/*
  Copyright (c) 2017, Matthias Schiffer <mschiffer@universe-factory.net>
  All rights reserved.

  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.

  THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS 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 COPYRIGHT HOLDER 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 <fcntl.h>
#include <stdbool.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <sys/mman.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <openssl/sha.h>


static const char * skip_bcode(const char *data, const char *end);


/** Prints a (printf-style) error message and exits */
#define exit_error(...) do { fprintf(stderr, __VA_ARGS__); exit(1); } while (0)

/** Prints an error messages about unexpected EOF and exits */
__attribute__((noreturn))
static void eof(void) {
	exit_error("unexptected EOF\n");
}


/** Returns the length of the file assiciated with file descriptor fd */
static size_t flen(int fd) {
	struct stat statbuf;
	if (fstat(fd, &statbuf) < 0)
		exit_error("fstat: %m\n");

	return statbuf.st_size;
}


/**
	Returns a pointer after the current integer

	An integer has the form "i42e". 'i' is already skipped by skip_bcode.
*/
static const char * skip_int(const char *data, const char *end) {
	if (data >= end)
		eof();

	const char *e = memchr(data, 'e', end-data);
	if (!e)
		eof();

	return e+1;
}

/**
	Returns a pointer after the current byte string

	A byte string has the form length:data (length is a decimal number
	and data can be arbitrary binary data).
*/
static const char * skip_string(const char *data, const char *end) {
	if (data >= end)
		eof();

	const char *colon = memchr(data, ':', end-data);
	if (!colon)
		eof();

	char *endp;
	size_t len = (size_t)strtoull(data, &endp, 10);
	if (endp != colon)
		exit_error("syntax error\n");

	const char *ret = colon + 1 + len;
	if (ret >= end || ret <= colon /* Check for overflow */)
		eof();

	return ret;
}

/**
	Returns a pointer after the current dictionary

	A dictionary has the form "dkeyvaluekeyvalue...e", where keys are
	byte strings and values are arbitrary bcode objects. 'd' is already
	skipped by skip_bcode.
*/
static const char * skip_dict(const char *key, const char *end) {
	while (key < end) {
		if (key[0] == 'e')
			return key+1;

		const char *value = skip_string(key, end);
		key = skip_bcode(value, end);
	}

	eof();
}

/**
	Returns a pointer after the current list

	A dictionary has the form "lvaluevalue...e", where values are arbitrary
	bcode objects. 'l' is already skipped by skip_bcode.
*/
static const char * skip_list(const char *data, const char *end) {
	while (data < end) {
		if (data[0] == 'e')
			return data+1;

		data = skip_bcode(data, end);
	}

	eof();
}

/**
	Returns a pointer after the current bencoded object

	Possible object types are integers, byte strings, dictionaries and
	lists.
*/
static const char * skip_bcode(const char *data, const char *end) {
	if (data >= end)
		eof();

	switch(data[0]) {
	case 'i':
		return skip_int(data+1, end);
	case '0' ... '9':
		return skip_string(data, end);
	case 'd':
		return skip_dict(data+1, end);
	case 'l':
		return skip_list(data+1, end);
	default:
		exit_error("unexpected character '%c'\n", data[0]);
	}
}


/**
	Prints the SHA1 hash of the data block bounded by the data and end
	arguments.
*/
static void print_hash(const char *data, const char *end) {
	SHA_CTX ctx;
	unsigned char hash[20];

	SHA1_Init(&ctx);
	SHA1_Update(&ctx, (unsigned char *)data, end-data);
	SHA1_Final(hash, &ctx);

	for (size_t i = 0; i < 20; i++)
		printf("%02x", hash[i]);

	printf("\n");
}


int main(int argc, char *argv[]) {
	if (argc != 2) {
		fprintf(stderr, "Usage: torrenthash <file>\n");
		return 1;
	}

	int fd = open(argv[1], O_RDONLY);
	if (fd < 0)
		exit_error("open: %m\n");
	size_t len = flen(fd);
	if (len == 0)
		exit_error("empty file\n");

	/* Map the file into memory */
	const char *data = mmap(NULL, len, PROT_READ, MAP_PRIVATE, fd, 0);
	if (data == MAP_FAILED)
		exit_error("mmap: %m\n");

	const char *end = data + len;

	/* The file must have a dictionary on the top level */
	if (data[0] != 'd')
		exit_error("unexpected character '%c'\n", data[0]);

	/* Iterate through the dictionary entries */
	for (const char *key = data+1; key < end;) {
		if (key[0] == 'e')
			exit_error("key 'info' not found\n");

		const char *value = skip_string(key, end);
		const char *next_key = skip_bcode(value, end);

		/* Check for "info" key */
		if ((value-key) == 6 && memcmp(key, "4:info", 6) == 0) {
			print_hash(value, next_key);
			return 0;
		}

		key = next_key;
	}

	eof();
}