premiere-libtorrent/include/libtorrent/lazy_entry.hpp

/*

Copyright (c) 2003-2013, Arvid Norberg
All rights reserved.

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modification, are permitted provided that the following conditions
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    * Redistributions in binary form must reproduce the above copyright
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      contributors may be used to endorse or promote products derived
      from this software without specific prior written permission.

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*/

#ifndef TORRENT_LAZY_ENTRY_HPP_INCLUDED
#define TORRENT_LAZY_ENTRY_HPP_INCLUDED

#include <utility>
#include <vector>
#include <string>
#include <cstring>
#include <algorithm>
#include "libtorrent/config.hpp"
#include "libtorrent/assert.hpp"
#include "libtorrent/error_code.hpp"

namespace libtorrent
{
	struct lazy_entry;

	// This function decodes bencoded_ data.
	// 
	// .. _bencoded: http://wiki.theory.org/index.php/BitTorrentSpecification
	// 
	// Whenever possible, ``lazy_bdecode()`` should be preferred over ``bdecode()``.
	// It is more efficient and more secure. It supports having constraints on the
	// amount of memory is consumed by the parser.
	// 
	// *lazy* refers to the fact that it doesn't copy any actual data out of the
	// bencoded buffer. It builds a tree of ``lazy_entry`` which has pointers into
	// the bencoded buffer. This makes it very fast and efficient. On top of that,
	// it is not recursive, which saves a lot of stack space when parsing deeply
	// nested trees. However, in order to protect against potential attacks, the
	// ``depth_limit`` and ``item_limit`` control how many levels deep the tree is
	// allowed to get. With recursive parser, a few thousand levels would be enough
	// to exhaust the threads stack and terminate the process. The ``item_limit``
	// protects against very large structures, not necessarily deep. Each bencoded
	// item in the structure causes the parser to allocate some amount of memory,
	// this memory is constant regardless of how much data actually is stored in
	// the item. One potential attack is to create a bencoded list of hundreds of
	// thousands empty strings, which would cause the parser to allocate a significant
	// amount of memory, perhaps more than is available on the machine, and effectively
	// provide a denial of service. The default item limit is set as a reasonable
	// upper limit for desktop computers. Very few torrents have more items in them.
	// The limit corresponds to about 25 MB, which might be a bit much for embedded
	// systems.
	// 
	// ``start`` and ``end`` defines the bencoded buffer to be decoded. ``ret`` is
	// the ``lazy_entry`` which is filled in with the whole decoded tree. ``ec``
	// is a reference to an ``error_code`` which is set to describe the error encountered
	// in case the function fails. ``error_pos`` is an optional pointer to an int,
	// which will be set to the byte offset into the buffer where an error occurred,
	// in case the function fails.
	TORRENT_EXPORT int lazy_bdecode(char const* start, char const* end
		, lazy_entry& ret, error_code& ec, int* error_pos = 0
		, int depth_limit = 1000, int item_limit = 1000000);

#ifndef TORRENT_NO_DEPRECATE
	// for backwards compatibility, does not report error code
	// deprecated in 0.16
	TORRENT_DEPRECATED_PREFIX
	TORRENT_EXPORT int lazy_bdecode(char const* start, char const* end
		, lazy_entry& ret, int depth_limit = 1000, int item_limit = 1000000) TORRENT_DEPRECATED;
#endif

	// this is a string that is not NULL-terminated. Instead it
	// comes with a length, specified in bytes. This is particularly
	// useful when parsing bencoded structures, because strings are
	// not NULL-terminated internally, and requiring NULL termination
	// would require copying the string.
	//
	// see lazy_entry::string_pstr().
	struct TORRENT_EXPORT pascal_string
	{
		// construct a string pointing to the characters at ``p``
		// of length ``l`` characters. No NULL termination is required.
		pascal_string(char const* p, int l): len(l), ptr(p) {}
		
		// the number of characters in the string.
		int len;

		// the pointer to the first character in the string. This is
		// not NULL terminated, but instead consult the ``len`` field
		// to know how many characters follow.
		char const* ptr;

		// lexicographical comparison of strings. Order is consisten
		// with memcmp.
		bool operator<(pascal_string const& rhs) const
		{
			return std::memcmp(ptr, rhs.ptr, (std::min)(len, rhs.len)) < 0
				|| len < rhs.len;
		}
	};

	struct lazy_dict_entry;

	// this object represent a node in a bencoded structure. It is a variant
	// type whose concrete type is one of:
	//
	// 1. dictionary (maps strings -> lazy_entry)
	// 2. list (sequence of lazy_entry, i.e. heterogenous)
	// 3. integer
	// 4. string
	//
	// There is also a ``none`` type, which is used for uninitialized
	// lazy_entries.
	struct TORRENT_EXPORT lazy_entry
	{
		// The different types a lazy_entry can have
		enum entry_type_t
		{
			none_t, dict_t, list_t, string_t, int_t
		};

		// internal
		lazy_entry() : m_begin(0), m_len(0), m_size(0), m_capacity(0), m_type(none_t)
		{ m_data.start = 0; }

		// tells you which specific type this lazy entry has.
		// See entry_type_t. The type determines which subset of
		// member functions are valid to use.
		entry_type_t type() const { return (entry_type_t)m_type; }

		// start points to the first decimal digit
		// length is the number of digits
		void construct_int(char const* start, int length)
		{
			TORRENT_ASSERT(m_type == none_t);
			m_type = int_t;
			m_data.start = start;
			m_size = length;
			m_begin = start - 1; // include 'i'
			m_len = length + 2; // include 'e'
		}

		// requires the type to be an integer. return the integer value
		boost::int64_t int_value() const;

		// internal
		void construct_string(char const* start, int length);

		// the string is not null-terminated!
		// use string_length() to determine how many bytes
		// are part of the string.
		char const* string_ptr() const
		{
			TORRENT_ASSERT(m_type == string_t);
			return m_data.start;
		}

		// this will return a null terminated string
		// it will write to the source buffer!
		char const* string_cstr() const
		{
			TORRENT_ASSERT(m_type == string_t);
			const_cast<char*>(m_data.start)[m_size] = 0;
			return m_data.start;
		}

		// if this is a string, returns a pascal_string
		// representing the string value.
		pascal_string string_pstr() const
		{
			TORRENT_ASSERT(m_type == string_t);
			return pascal_string(m_data.start, m_size);
		}

		// if this is a string, returns the string as a std::string.
		// (which requires a copy)
		std::string string_value() const
		{
			TORRENT_ASSERT(m_type == string_t);
			return std::string(m_data.start, m_size);
		}

		// if the lazy_entry is a string, returns the
		// length of the string, in bytes.
		int string_length() const
		{ return m_size; }

		// internal
		void construct_dict(char const* begin)
		{
			TORRENT_ASSERT(m_type == none_t);
			m_type = dict_t;
			m_size = 0;
			m_capacity = 0;
			m_begin = begin;
		}

		// internal
		lazy_entry* dict_append(char const* name);
		// internal
		void pop();

		// if this is a dictionary, look for a key ``name``, and return
		// a pointer to its value, or NULL if there is none.
		lazy_entry* dict_find(char const* name);
		lazy_entry const* dict_find(char const* name) const
		{ return const_cast<lazy_entry*>(this)->dict_find(name); }
		lazy_entry const* dict_find_string(char const* name) const;

		// if this is a dictionary, look for a key ``name`` whose value
		// is a string. If such key exist, return a pointer to
		// its value, otherwise NULL.
		std::string dict_find_string_value(char const* name) const;
		pascal_string dict_find_pstr(char const* name) const;

		// if this is a dictionary, look for a key ``name`` whose value
		// is an int. If such key exist, return a pointer to its value,
		// otherwise NULL.
		boost::int64_t dict_find_int_value(char const* name, boost::int64_t default_val = 0) const;
		lazy_entry const* dict_find_int(char const* name) const;

		// these functions require that ``this`` is a dictionary.
		// (this->type() == dict_t). They look for an element with the
		// specified name in the dictionary. ``dict_find_dict`` only
		// finds dictionaries and ``dict_find_list`` only finds lists.
		// if no key with the corresponding value of the right type is
		// found, NULL is returned.
		lazy_entry const* dict_find_dict(char const* name) const;
		lazy_entry const* dict_find_list(char const* name) const;

		// if this is a dictionary, return the key value pair at
		// position ``i`` from the dictionary.
		std::pair<std::string, lazy_entry const*> dict_at(int i) const;

		// requires that ``this`` is a dictionary. return the
		// number of items in it
		int dict_size() const
		{
			TORRENT_ASSERT(m_type == dict_t);
			return m_size;
		}

		// internal
		void construct_list(char const* begin)
		{
			TORRENT_ASSERT(m_type == none_t);
			m_type = list_t;
			m_size = 0;
			m_capacity = 0;
			m_begin = begin;
		}

		// internal
		lazy_entry* list_append();

		// requires that ``this`` is a list. return
		// the item at index ``i``.
		lazy_entry* list_at(int i)
		{
			TORRENT_ASSERT(m_type == list_t);
			TORRENT_ASSERT(i < int(m_size));
			return &m_data.list[i];
		}
		lazy_entry const* list_at(int i) const
		{ return const_cast<lazy_entry*>(this)->list_at(i); }

		// these functions require ``this`` to have the type list.
		// (this->type() == list_t). ``list_string_value_at`` returns
		// the string at index ``i``. ``list_pstr_at``
		// returns a pascal_string of the string value at index ``i``.
		// if the element at ``i`` is not a string, an empty string
		// is returned.
		std::string list_string_value_at(int i) const;
		pascal_string list_pstr_at(int i) const;

		// this function require ``this`` to have the type list.
		// (this->type() == list_t). returns the integer value at
		// index ``i``. If the element at ``i`` is not an integer
		// ``default_val`` is returned, which defaults to 0.
		boost::int64_t list_int_value_at(int i, boost::int64_t default_val = 0) const;

		// if this is a list, return the number of items in it.
		int list_size() const
		{
			TORRENT_ASSERT(m_type == list_t);
			return int(m_size);
		}

		// internal: end points one byte passed last byte in the source
		// buffer backing the bencoded structure.
		void set_end(char const* end)
		{
			TORRENT_ASSERT(end > m_begin);
			m_len = end - m_begin;
		}
		
		// internal
		void clear();

		// internal: releases ownership of any memory allocated
		void release()
		{
			m_data.start = 0;
			m_size = 0;
			m_capacity = 0;
			m_type = none_t;
		}

		// internal
		~lazy_entry()
		{ clear(); }

		// returns pointers into the source buffer where
		// this entry has its bencoded data
		std::pair<char const*, int> data_section() const;

		// swap values of ``this`` and ``e``.
		void swap(lazy_entry& e)
		{
			using std::swap;
			boost::uint32_t tmp = e.m_type;
			e.m_type = m_type;
			m_type = tmp;
			tmp = e.m_capacity;
			e.m_capacity = m_capacity;
			m_capacity = tmp;
			swap(m_data.start, e.m_data.start);
			swap(m_size, e.m_size);
			swap(m_begin, e.m_begin);
			swap(m_len, e.m_len);
		}

	private:

		union data_t
		{
			lazy_dict_entry* dict;
			lazy_entry* list;
			char const* start;
		} m_data;

		// used for dictionaries and lists to record the range
		// in the original buffer they are based on
		char const* m_begin;
		// the number of bytes this entry extends in the
		// bencoded byffer
		boost::uint32_t m_len;

		// if list or dictionary, the number of items
		boost::uint32_t m_size;
		// if list or dictionary, allocated number of items
		boost::uint32_t m_capacity:29;
		// element type (dict, list, int, string)
		boost::uint32_t m_type:3;

		// non-copyable
		lazy_entry(lazy_entry const&);
		lazy_entry const& operator=(lazy_entry const&);
	};

	struct lazy_dict_entry
	{
		char const* name;
		lazy_entry val;
	};

	TORRENT_EXTRA_EXPORT std::string print_entry(lazy_entry const& e
		, bool single_line = false, int indent = 0);

	// get the ``error_category`` for bdecode errors
	TORRENT_EXPORT boost::system::error_category& get_bdecode_category();

	namespace bdecode_errors
	{
		// libtorrent uses boost.system's ``error_code`` class to represent errors. libtorrent has
		// its own error category get_bdecode_category() whith the error codes defined by error_code_enum.
		enum error_code_enum
		{
			// Not an error
			no_error = 0,
			// expected string in bencoded string
			expected_string,
			// expected colon in bencoded string
			expected_colon,
			// unexpected end of file in bencoded string
			unexpected_eof,
			// expected value (list, dict, int or string) in bencoded string
			expected_value,
			// bencoded recursion depth limit exceeded
			depth_exceeded,
			// bencoded item count limit exceeded
			limit_exceeded,

			// the number of error codes
			error_code_max
		};

		// hidden
		inline boost::system::error_code make_error_code(error_code_enum e)
		{
			return boost::system::error_code(e, get_bdecode_category());
		}
	}
}

#endif