/*
 * This program reads a binary file containing a G.711 PCM stream capture
 * and looks for an IS_Header pattern as defined in ETSI TS 101 504
 * (GSM 08.62) section A.1.2.  The objective is to analyze PCM streams
 * originating from extant commercial GSM network operators and see if
 * they implement in-band TFO.
 */

#include <sys/types.h>
#include <sys/file.h>
#include <sys/stat.h>
#include <sys/mman.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>

static char *pcmfile;
static size_t pcm_file_size;
static u_char *filemap;

static const u_char hdr_pattern[20] =	{0, 1, 0, 1, 0, 1, 1, 0, 1, 0,
					 0, 1, 1, 0, 1, 0, 1, 0, 0, 1};

static void
mmap_pcm_file()
{
	int fd;
	struct stat st;

	fd = open(pcmfile, O_RDONLY);
	if (fd < 0) {
		perror(pcmfile);
		exit(1);
	}
	fstat(fd, &st);
	if (!S_ISREG(st.st_mode)) {
		fprintf(stderr, "error: %s is not a regular file\n", pcmfile);
		exit(1);
	}
	pcm_file_size = st.st_size;
	if (pcm_file_size < 320) {
		fprintf(stderr, "error: %s is too short\n", pcmfile);
		exit(1);
	}
	filemap = mmap(NULL, pcm_file_size, PROT_READ, MAP_PRIVATE, fd, 0L);
	if (filemap == MAP_FAILED) {
		perror("mmap");
		exit(1);
	}
	close(fd);
}

static void
try_offset(offset)
	size_t offset;
{
	unsigned n;

	for (n = 0; n < 20; n++) {
		if ((filemap[offset + n * 16] & 1) != hdr_pattern[n])
			return;
	}
	printf("Found IS_Header at offset %lu (0x%lx)\n", (u_long) offset,
		(u_long) offset);
}

main(argc, argv)
	char **argv;
{
	size_t offset, endoff;

	if (argc != 2) {
		fprintf(stderr, "usage: %s pcm-capture-file\n", argv[0]);
		exit(1);
	}
	pcmfile = argv[1];
	mmap_pcm_file();
	endoff = pcm_file_size - 320;
	for (offset = 0; offset <= endoff; offset++)
		try_offset(offset);
	exit(0);
}