/*
 * This file is in the public domain.  Use it as you see fit.
 */

/*
 * "untar" is an extremely simple tar extractor:
 *  * A single C source file, so it should be easy to compile
 *    and run on any system with a C compiler.
 *  * Extremely portable standard C.  The only non-ANSI function
 *    used is mkdir().
 *  * Reads basic ustar tar archives.
 *  * Does not require libarchive or any other special library.
 *
 * To compile: cc -o untar untar.c
 *
 * Usage:  untar <archive>
 *
 * In particular, this program should be sufficient to extract the
 * distribution for libarchive, allowing people to bootstrap
 * libarchive on systems that do not already have a tar program.
 *
 * To unpack libarchive-x.y.z.tar.gz:
 *    * gunzip libarchive-x.y.z.tar.gz
 *    * untar libarchive-x.y.z.tar
 *
 * Written by Tim Kientzle, March 2009.
 *
 * Released into the public domain.
 */

/* These are all highly standard and portable headers. */
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

/* This is for mkdir(); this may need to be changed for some platforms. */
#include <sys/stat.h>  /* For mkdir() */

#if defined(_WIN32) && !defined(__CYGWIN__)
#define NUM_FORMAT "zu"
#include <direct.h>
#elif defined(__linux__) || defined(linux) || defined(__linux)
#define NUM_FORMAT "d"
#else
#define NUM_FORMAT "lu"
#endif /* defined(_WIN32) && !defined(__CYGWIN__) */

#define TO_STRING(x) #x
#define STR(x) TO_STRING(x)

/* Parse an octal number, ignoring leading and trailing nonsense. */
static int
parseoct(const char *p, size_t n)
{
	int i = 0;

	while ((*p < '0' || *p > '7') && n > 0) {
		++p;
		--n;
	}
	while (*p >= '0' && *p <= '7' && n > 0) {
		i *= 8;
		i += *p - '0';
		++p;
		--n;
	}
	return (i);
}

/* Returns true if this is 512 zero bytes. */
static int
is_end_of_archive(const char *p)
{
	int n;
	for (n = 511; n >= 0; --n)
		if (p[n] != '\0')
			return (0);
	return (1);
}

/* Create a directory, including parent directories as necessary. */
static void
create_dir(char *pathname, int mode)
{
	char *p;
	int r;

	/* Strip trailing '/' */
	if (pathname[strlen(pathname) - 1] == '/')
		pathname[strlen(pathname) - 1] = '\0';

	/* Try creating the directory. */
#if defined(_WIN32) && !defined(__CYGWIN__)
	r = _mkdir(pathname);
#else
	r = mkdir(pathname, mode);
#endif

	if (r != 0) {
		/* On failure, try creating parent directory. */
		p = strrchr(pathname, '/');
		if (p != NULL) {
			*p = '\0';
			create_dir(pathname, 0755);
			*p = '/';
#if defined(_WIN32) && !defined(__CYGWIN__)
			r = _mkdir(pathname);
#else
			r = mkdir(pathname, mode);
#endif
		}
	}
	if (r != 0)
		fprintf(stderr, "Could not create directory %s\n", pathname);
}

/* Create a file, including parent directory as necessary. */
static FILE *
create_file(char *pathname, int mode)
{
	FILE *f;
	f = fopen(pathname, "wb+");
	if (f == NULL) {
		/* Try creating parent dir and then creating file. */
		char *p = strrchr(pathname, '/');
		if (p != NULL) {
			*p = '\0';
			create_dir(pathname, 0755);
			*p = '/';
			f = fopen(pathname, "wb+");
		}
	}
	return (f);
}

/* Verify the tar checksum. */
static int
verify_checksum(const char *p)
{
	int n, u = 0;
	for (n = 0; n < 512; ++n) {
		if (n < 148 || n > 155)
			/* Standard tar checksum adds unsigned bytes. */
			u += ((unsigned char *)p)[n];
		else
			u += 0x20;

	}
	return (u == parseoct(p + 148, 8));
}

/* Extract a tar archive. */
static void
untar(FILE *a, const char *path)
{
    enum { BUF_SIZE=512 };
	char buff[BUF_SIZE];
	FILE *f = NULL;
	size_t bytes_read;
	off_t filesize;

	printf("Extracting from %s\n", path);
	for (;;) {
		bytes_read = fread(buff, 1, BUF_SIZE, a);
		if (bytes_read < BUF_SIZE) {
			fprintf(stderr,
					"Short read on %s: expected " STR(BUF_SIZE) ", got %"NUM_FORMAT"\n",
					path, bytes_read);
			return;
		}
		if (is_end_of_archive(buff)) {
			printf("End of %s\n", path);
			return;
		}
		if (!verify_checksum(buff)) {
			fprintf(stderr, "Checksum failure\n");
			return;
		}
		filesize = parseoct(buff + 124, 12);
		switch (buff[156]) {
			case '1':
				printf(" Ignoring hardlink %s\n", buff);
				break;
			case '2':
				printf(" Ignoring symlink %s\n", buff);
				break;
			case '3':
				printf(" Ignoring character device %s\n", buff);
				break;
			case '4':
				printf(" Ignoring block device %s\n", buff);
				break;
			case '5':
				printf(" Extracting dir %s\n", buff);
				create_dir(buff, parseoct(buff + 100, 8));
				filesize = 0;
				break;
			case '6':
				printf(" Ignoring FIFO %s\n", buff);
				break;
			default:
				printf(" Extracting file %s\n", buff);
				f = create_file(buff, parseoct(buff + 100, 8));
				break;
		}
		while (filesize > 0) {
			bytes_read = fread(buff, 1, BUF_SIZE, a);
			if (bytes_read < BUF_SIZE) {
				fprintf(stderr,
						"Short read on %s: Expected " STR(BUF_SIZE) ", got %"NUM_FORMAT"\n",
						path, bytes_read);
				return;
			}
			if (filesize < BUF_SIZE)
				bytes_read = filesize;
			if (f != NULL) {
				if (fwrite(buff, 1, bytes_read, f)
					!= bytes_read)
				{
					fprintf(stderr, "Failed write\n");
					fclose(f);
					f = NULL;
				}
			}
			filesize -= (off_t)bytes_read;
		}
		if (f != NULL) {
			fclose(f);
			f = NULL;
		}
	}
}

int
main(int argc, char **argv)
{
	FILE *a;

	++argv; /* Skip program name */
	for ( ;*argv != NULL; ++argv) {
		a = fopen(*argv, "rb");
		if (a == NULL)
			fprintf(stderr, "Unable to open %s\n", *argv);
		else {
			untar(a, *argv);
			fclose(a);
		}
	}
	return (0);
}
