
                                score_add_split(&m, &score);
                                if (best_shift == -1 ||
                                    score_cmp(&score, &best_score) <= 0) {
                                        best_score.effective_indent = score.effective_indent;
                                        best_score.penalty = score.penalty;
                                        best_shift = shift;
                                }
                        }

                        while (g.end > best_shift) {
                                if (group_slide_up(xdf, &g, flags))
                                        xdl_bug("best shift unreached");
                                if (group_previous(xdfo, &go))
                                        xdl_bug("group sync broken sliding to blank line");
                        }
                }

        next:
                /* Move past the just-processed group: */
                if (group_next(xdf, &g))
                        break;
                if (group_next(xdfo, &go))
                        xdl_bug("group sync broken moving to next group");
        }

        if (!group_next(xdfo, &go))
                xdl_bug("group sync broken at end of file");

        return 0;
}


int xdl_build_script(xdfenv_t *xe, xdchange_t **xscr) {
        xdchange_t *cscr = NULL, *xch;
        char *rchg1 = xe->xdf1.rchg, *rchg2 = xe->xdf2.rchg;
        long i1, i2, l1, l2;

        /*
         * Trivial. Collects "groups" of changes and creates an edit script.
         */
        for (i1 = xe->xdf1.nrec, i2 = xe->xdf2.nrec; i1 >= 0 || i2 >= 0; i1--, i2--)
                if (rchg1[i1 - 1] || rchg2[i2 - 1]) {
                        for (l1 = i1; rchg1[i1 - 1]; i1--);
                        for (l2 = i2; rchg2[i2 - 1]; i2--);

                        if (!(xch = xdl_add_change(cscr, i1, i2, l1 - i1, l2 - i2))) {
                                xdl_free_script(cscr);
                                return -1;
                        }
                        cscr = xch;
                }

        *xscr = cscr;

        return 0;
}


void xdl_free_script(xdchange_t *xscr) {
        xdchange_t *xch;

        while ((xch = xscr) != NULL) {
                xscr = xscr->next;
                xdl_free(xch);
        }
}

static int xdl_call_hunk_func(xdfenv_t *xe, xdchange_t *xscr, xdemitcb_t *ecb,
                              xdemitconf_t const *xecfg)
{
        xdchange_t *xch, *xche;

        for (xch = xscr; xch; xch = xche->next) {
                xche = xdl_get_hunk(&xch, xecfg);
                if (!xch)
                        break;
                if (xecfg->hunk_func(xch->i1, xche->i1 + xche->chg1 - xch->i1,
                                     xch->i2, xche->i2 + xche->chg2 - xch->i2,
                                     ecb->priv) < 0)
                        return -1;
        }
        return 0;
}

static void xdl_mark_ignorable(xdchange_t *xscr, xdfenv_t *xe, long flags)
{
        xdchange_t *xch;

        for (xch = xscr; xch; xch = xch->next) {
                int ignore = 1;
                xrecord_t **rec;
                long i;

                rec = &xe->xdf1.recs[xch->i1];
                for (i = 0; i < xch->chg1 && ignore; i++)
                        ignore = xdl_blankline(rec[i]->ptr, rec[i]->size, flags);

                rec = &xe->xdf2.recs[xch->i2];
                for (i = 0; i < xch->chg2 && ignore; i++)
                        ignore = xdl_blankline(rec[i]->ptr, rec[i]->size, flags);

                xch->ignore = ignore;
        }
}

int xdl_diff(mmfile_t *mf1, mmfile_t *mf2, xpparam_t const *xpp,
             xdemitconf_t const *xecfg, xdemitcb_t *ecb) {
        xdchange_t *xscr;
        xdfenv_t xe;
        emit_func_t ef = xecfg->hunk_func ? xdl_call_hunk_func : xdl_emit_diff;

        if (xdl_do_diff(mf1, mf2, xpp, &xe) < 0) {

                return -1;
        }
        if (xdl_change_compact(&xe.xdf1, &xe.xdf2, xpp->flags) < 0 ||
            xdl_change_compact(&xe.xdf2, &xe.xdf1, xpp->flags) < 0 ||
            xdl_build_script(&xe, &xscr) < 0) {

                xdl_free_env(&xe);
                return -1;
        }
        if (xscr) {
                if (xpp->flags & XDF_IGNORE_BLANK_LINES)
                        xdl_mark_ignorable(xscr, &xe, xpp->flags);

                if (ef(&xe, xscr, ecb, xecfg) < 0) {

                        xdl_free_script(xscr);
                        xdl_free_env(&xe);
                        return -1;
                }
                xdl_free_script(xscr);
        }
        xdl_free_env(&xe);

        return 0;
}