# = bencode.rb - Bencode Library
# 
# Bencode is a Ruby implementation of the Bencode data serialization 
# format used in the BitTorrent protocol.
#
# == Synopsis
#
# Bencoding (pronounced <i>bee-encode</i>) is a simple protocol, consisting of
# only 4 value types.
#
# === Integers
#
# An integer is encoded as an _i_ followed by the numeral itself, followed 
# by an _e_. Leading zeros are not allowed. Negative values are prefixed
# with a minus sign.
#
#   42.bencode  #=> "i42e"
#   -2.bencode  #=> "i-2e"
#   0.bencode   #=> "i0e"
#
# === Strings
#   
# Strings are sequences of zero or more bytes. They are encoded as 
# <i><length>:<contents></i>, where _length_ is the length of _contents_. _length_
# must be non-negative.
#
#   "".bencode     #=> "0:"
#   "foo".bencode  #=> "3:foo"
#
# === Lists
#
# Lists are encoded as _l_ followed by the elements, followed by _e_.
# There is no element seperator.
#
#   [1, 2, 3].bencode  #=> "li1ei2ei3ee"
#
# === Dictionaries
#
# Dictionaries are encoded as _d_ followed by a sequence of key-value pairs, followed by _e_. 
# Each value must be immediately preceded by a key. Keys must be strings, and must appear in 
# lexicographical order.
#
#   {"foo" => 3, "bar" => 1, "baz" => 2}.bencode  
#     #=> "d3:bari1e3:bazi2e3:fooi3ee"
# 
# 
# == Authors
#
# * Daniel Schierbeck
#
# == Contributors
#
# * Daniel Martin
# * Phrogz
# * Julien Pervillé
#
# == Copyright
#
# Bencode is free software; you can redistribute it and/or modify it under the
# terms of the GNU General Public License as published by the Free Software
# Foundation; either version 2 of the License, or (at your option) any later
# version.
# 
# Bencode is distributed in the hope that it will be useful, but WITHOUT ANY
# WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR
# A PARTICULAR PURPOSE.  See the GNU General Public License for more details.
#

class Object
  #
  # Raises an exception. Subclasses of Object must themselves
  # define meaningful #bencode methods.
  def bencode
    raise BencodeError, self.class
  end
end

class Integer
  #
  # Bencodes the Integer object. Bencoded integers are represented
  # as +ixe+, where +x+ is the integer with an optional
  # hyphen prepended, indicating negativity.
  #
  #   42.bencode   #=> "i42e"
  #   -7.bencode   #=> "i-7e"
  def bencode
    "i#{self}e"
  end
end

class String
  #
  # Bencodes the String object. Bencoded strings are represented
  # as +x+:+y+, where +y+ is the string and +x+ is the length of the
  # string.
  #   
  #   "foo".bencode   #=> "3:foo"
  #   "".bencode      #=> "0:"
  #
  def bencode
    "#{length}:#{self}"
  end

  #
  # Bdecodes the String object and returns the data serialized
  # through bencoding.
  #
  #   "li1ei2ei3ee".bdecode   #=> [1, 2, 3]
  #
  def bdecode
    Bencode.load(self)
  end

  #
  # Tests whether the String object is a valid bencoded string.
  def bencoded?
    bdecode
  rescue BdecodeError
    false
  else
    true
  end
end

class Array
  #
  # Bencodes the Array object. Bencoded arrays are represented as
  # +lxe+, where +x+ is zero or more bencoded objects.
  #
  #   [1, "foo"].bencode   #=> "li1e3:fooe"
  #
  def bencode
    "l#{map{|obj| obj.bencode}.join('') }e"
  end
end

class Hash
  #
  # Bencodes the Hash object. Bencoded hashes are represented as
  # +dxe+, where +x+ is zero or a power of two bencoded objects. 
  # each key is immediately followed by its associated value.
  # All keys must be strings. The keys of the bencoded hash will
  # be in lexicographical order.
  def bencode
    pairs = sort.map{|key, val| [key.to_str.bencode, val.bencode]}
    "d#{pairs.join('')}e"
  end
end

class IO
  def self.bdecode(filename)
    open(filename, 'r').bdecode
  end

  def self.bencode(filename)
    open(filename, 'r').bencode
  end

  def bdecode
    read.chomp.bdecode
  end

  def bencode
    read.chomp.bencode
  end
end

class BencodeError < StandardError
  def initialize(object_class = nil) # :nodoc:
    @object_class = object_class
  end

  def to_s # :nodoc:
    "could not bencode #{@object_class}"
  end
end

class BdecodeError < StandardError; end

module Bencode
  class << self
    # Bencodes +obj+
    def dump(obj)
      obj.bencode
    end

    # Bdecodes +str+
    def load(str)
      require 'strscan'

      scanner = StringScanner.new(str)
      obj = parse(scanner)
      raise BdecodeError unless scanner.eos?
      return obj
    end

    # Bdecodes the file located at +path+
    def load_file(path)
      load(File.open(path).read)
    end

    def parse(scanner) # :nodoc:
      case token = scanner.scan(/[ild]|\d+:|\s/)
      when nil
        raise BdecodeError
      when "i"
        number = scanner.scan(/0|(?:-?[1-9][0-9]*)/)
        raise BdecodeError unless number and scanner.scan(/e/)
        return number.to_i
      when "l"
        ary = []
        until scanner.peek(1) == "e"
          ary.push(parse(scanner))
        end
        scanner.pos += 1
        return ary
      when "d"
        hsh = {}
        until scanner.peek(1) == "e"
          key, value = parse(scanner), parse(scanner)
          raise BdecodeError, "key must be a string" unless key.is_a? String
          hsh.store(key, value)
        end
        scanner.pos += 1
        return hsh
      when /\d+:/
        length = token.chop.to_i
        str = scanner.peek(length)
        scanner.pos += length
        return str
      when /\s/
        nil
      else
        raise BdecodeError
      end
    end

    private :parse
  end
end