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rcsid[] and sccsid[] and copyright[] are essentially unmaintained (and
unmaintainable).  these days, people use source.  these id's do not provide
any benefit, and do hurt the small install media
(the 33,000 line diff is essentially mechanical)
ok with the idea millert, ok dms

/*	$OpenBSD: comm.c,v 1.8 2009/10/27 23:59:37 deraadt Exp $	*/
/*	$NetBSD: comm.c,v 1.10 1995/09/05 19:57:43 jtc Exp $	*/

/*
 * Copyright (c) 1989, 1993, 1994
 *	The Regents of the University of California.  All rights reserved.
 *
 * This code is derived from software contributed to Berkeley by
 * Case Larsen.
 *
 * 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.
 */

#include <err.h>
#include <limits.h>
#include <locale.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>

#define	MAXLINELEN	(LINE_MAX + 1)

char *tabs[] = { "", "\t", "\t\t" };

FILE   *file(const char *);
void	show(FILE *, char *, char *);
void	usage(void);

int
main(int argc, char *argv[])
{
	int comp, file1done, file2done, read1, read2;
	int ch, flag1, flag2, flag3;
	FILE *fp1, *fp2;
	char *col1, *col2, *col3;
	char **p, line1[MAXLINELEN], line2[MAXLINELEN];
	int (*compare)(const char * ,const char *);

	setlocale(LC_ALL, "");

	flag1 = flag2 = flag3 = 1;
	compare = strcoll;
	while ((ch = getopt(argc, argv, "123f")) != -1)
		switch(ch) {
		case '1':
			flag1 = 0;
			break;
		case '2':
			flag2 = 0;
			break;
		case '3':
			flag3 = 0;
			break;
		case 'f':
			compare = strcasecmp;
			break;
		case '?':
		default:
			usage();
		}
	argc -= optind;
	argv += optind;

	if (argc != 2)
		usage();

	fp1 = file(argv[0]);
	fp2 = file(argv[1]);

	/* for each column printed, add another tab offset */
	p = tabs;
	col1 = col2 = col3 = NULL;
	if (flag1)
		col1 = *p++;
	if (flag2)
		col2 = *p++;
	if (flag3)
		col3 = *p;

	for (read1 = read2 = 1;;) {
		/* read next line, check for EOF */
		if (read1)
			file1done = !fgets(line1, MAXLINELEN, fp1);
		if (read2)
			file2done = !fgets(line2, MAXLINELEN, fp2);

		/* if one file done, display the rest of the other file */
		if (file1done) {
			if (!file2done && col2)
				show(fp2, col2, line2);
			break;
		}
		if (file2done) {
			if (!file1done && col1)
				show(fp1, col1, line1);
			break;
		}

		/* lines are the same */
		if (!(comp = compare(line1, line2))) {
			read1 = read2 = 1;
			if (col3)
				if (printf("%s%s", col3, line1) < 0)
					break;
			continue;
		}

		/* lines are different */
		if (comp < 0) {
			read1 = 1;
			read2 = 0;
			if (col1)
				if (printf("%s%s", col1, line1) < 0)
					break;
		} else {
			read1 = 0;
			read2 = 1;
			if (col2)
				if (printf("%s%s", col2, line2) < 0)
					break;
		}
	}

	if (ferror (stdout) || fclose (stdout) == EOF)
		err(1, "stdout");

	exit(0);
}

void
show(FILE *fp, char *offset, char *buf)
{
	while (printf("%s%s", offset, buf) >= 0 && fgets(buf, MAXLINELEN, fp))
		;
}

FILE *
file(const char *name)
{
	FILE *fp;

	if (!strcmp(name, "-"))
		return (stdin);
	if ((fp = fopen(name, "r")) == NULL)
		err(1, "%s", name);
	return (fp);
}

void
usage(void)
{
	(void)fprintf(stderr, "usage: comm [-123f] file1 file2\n");
	exit(1);
}