/*

Copyright (c) 2003-2016, Arvid Norberg
All rights reserved.

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modification, are permitted provided that the following conditions
are met:

    * Redistributions of source code must retain the above copyright
      notice, this list of conditions and the following disclaimer.
    * 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.
    * Neither the name of the author nor the names of its
      contributors may be used to endorse or promote products derived
      from this software without specific prior written permission.

THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
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*/

#ifndef TORRENT_BENCODE_HPP_INCLUDED
#define TORRENT_BENCODE_HPP_INCLUDED

// OVERVIEW
//
// Bencoding is a common representation in bittorrent used for for dictionary,
// list, int and string hierarchies. It's used to encode .torrent files and
// some messages in the network protocol. libtorrent also uses it to store
// settings, resume data and other state between sessions.
//
// Strings in bencoded structures are not necessarily representing text.
// Strings are raw byte buffers of a certain length. If a string is meant to be
// interpreted as text, it is required to be UTF-8 encoded. See `BEP 3`_.
//
// There are two mechanisms to *decode* bencoded buffers in libtorrent.
//
// The most flexible one is `bdecode() bencode()`_, which returns a structure
// represented by entry. Once a buffer has been decoded with this function, it
// can be discarded. The entry does not contain any references back to it. This
// means that bdecode() copies all the data out of the buffer and into its own
// hierarchy. This makes this function expensive, which might matter if you're
// parsing large amounts of data.
//
// Another consideration is that `bdecode() bencode()`_ is a recursive parser.
// For this reason, in order to avoid DoS attacks by triggering a stack
// overflow, there is a recursion limit. This limit is a sanity check to make
// sure it doesn't run the risk of busting the stack.
//
// The second mechanism is the decode function for bdecode_node. This function
// builds a tree that points back into the original buffer. The returned
// bdecode_node will not be valid once the buffer it was parsed out of is
// discarded.
//
// Not only is this function more efficient because of less memory allocation
// and data copy, the parser is also not recursive, which means it probably
// performs a little bit better and can have a higher recursion limit on the
// structures it's parsing.

#include <string>
#include <iterator> // for distance

#include "libtorrent/config.hpp"
#include "libtorrent/entry.hpp"
#include "libtorrent/assert.hpp"
#include "libtorrent/io.hpp" // for write_string
#include "libtorrent/string_util.hpp" // for is_digit

namespace libtorrent {

#ifndef TORRENT_NO_DEPRECATE
	using invalid_encoding = system_error;
#endif

namespace detail {

		template <class OutIt, class In, typename Cond
			= typename std::enable_if<std::is_integral<In>::value>::type>
		int write_integer(OutIt& out, In data)
		{
			entry::integer_type const val = entry::integer_type(data);
			TORRENT_ASSERT(data == In(val));
			// the stack allocated buffer for keeping the
			// decimal representation of the number can
			// not hold number bigger than this:
			static_assert(sizeof(entry::integer_type) <= 8, "64 bit integers required");
			static_assert(sizeof(data) <= sizeof(entry::integer_type), "input data too big, see entry::integer_type");
			char buf[21];
			int ret = 0;
			for (char const* str = integer_to_str(buf, 21, val);
				*str != 0; ++str)
			{
				*out = *str;
				++out;
				++ret;
			}
			return ret;
		}

		template <class OutIt>
		void write_char(OutIt& out, char c)
		{
			*out = c;
			++out;
		}

		template <class InIt>
		std::string read_until(InIt& in, InIt end, char end_token, bool& err)
		{
			std::string ret;
			if (in == end)
			{
				err = true;
				return ret;
			}
			while (*in != end_token)
			{
				ret += *in;
				++in;
				if (in == end)
				{
					err = true;
					return ret;
				}
			}
			return ret;
		}

		template<class InIt>
		void read_string(InIt& in, InIt end, int len, std::string& str, bool& err)
		{
			TORRENT_ASSERT(len >= 0);
			for (int i = 0; i < len; ++i)
			{
				if (in == end)
				{
					err = true;
					return;
				}
				str += *in;
				++in;
			}
		}

		template<class OutIt>
		int bencode_recursive(OutIt& out, const entry& e)
		{
			int ret = 0;
			switch(e.type())
			{
			case entry::int_t:
				write_char(out, 'i');
				ret += write_integer(out, e.integer());
				write_char(out, 'e');
				ret += 2;
				break;
			case entry::string_t:
				ret += write_integer(out, e.string().length());
				write_char(out, ':');
				ret += write_string(e.string(), out);
				ret += 1;
				break;
			case entry::list_t:
				write_char(out, 'l');
				for (entry::list_type::const_iterator i = e.list().begin(); i != e.list().end(); ++i)
					ret += bencode_recursive(out, *i);
				write_char(out, 'e');
				ret += 2;
				break;
			case entry::dictionary_t:
				write_char(out, 'd');
				for (entry::dictionary_type::const_iterator i = e.dict().begin();
					i != e.dict().end(); ++i)
				{
					// write key
					ret += write_integer(out, i->first.length());
					write_char(out, ':');
					ret += write_string(i->first, out);
					// write value
					ret += bencode_recursive(out, i->second);
					ret += 1;
				}
				write_char(out, 'e');
				ret += 2;
				break;
			case entry::preformatted_t:
				std::copy(e.preformatted().begin(), e.preformatted().end(), out);
				ret += int(e.preformatted().size());
				break;
			case entry::undefined_t:

				// empty string
				write_char(out, '0');
				write_char(out, ':');

				ret += 2;
				break;
			}
			return ret;
		}

		template<class InIt>
		void bdecode_recursive(InIt& in, InIt end, entry& ret, bool& err, int depth)
		{
			if (depth >= 100)
			{
				err = true;
				return;
			}

			if (in == end)
			{
				err = true;
#if TORRENT_USE_ASSERTS
				ret.m_type_queried = false;
#endif
				return;
			}
			switch (*in)
			{

			// ----------------------------------------------
			// integer
			case 'i':
				{
				++in; // 'i'
				std::string val = read_until(in, end, 'e', err);
				if (err) return;
				TORRENT_ASSERT(*in == 'e');
				++in; // 'e'
				ret = entry(entry::int_t);
				char* end_pointer;
				ret.integer() = std::strtoll(val.c_str(), &end_pointer, 10);
#if TORRENT_USE_ASSERTS
				ret.m_type_queried = false;
#endif
				if (end_pointer == val.c_str())
				{
					err = true;
					return;
				}
				} break;

			// ----------------------------------------------
			// list
			case 'l':
				{
				ret = entry(entry::list_t);
				++in; // 'l'
				while (*in != 'e')
				{
					ret.list().push_back(entry());
					entry& e = ret.list().back();
					bdecode_recursive(in, end, e, err, depth + 1);
					if (err)
					{
#if TORRENT_USE_ASSERTS
						ret.m_type_queried = false;
#endif
						return;
					}
					if (in == end)
					{
						err = true;
#if TORRENT_USE_ASSERTS
						ret.m_type_queried = false;
#endif
						return;
					}
				}
#if TORRENT_USE_ASSERTS
				ret.m_type_queried = false;
#endif
				TORRENT_ASSERT(*in == 'e');
				++in; // 'e'
				} break;

			// ----------------------------------------------
			// dictionary
			case 'd':
				{
				ret = entry(entry::dictionary_t);
				++in; // 'd'
				while (*in != 'e')
				{
					entry key;
					bdecode_recursive(in, end, key, err, depth + 1);
					if (err || key.type() != entry::string_t)
					{
#if TORRENT_USE_ASSERTS
						ret.m_type_queried = false;
#endif
						return;
					}
					entry& e = ret[key.string()];
					bdecode_recursive(in, end, e, err, depth + 1);
					if (err)
					{
#if TORRENT_USE_ASSERTS
						ret.m_type_queried = false;
#endif
						return;
					}
					if (in == end)
					{
						err = true;
#if TORRENT_USE_ASSERTS
						ret.m_type_queried = false;
#endif
						return;
					}
				}
#if TORRENT_USE_ASSERTS
				ret.m_type_queried = false;
#endif
				TORRENT_ASSERT(*in == 'e');
				++in; // 'e'
				} break;

			// ----------------------------------------------
			// string
			default:
				static_assert(sizeof(*in) == 1, "Input iterator to 8 bit data required");
				if (is_digit(char(*in)))
				{
					std::string len_s = read_until(in, end, ':', err);
					if (err)
					{
#if TORRENT_USE_ASSERTS
						ret.m_type_queried = false;
#endif
						return;
					}
					TORRENT_ASSERT(*in == ':');
					++in; // ':'
					int len = atoi(len_s.c_str());
					ret = entry(entry::string_t);
					read_string(in, end, len, ret.string(), err);
					if (err)
					{
#if TORRENT_USE_ASSERTS
						ret.m_type_queried = false;
#endif
						return;
					}
				}
				else
				{
					err = true;
#if TORRENT_USE_ASSERTS
					ret.m_type_queried = false;
#endif
					return;
				}
#if TORRENT_USE_ASSERTS
				ret.m_type_queried = false;
#endif
			}
		}
	}

	// These functions will encode data to bencoded or decode bencoded data.
	//
	// If possible, ``bdecode()`` producing a bdecode_node should be preferred
	// over this function.
	//
	// The entry_ class is the internal representation of the bencoded data
	// and it can be used to retrieve information, an entry_ can also be build by
	// the program and given to ``bencode()`` to encode it into the ``OutIt``
	// iterator.
	//
	// The ``OutIt`` and ``InIt`` are iterators
	// (InputIterator_ and OutputIterator_ respectively). They
	// are templates and are usually instantiated as ostream_iterator_,
	// back_insert_iterator_ or istream_iterator_. These
	// functions will assume that the iterator refers to a character
	// (``char``). So, if you want to encode entry ``e`` into a buffer
	// in memory, you can do it like this::
	//
	//	std::vector<char> buffer;
	//	bencode(std::back_inserter(buf), e);
	//
	// .. _InputIterator: http://www.sgi.com/tech/stl/InputIterator.html
	// .. _OutputIterator: http://www.sgi.com/tech/stl/OutputIterator.html
	// .. _ostream_iterator: http://www.sgi.com/tech/stl/ostream_iterator.html
	// .. _back_insert_iterator: http://www.sgi.com/tech/stl/back_insert_iterator.html
	// .. _istream_iterator: http://www.sgi.com/tech/stl/istream_iterator.html
	//
	// If you want to decode a torrent file from a buffer in memory, you can do it like this::
	//
	//	std::vector<char> buffer;
	//	// ...
	//	entry e = bdecode(buf.begin(), buf.end());
	//
	// Or, if you have a raw char buffer::
	//
	//	const char* buf;
	//	// ...
	//	entry e = bdecode(buf, buf + data_size);
	//
	// Now we just need to know how to retrieve information from the entry.
	//
	// If ``bdecode()`` encounters invalid encoded data in the range given to it
	// it will return a default constructed ``entry`` object.
	template<class OutIt> int bencode(OutIt out, const entry& e)
	{
		return detail::bencode_recursive(out, e);
	}
	template<class InIt> entry bdecode(InIt start, InIt end)
	{
		entry e;
		bool err = false;
		detail::bdecode_recursive(start, end, e, err, 0);
		TORRENT_ASSERT(e.m_type_queried == false);
		if (err) return entry();
		return e;
	}
	template<class InIt> entry bdecode(InIt start, InIt end
		, typename std::iterator_traits<InIt>::difference_type& len)
	{
		entry e;
		bool err = false;
		InIt s = start;
		detail::bdecode_recursive(start, end, e, err, 0);
		len = std::distance(s, start);
		TORRENT_ASSERT(len >= 0);
		if (err) return entry();
		return e;
	}
}

#endif // TORRENT_BENCODE_HPP_INCLUDED