/*
 * Copyright (c) 1999 Apple Computer, Inc. All rights reserved.
 *
 * @APPLE_LICENSE_HEADER_START@
 * 
 * Portions Copyright (c) 1999 Apple Computer, Inc.  All Rights
 * Reserved.  This file contains Original Code and/or Modifications of
 * Original Code as defined in and that are subject to the Apple Public
 * Source License Version 1.1 (the "License").  You may not use this file
 * except in compliance with the License.  Please obtain a copy of the
 * License at http://www.apple.com/publicsource and read it before using
 * this file.
 * 
 * The Original Code and all software distributed under the License are
 * distributed on an "AS IS" basis, WITHOUT WARRANTY OF ANY KIND, EITHER
 * EXPRESS OR IMPLIED, AND APPLE HEREBY DISCLAIMS ALL SUCH WARRANTIES,
 * INCLUDING WITHOUT LIMITATION, ANY WARRANTIES OF MERCHANTABILITY,
 * FITNESS FOR A PARTICULAR PURPOSE OR NON- INFRINGEMENT.  Please see the
 * License for the specific language governing rights and limitations
 * under the License.
 * 
 * @APPLE_LICENSE_HEADER_END@
 */
/*	$OpenBSD: is_tar.c,v 1.3 1997/02/09 23:58:27 millert Exp $	*/

/*
 * is_tar() -- figure out whether file is a tar archive.
 *
 * Stolen (by the author!) from the public domain tar program:
 * Pubic Domain version written 26 Aug 1985 John Gilmore (ihnp4!hoptoad!gnu).
 *
 * @(#)list.c 1.18 9/23/86 Public Domain - gnu
 *
 * Comments changed and some code/comments reformatted
 * for file command by Ian Darwin.
 */

#include <string.h>
#include <ctype.h>
#include <sys/types.h>
#include "tar.h"

#define	isodigit(c)	( ((c) >= '0') && ((c) <= '7') )

#if	defined(__STDC__) || defined(__cplusplus)
static int from_oct(int, char*);	/* Decode octal number */
#else
static int from_oct();
#endif

/*
 * Return 
 *	0 if the checksum is bad (i.e., probably not a tar archive), 
 *	1 for old UNIX tar file,
 *	2 for Unix Std (POSIX) tar file.
 */
int
is_tar(buf, nbytes)
unsigned char *buf;
int nbytes;
{
	register union record *header = (union record *)buf;
	register int	i;
	register int	sum, recsum;
	register char	*p;

	if (nbytes < sizeof(union record))
		return 0;

	recsum = from_oct(8,  header->header.chksum);

	sum = 0;
	p = header->charptr;
	for (i = sizeof(union record); --i >= 0;) {
		/*
		 * We can't use unsigned char here because of old compilers,
		 * e.g. V7.
		 */
		sum += 0xFF & *p++;
	}

	/* Adjust checksum to count the "chksum" field as blanks. */
	for (i = sizeof(header->header.chksum); --i >= 0;)
		sum -= 0xFF & header->header.chksum[i];
	sum += ' '* sizeof header->header.chksum;	

	if (sum != recsum)
		return 0;	/* Not a tar archive */
	
	if (0==strcmp(header->header.magic, TMAGIC)) 
		return 2;		/* Unix Standard tar archive */

	return 1;			/* Old fashioned tar archive */
}


/*
 * Quick and dirty octal conversion.
 *
 * Result is -1 if the field is invalid (all blank, or nonoctal).
 */
static int
from_oct(digs, where)
	register int	digs;
	register char	*where;
{
	register int	value;

	while (isspace(*where)) {		/* Skip spaces */
		where++;
		if (--digs <= 0)
			return -1;		/* All blank field */
	}
	value = 0;
	while (digs > 0 && isodigit(*where)) {	/* Scan til nonoctal */
		value = (value << 3) | (*where++ - '0');
		--digs;
	}

	if (digs > 0 && *where && !isspace(*where))
		return -1;			/* Ended on non-space/nul */

	return value;
}
