root/usr.bin/rcs/diff3.c
/*      $OpenBSD: diff3.c,v 1.45 2021/04/13 14:20:24 stsp Exp $ */

/*
 * Copyright (C) Caldera International Inc.  2001-2002.
 * All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 * 1. Redistributions of source code and documentation must retain the above
 *    copyright notice, this list of conditions and the following disclaimer.
 * 2. Redistributions in binary form must reproduce the above copyright
 *    notice, this list of conditions and the following disclaimer in the
 *    documentation and/or other materials provided with the distribution.
 * 3. All advertising materials mentioning features or use of this software
 *    must display the following acknowledgement:
 *      This product includes software developed or owned by Caldera
 *      International, Inc.
 * 4. Neither the name of Caldera International, Inc. nor the names of other
 *    contributors may be used to endorse or promote products derived from
 *    this software without specific prior written permission.
 *
 * USE OF THE SOFTWARE PROVIDED FOR UNDER THIS LICENSE BY CALDERA
 * INTERNATIONAL, INC. AND CONTRIBUTORS ``AS IS'' AND ANY EXPRESS OR
 * IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
 * OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
 * IN NO EVENT SHALL CALDERA INTERNATIONAL, INC. BE LIABLE FOR ANY DIRECT,
 * INDIRECT INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES
 * (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
 * SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
 * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT,
 * STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING
 * IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
 * POSSIBILITY OF SUCH DAMAGE.
 */
/*-
 * Copyright (c) 1991, 1993
 *      The Regents of the University of California.  All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 * 1. Redistributions of source code must retain the above copyright
 *    notice, this list of conditions and the following disclaimer.
 * 2. Redistributions in binary form must reproduce the above copyright
 *    notice, this list of conditions and the following disclaimer in the
 *    documentation and/or other materials provided with the distribution.
 * 3. Neither the name of the University nor the names of its contributors
 *    may be used to endorse or promote products derived from this software
 *    without specific prior written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
 * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
 * ARE DISCLAIMED.  IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
 * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
 * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
 * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
 * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
 * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
 * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 * SUCH DAMAGE.
 *
 *      @(#)diff3.c     8.1 (Berkeley) 6/6/93
 */

#include <ctype.h>
#include <err.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <time.h>
#include <unistd.h>

#include "diff.h"
#include "rcsprog.h"

/* diff3 - 3-way differential file comparison */

/* diff3 [-ex3EX] d13 d23 f1 f2 f3 [m1 m3]
 *
 * d13 = diff report on f1 vs f3
 * d23 = diff report on f2 vs f3
 * f1, f2, f3 the 3 files
 * if changes in f1 overlap with changes in f3, m1 and m3 are used
 * to mark the overlaps; otherwise, the file names f1 and f3 are used
 * (only for options E and X).
 */

/*
 * "from" is first in range of changed lines; "to" is last+1
 * from=to=line after point of insertion for added lines.
 */
struct range {
        int from;
        int to;
};

struct diff {
        struct range old;
        struct range new;
};

static size_t szchanges;

static struct diff *d13;
static struct diff *d23;

/*
 * "de" is used to gather editing scripts.  These are later spewed out in
 * reverse order.  Its first element must be all zero, the "new" component
 * of "de" contains line positions or byte positions depending on when you
 * look (!?).  Array overlap indicates which sections in "de" correspond to
 * lines that are different in all three files.
 */
static struct diff *de;
static char *overlap;
static int overlapcnt = 0;
static FILE *fp[3];
static int cline[3];            /* # of the last-read line in each file (0-2) */

/*
 * the latest known correspondence between line numbers of the 3 files
 * is stored in last[1-3];
 */
static int last[4];
static int eflag = 3;   /* default -E for compatibility with former RCS */
static int oflag = 1;   /* default -E for compatibility with former RCS */
static int debug  = 0;
static char f1mark[PATH_MAX], f3mark[PATH_MAX]; /* markers for -E and -X */

static int duplicate(struct range *, struct range *);
static int edit(struct diff *, int, int);
static char *getchange(FILE *);
static char *get_line(FILE *, size_t *);
static int number(char **);
static ssize_t readin(char *, struct diff **);
static int skip(int, int, char *);
static int edscript(int);
static int merge(size_t, size_t);
static void change(int, struct range *, int);
static void keep(int, struct range *);
static void prange(struct range *);
static void repos(int);
static void separate(const char *);
static void increase(void);
static int diff3_internal(int, char **, const char *, const char *);

int diff3_conflicts = 0;

/*
 * For merge(1).
 */
BUF *
merge_diff3(char **av, int flags)
{
        int argc;
        char *argv[5], *dp13, *dp23, *path1, *path2, *path3;
        BUF *b1, *b2, *b3, *d1, *d2, *diffb;
        u_char *data, *patch;
        size_t dlen, plen;

        b1 = b2 = b3 = d1 = d2 = diffb = NULL;
        dp13 = dp23 = path1 = path2 = path3 = NULL;
        data = patch = NULL;

        if ((flags & MERGE_EFLAG) && !(flags & MERGE_OFLAG))
                oflag = 0;

        if ((b1 = buf_load(av[0])) == NULL)
                goto out;
        if ((b2 = buf_load(av[1])) == NULL)
                goto out;
        if ((b3 = buf_load(av[2])) == NULL)
                goto out;

        d1 = buf_alloc(128);
        d2 = buf_alloc(128);
        diffb = buf_alloc(128);

        (void)xasprintf(&path1, "%s/diff1.XXXXXXXXXX", rcs_tmpdir);
        (void)xasprintf(&path2, "%s/diff2.XXXXXXXXXX", rcs_tmpdir);
        (void)xasprintf(&path3, "%s/diff3.XXXXXXXXXX", rcs_tmpdir);

        buf_write_stmp(b1, path1);
        buf_write_stmp(b2, path2);
        buf_write_stmp(b3, path3);

        buf_free(b2);
        b2 = NULL;

        if ((diffreg(path1, path3, d1, D_FORCEASCII) == D_ERROR) ||
            (diffreg(path2, path3, d2, D_FORCEASCII) == D_ERROR)) {
                buf_free(diffb);
                diffb = NULL;
                goto out;
        }

        (void)xasprintf(&dp13, "%s/d13.XXXXXXXXXX", rcs_tmpdir);
        buf_write_stmp(d1, dp13);

        buf_free(d1);
        d1 = NULL;

        (void)xasprintf(&dp23, "%s/d23.XXXXXXXXXX", rcs_tmpdir);
        buf_write_stmp(d2, dp23);

        buf_free(d2);
        d2 = NULL;

        argc = 0;
        diffbuf = diffb;
        argv[argc++] = dp13;
        argv[argc++] = dp23;
        argv[argc++] = path1;
        argv[argc++] = path2;
        argv[argc++] = path3;

        diff3_conflicts = diff3_internal(argc, argv, av[0], av[2]);
        if (diff3_conflicts < 0) {
                buf_free(diffb);
                diffb = NULL;
                goto out;
        }

        plen = buf_len(diffb);
        patch = buf_release(diffb);
        dlen = buf_len(b1);
        data = buf_release(b1);

        if ((diffb = rcs_patchfile(data, dlen, patch, plen, ed_patch_lines)) == NULL)
                goto out;

        if (!(flags & QUIET) && diff3_conflicts != 0)
                warnx("warning: overlaps or other problems during merge");

out:
        buf_free(b2);
        buf_free(b3);
        buf_free(d1);
        buf_free(d2);

        (void)unlink(path1);
        (void)unlink(path2);
        (void)unlink(path3);
        (void)unlink(dp13);
        (void)unlink(dp23);

        free(path1);
        free(path2);
        free(path3);
        free(dp13);
        free(dp23);
        free(data);
        free(patch);

        return (diffb);
}

BUF *
rcs_diff3(RCSFILE *rf, char *workfile, RCSNUM *rev1, RCSNUM *rev2, int flags)
{
        int argc;
        char *argv[5], r1[RCS_REV_BUFSZ], r2[RCS_REV_BUFSZ];
        char *dp13, *dp23, *path1, *path2, *path3;
        BUF *b1, *b2, *b3, *d1, *d2, *diffb;
        size_t dlen, plen;
        u_char *data, *patch;

        b1 = b2 = b3 = d1 = d2 = diffb = NULL;
        dp13 = dp23 = path1 = path2 = path3 = NULL;
        data = patch = NULL;

        if ((flags & MERGE_EFLAG) && !(flags & MERGE_OFLAG))
                oflag = 0;

        rcsnum_tostr(rev1, r1, sizeof(r1));
        rcsnum_tostr(rev2, r2, sizeof(r2));

        if ((b1 = buf_load(workfile)) == NULL)
                goto out;

        if (!(flags & QUIET))
                (void)fprintf(stderr, "retrieving revision %s\n", r1);
        if ((b2 = rcs_getrev(rf, rev1)) == NULL)
                goto out;

        if (!(flags & QUIET))
                (void)fprintf(stderr, "retrieving revision %s\n", r2);
        if ((b3 = rcs_getrev(rf, rev2)) == NULL)
                goto out;

        d1 = buf_alloc(128);
        d2 = buf_alloc(128);
        diffb = buf_alloc(128);

        (void)xasprintf(&path1, "%s/diff1.XXXXXXXXXX", rcs_tmpdir);
        (void)xasprintf(&path2, "%s/diff2.XXXXXXXXXX", rcs_tmpdir);
        (void)xasprintf(&path3, "%s/diff3.XXXXXXXXXX", rcs_tmpdir);

        buf_write_stmp(b1, path1);
        buf_write_stmp(b2, path2);
        buf_write_stmp(b3, path3);

        buf_free(b2);
        b2 = NULL;

        if ((diffreg(path1, path3, d1, D_FORCEASCII) == D_ERROR) ||
            (diffreg(path2, path3, d2, D_FORCEASCII) == D_ERROR)) {
                buf_free(diffb);
                diffb = NULL;
                goto out;
        }

        (void)xasprintf(&dp13, "%s/d13.XXXXXXXXXX", rcs_tmpdir);
        buf_write_stmp(d1, dp13);

        buf_free(d1);
        d1 = NULL;

        (void)xasprintf(&dp23, "%s/d23.XXXXXXXXXX", rcs_tmpdir);
        buf_write_stmp(d2, dp23);

        buf_free(d2);
        d2 = NULL;

        argc = 0;
        diffbuf = diffb;
        argv[argc++] = dp13;
        argv[argc++] = dp23;
        argv[argc++] = path1;
        argv[argc++] = path2;
        argv[argc++] = path3;

        diff3_conflicts = diff3_internal(argc, argv, workfile, r2);
        if (diff3_conflicts < 0) {
                buf_free(diffb);
                diffb = NULL;
                goto out;
        }

        plen = buf_len(diffb);
        patch = buf_release(diffb);
        dlen = buf_len(b1);
        data = buf_release(b1);

        if ((diffb = rcs_patchfile(data, dlen, patch, plen, ed_patch_lines)) == NULL)
                goto out;

        if (!(flags & QUIET) && diff3_conflicts != 0)
                warnx("warning: overlaps or other problems during merge");

out:
        buf_free(b2);
        buf_free(b3);
        buf_free(d1);
        buf_free(d2);

        (void)unlink(path1);
        (void)unlink(path2);
        (void)unlink(path3);
        (void)unlink(dp13);
        (void)unlink(dp23);

        free(path1);
        free(path2);
        free(path3);
        free(dp13);
        free(dp23);
        free(data);
        free(patch);

        return (diffb);
}

static int
diff3_internal(int argc, char **argv, const char *fmark, const char *rmark)
{
        ssize_t m, n;
        int i;

        if (argc < 5)
                return (-1);

        if (oflag) {
                i = snprintf(f1mark, sizeof(f1mark), "<<<<<<< %s", fmark);
                if (i < 0 || i >= (int)sizeof(f1mark))
                        errx(1, "diff3_internal: string truncated");

                i = snprintf(f3mark, sizeof(f3mark), ">>>>>>> %s", rmark);
                if (i < 0 || i >= (int)sizeof(f3mark))
                        errx(1, "diff3_internal: string truncated");
        }

        increase();
        if ((m = readin(argv[0], &d13)) < 0) {
                warn("%s", argv[0]);
                return (-1);
        }
        if ((n = readin(argv[1], &d23)) < 0) {
                warn("%s", argv[1]);
                return (-1);
        }

        for (i = 0; i <= 2; i++)
                if ((fp[i] = fopen(argv[i + 2], "r")) == NULL) {
                        warn("%s", argv[i + 2]);
                        return (-1);
                }

        return (merge(m, n));
}

int
ed_patch_lines(struct rcs_lines *dlines, struct rcs_lines *plines)
{
        char op, *ep;
        struct rcs_line *sort, *lp, *dlp, *ndlp, *insert_after;
        int start, end, i, lineno;
        u_char tmp;

        dlp = TAILQ_FIRST(&(dlines->l_lines));
        lp = TAILQ_FIRST(&(plines->l_lines));

        end = 0;
        for (lp = TAILQ_NEXT(lp, l_list); lp != NULL;
            lp = TAILQ_NEXT(lp, l_list)) {
                /* Skip blank lines */
                if (lp->l_len < 2)
                        continue;

                /* NUL-terminate line buffer for strtol() safety. */
                tmp = lp->l_line[lp->l_len - 1];
                lp->l_line[lp->l_len - 1] = '\0';

                /* len - 1 is NUL terminator so we use len - 2 for 'op' */
                op = lp->l_line[lp->l_len - 2];
                start = (int)strtol(lp->l_line, &ep, 10);

                /* Restore the last byte of the buffer */
                lp->l_line[lp->l_len - 1] = tmp;

                if (op == 'a') {
                        if (start > dlines->l_nblines ||
                            start < 0 || *ep != 'a')
                                errx(1, "ed_patch_lines");
                } else if (op == 'c') {
                        if (start > dlines->l_nblines ||
                            start < 0 || (*ep != ',' && *ep != 'c'))
                                errx(1, "ed_patch_lines");

                        if (*ep == ',') {
                                ep++;
                                end = (int)strtol(ep, &ep, 10);
                                if (end < 0 || *ep != 'c')
                                        errx(1, "ed_patch_lines");
                        } else {
                                end = start;
                        }
                }


                for (;;) {
                        if (dlp == NULL)
                                break;
                        if (dlp->l_lineno == start)
                                break;
                        if (dlp->l_lineno > start) {
                                dlp = TAILQ_PREV(dlp, tqh, l_list);
                        } else if (dlp->l_lineno < start) {
                                ndlp = TAILQ_NEXT(dlp, l_list);
                                if (ndlp->l_lineno > start)
                                        break;
                                dlp = ndlp;
                        }
                }

                if (dlp == NULL)
                        errx(1, "ed_patch_lines");


                if (op == 'c') {
                        insert_after = TAILQ_PREV(dlp, tqh, l_list);
                        for (i = 0; i <= (end - start); i++) {
                                ndlp = TAILQ_NEXT(dlp, l_list);
                                TAILQ_REMOVE(&(dlines->l_lines), dlp, l_list);
                                dlp = ndlp;
                        }
                        dlp = insert_after;
                }

                if (op == 'a' || op == 'c') {
                        for (;;) {
                                ndlp = lp;
                                lp = TAILQ_NEXT(lp, l_list);
                                if (lp == NULL)
                                        errx(1, "ed_patch_lines");

                                if (lp->l_len == 2 &&
                                    lp->l_line[0] == '.' &&
                                    lp->l_line[1] == '\n')
                                        break;

                                TAILQ_REMOVE(&(plines->l_lines), lp, l_list);
                                TAILQ_INSERT_AFTER(&(dlines->l_lines), dlp,
                                    lp, l_list);
                                dlp = lp;

                                lp->l_lineno = start;
                                lp = ndlp;
                        }
                }

                /*
                 * always resort lines as the markers might be put at the
                 * same line as we first started editing.
                 */
                lineno = 0;
                TAILQ_FOREACH(sort, &(dlines->l_lines), l_list)
                        sort->l_lineno = lineno++;
                dlines->l_nblines = lineno - 1;
        }

        return (0);
}

/*
 * Pick up the line numbers of all changes from one change file.
 * (This puts the numbers in a vector, which is not strictly necessary,
 * since the vector is processed in one sequential pass.
 * The vector could be optimized out of existence)
 */
static ssize_t
readin(char *name, struct diff **dd)
{
        int a, b, c, d;
        char kind, *p;
        size_t i;

        fp[0] = fopen(name, "r");
        if (fp[0] == NULL)
                return (-1);
        for (i = 0; (p = getchange(fp[0])); i++) {
                if (i >= szchanges - 1)
                        increase();
                a = b = number(&p);
                if (*p == ',') {
                        p++;
                        b = number(&p);
                }
                kind = *p++;
                c = d = number(&p);
                if (*p==',') {
                        p++;
                        d = number(&p);
                }
                if (kind == 'a')
                        a++;
                if (kind == 'd')
                        c++;
                b++;
                d++;
                (*dd)[i].old.from = a;
                (*dd)[i].old.to = b;
                (*dd)[i].new.from = c;
                (*dd)[i].new.to = d;
        }

        if (i) {
                (*dd)[i].old.from = (*dd)[i-1].old.to;
                (*dd)[i].new.from = (*dd)[i-1].new.to;
        }

        (void)fclose(fp[0]);

        return (i);
}

static int
number(char **lc)
{
        int nn;

        nn = 0;
        while (isdigit((unsigned char)(**lc)))
                nn = nn*10 + *(*lc)++ - '0';

        return (nn);
}

static char *
getchange(FILE *b)
{
        char *line;

        while ((line = get_line(b, NULL))) {
                if (isdigit((unsigned char)line[0]))
                        return (line);
        }

        return (NULL);
}

static char *
get_line(FILE *b, size_t *n)
{
        char *cp;
        size_t len;
        static char *buf;
        static size_t bufsize;

        if ((cp = fgetln(b, &len)) == NULL)
                return (NULL);

        if (cp[len - 1] != '\n')
                len++;
        if (len + 1 > bufsize) {
                do {
                        bufsize += 1024;
                } while (len + 1 > bufsize);
                buf = xreallocarray(buf, 1, bufsize);
        }
        memcpy(buf, cp, len - 1);
        buf[len - 1] = '\n';
        buf[len] = '\0';
        if (n != NULL)
                *n = len;

        return (buf);
}

static int
merge(size_t m1, size_t m2)
{
        struct diff *d1, *d2, *d3;
        int dpl, j, t1, t2;

        d1 = d13;
        d2 = d23;
        j = 0;
        for (;;) {
                t1 = (d1 < d13 + m1);
                t2 = (d2 < d23 + m2);
                if (!t1 && !t2)
                        break;

                if (debug) {
                        printf("%d,%d=%d,%d %d,%d=%d,%d\n",
                        d1->old.from, d1->old.to,
                        d1->new.from, d1->new.to,
                        d2->old.from, d2->old.to,
                        d2->new.from, d2->new.to);
                }

                /* first file is different from others */
                if (!t2 || (t1 && d1->new.to < d2->new.from)) {
                        /* stuff peculiar to 1st file */
                        if (eflag==0) {
                                separate("1");
                                change(1, &d1->old, 0);
                                keep(2, &d1->new);
                                change(3, &d1->new, 0);
                        }
                        d1++;
                        continue;
                }

                /* second file is different from others */
                if (!t1 || (t2 && d2->new.to < d1->new.from)) {
                        if (eflag==0) {
                                separate("2");
                                keep(1, &d2->new);
                                change(2, &d2->old, 0);
                                change(3, &d2->new, 0);
                        }
                        d2++;
                        continue;
                }

                /*
                 * Merge overlapping changes in first file
                 * this happens after extension (see below).
                 */
                if (d1 + 1 < d13 + m1 && d1->new.to >= d1[1].new.from) {
                        d1[1].old.from = d1->old.from;
                        d1[1].new.from = d1->new.from;
                        d1++;
                        continue;
                }

                /* merge overlapping changes in second */
                if (d2 + 1 < d23 + m2 && d2->new.to >= d2[1].new.from) {
                        d2[1].old.from = d2->old.from;
                        d2[1].new.from = d2->new.from;
                        d2++;
                        continue;
                }
                /* stuff peculiar to third file or different in all */
                if (d1->new.from == d2->new.from && d1->new.to == d2->new.to) {
                        dpl = duplicate(&d1->old,&d2->old);
                        if (dpl == -1)
                                return (-1);

                        /*
                         * dpl = 0 means all files differ
                         * dpl = 1 means files 1 and 2 identical
                         */
                        if (eflag==0) {
                                separate(dpl ? "3" : "");
                                change(1, &d1->old, dpl);
                                change(2, &d2->old, 0);
                                d3 = d1->old.to > d1->old.from ? d1 : d2;
                                change(3, &d3->new, 0);
                        } else
                                j = edit(d1, dpl, j);
                        d1++;
                        d2++;
                        continue;
                }

                /*
                 * Overlapping changes from file 1 and 2; extend changes
                 * appropriately to make them coincide.
                 */
                if (d1->new.from < d2->new.from) {
                        d2->old.from -= d2->new.from-d1->new.from;
                        d2->new.from = d1->new.from;
                } else if (d2->new.from < d1->new.from) {
                        d1->old.from -= d1->new.from-d2->new.from;
                        d1->new.from = d2->new.from;
                }
                if (d1->new.to > d2->new.to) {
                        d2->old.to += d1->new.to - d2->new.to;
                        d2->new.to = d1->new.to;
                } else if (d2->new.to > d1->new.to) {
                        d1->old.to += d2->new.to - d1->new.to;
                        d1->new.to = d2->new.to;
                }
        }

        return (edscript(j));
}

static void
separate(const char *s)
{
        diff_output("====%s\n", s);
}

/*
 * The range of lines rold.from thru rold.to in file i is to be changed.
 * It is to be printed only if it does not duplicate something to be
 * printed later.
 */
static void
change(int i, struct range *rold, int fdup)
{
        diff_output("%d:", i);
        last[i] = rold->to;
        prange(rold);
        if (fdup || debug)
                return;
        i--;
        (void)skip(i, rold->from, NULL);
        (void)skip(i, rold->to, "  ");
}

/*
 * print the range of line numbers, rold.from thru rold.to, as n1,n2 or n1
 */
static void
prange(struct range *rold)
{
        if (rold->to <= rold->from)
                diff_output("%da\n", rold->from - 1);
        else {
                diff_output("%d", rold->from);
                if (rold->to > rold->from+1)
                        diff_output(",%d", rold->to - 1);
                diff_output("c\n");
        }
}

/*
 * No difference was reported by diff between file 1 (or 2) and file 3,
 * and an artificial dummy difference (trange) must be ginned up to
 * correspond to the change reported in the other file.
 */
static void
keep(int i, struct range *rnew)
{
        int delta;
        struct range trange;

        delta = last[3] - last[i];
        trange.from = rnew->from - delta;
        trange.to = rnew->to - delta;
        change(i, &trange, 1);
}

/*
 * skip to just before line number from in file "i".  If "pr" is non-NULL,
 * print all skipped stuff with string pr as a prefix.
 */
static int
skip(int i, int from, char *pr)
{
        size_t j, n;
        char *line;

        for (n = 0; cline[i] < from - 1; n += j) {
                if ((line = get_line(fp[i], &j)) == NULL)
                        return (-1);
                if (pr != NULL)
                        diff_output("%s%s", pr, line);
                cline[i]++;
        }
        return ((int) n);
}

/*
 * Return 1 or 0 according as the old range (in file 1) contains exactly
 * the same data as the new range (in file 2).
 */
static int
duplicate(struct range *r1, struct range *r2)
{
        int c,d;
        int nchar;
        int nline;

        if (r1->to-r1->from != r2->to-r2->from)
                return (0);
        (void)skip(0, r1->from, NULL);
        (void)skip(1, r2->from, NULL);
        nchar = 0;
        for (nline=0; nline < r1->to - r1->from; nline++) {
                do {
                        c = getc(fp[0]);
                        d = getc(fp[1]);
                        if (c == -1 && d == -1)
                                break;
                        if (c == -1 || d== -1)
                                return (-1);
                        nchar++;
                        if (c != d) {
                                repos(nchar);
                                return (0);
                        }
                } while (c != '\n');
        }
        repos(nchar);
        return (1);
}

static void
repos(int nchar)
{
        int i;

        for (i = 0; i < 2; i++)
                (void)fseek(fp[i], (long)-nchar, SEEK_CUR);
}

/*
 * collect an editing script for later regurgitation
 */
static int
edit(struct diff *diff, int fdup, int j)
{
        if (((fdup + 1) & eflag) == 0)
                return (j);
        j++;
        overlap[j] = !fdup;
        if (!fdup)
                overlapcnt++;
        de[j].old.from = diff->old.from;
        de[j].old.to = diff->old.to;
        de[j].new.from = de[j-1].new.to + skip(2, diff->new.from, NULL);
        de[j].new.to = de[j].new.from + skip(2, diff->new.to, NULL);
        return (j);
}

/* regurgitate */
static int
edscript(int n)
{
        int j, k;
        char block[BUFSIZ+1];

        for (; n > 0; n--) {
                if (!oflag || !overlap[n])
                        prange(&de[n].old);
                else
                        diff_output("%da\n=======\n", de[n].old.to -1);
                (void)fseek(fp[2], (long)de[n].new.from, SEEK_SET);
                for (k = de[n].new.to-de[n].new.from; k > 0; k-= j) {
                        size_t r;
                        j = k > BUFSIZ ? BUFSIZ : k;
                        r = fread(block, 1, j, fp[2]);
                        if (r == 0) {
                                if (feof(fp[2]))
                                        break;
                                return -1;
                        }
                        if (r != (size_t)j)
                                j = r;
                        block[j] = '\0';
                        diff_output("%s", block);
                }

                if (!oflag || !overlap[n])
                        diff_output(".\n");
                else {
                        diff_output("%s\n.\n", f3mark);
                        diff_output("%da\n%s\n.\n", de[n].old.from - 1, f1mark);
                }
        }

        return (overlapcnt);
}

static void
increase(void)
{
        size_t newsz, incr;

        /* are the memset(3) calls needed? */
        newsz = szchanges == 0 ? 64 : 2 * szchanges;
        incr = newsz - szchanges;

        d13 = xreallocarray(d13, newsz, sizeof(*d13));
        memset(d13 + szchanges, 0, incr * sizeof(*d13));
        d23 = xreallocarray(d23, newsz, sizeof(*d23));
        memset(d23 + szchanges, 0, incr * sizeof(*d23));
        de = xreallocarray(de, newsz, sizeof(*de));
        memset(de + szchanges, 0, incr * sizeof(*de));
        overlap = xreallocarray(overlap, newsz, sizeof(*overlap));
        memset(overlap + szchanges, 0, incr * sizeof(*overlap));
        szchanges = newsz;
}