/*
 * is_tar() -- figure out whether file is a tar archive.
 *
 * Stolen (by the author of the file utility!) from the public domain tar program:
 * Public Domain version written 26 Aug 1985 John Gilmore (ihnp4!hoptoad!gnu).
 *
 * @(#)list.c 1.18 9/23/86 Public Domain - gnu
 * $Id: is_tar.c,v 1.2 2006/06/17 21:00:44 tkojm Exp $
 *
 * Comments changed and some code/comments reformatted
 * for file command by Ian Darwin.
 */

#if HAVE_CONFIG_H
#include "clamav-config.h"
#endif

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

#include "clamav.h"
#include "others.h"

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

static int from_oct(int digs, char *where);

/*
 * 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(const unsigned char *buf, unsigned int nbytes)
{
    union record *header = (union record *)buf;
    int i;
    int sum, recsum;
    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(int digs, char *where)
{
    int value;

    while (isspace((unsigned char)*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((unsigned char)*where))
        return -1; /* Ended on non-space/nul */

    return value;
}
