# Standard library dependencies
require 'set'
require 'zlib'

# Gem dependencies
require 'uuidtools'

module HasGlobalSession 
  # Ladies and gentlemen: the one and only, star of the show, GLOBAL SESSION!
  #
  # GlobalSession is designed to act as much like a Hash as possible. You can use
  # most of the methods you would use with Hash: [], has_key?, each, etc. It has a
  # few additional methods that are specific to itself, mostly involving whether
  # it's expired, valid, supports a certain key, etc.
  #
  class GlobalSession
    attr_reader :id, :authority, :created_at, :expired_at, :directory

    # Create a new global session object.
    #
    # === Parameters
    # directory(Directory):: directory implementation that the session should use for various operations
    # cookie(String):: Optional, serialized global session cookie. If none is supplied, a new session is created.
    # valid_signature_digest(String):: Optional, already-trusted signature. If supplied, the expensive RSA-verify operation will be skipped if the cookie's signature matches the value supplied.
    #
    # ===Raise
    # InvalidSession:: if the session contained in the cookie has been invalidated
    # ExpiredSession:: if the session contained in the cookie has expired
    # MalformedCookie:: if the cookie was corrupt or malformed
    # SecurityError:: if signature is invalid or cookie is not signed by a trusted authority
    def initialize(directory, cookie=nil, valid_signature_digest=nil)
      @configuration   = directory.configuration
      @schema_signed   = Set.new((@configuration['attributes']['signed']))
      @schema_insecure = Set.new((@configuration['attributes']['insecure']))
      @directory       = directory

      if cookie && !cookie.empty?
        load_from_cookie(cookie, valid_signature_digest)
      elsif @directory.local_authority_name
        create_from_scratch
      else
        create_invalid
      end
    end

    # Determine whether the session is valid. This method simply delegates to the
    # directory associated with this session.
    #
    # === Return
    # valid(true|false):: True if the session is valid, false otherwise
    def valid?
      @directory.valid_session?(@id, @expired_at)
    end

    # Serialize the session to a form suitable for use with HTTP cookies. If any
    # secure attributes have changed since the session was instantiated, compute
    # a fresh RSA signature.
    #
    # === Return
    # cookie(String):: The B64cookie-encoded Zlib-compressed JSON-serialized global session hash
    def to_s
      if @cookie && !@dirty_insecure && !@dirty_secure
        #use cached cookie if nothing has changed
        return @cookie
      end

      hash = {'id'=>@id,
              'tc'=>@created_at.to_i, 'te'=>@expired_at.to_i,
              'ds'=>@signed}

      if @signature && !@dirty_secure
        #use cached signature unless we've changed secure state
        authority = @authority
      else
        authority_check
        authority = @directory.local_authority_name
        hash['a'] = authority
        digest    = canonical_digest(hash)
        @signature = Encoding::Base64Cookie.dump(@directory.private_key.private_encrypt(digest))
      end

      hash['dx'] = @insecure
      hash['s']  = @signature
      hash['a']  = authority
      
      json = Encoding::JSON.dump(hash)
      zbin = Zlib::Deflate.deflate(json, Zlib::BEST_COMPRESSION)
      return Encoding::Base64Cookie.dump(zbin)
    end

    # Determine whether the global session schema allows a given key to be placed
    # in the global session.
    #
    # === Parameters
    # key(String):: The name of the key
    #
    # === Return
    # supported(true|false):: Whether the specified key is supported
    def supports_key?(key)
      @schema_signed.include?(key) || @schema_insecure.include?(key)
    end

    # Determine whether this session contains a value with the specified key.
    #
    # === Parameters
    # key(String):: The name of the key
    #
    # === Return
    # contained(true|false):: Whether the session currently has a value for the specified key.
    def has_key?(key)
      @signed.has_key(key) || @insecure.has_key?(key)
    end

    # Return the keys that are currently present in the global session.
    #
    # === Return
    # keys(Array):: List of keys contained in the global session
    def keys
      @signed.keys + @insecure.keys
    end

    # Return the values that are currently present in the global session.
    #
    # === Return
    # values(Array):: List of values contained in the global session
    def values
      @signed.values + @insecure.values
    end

    # Iterate over each key/value pair
    #
    # === Block
    # An iterator which will be called with each key/value pair
    #
    # === Return
    # Returns the value of the last expression evaluated by the block
    def each_pair(&block) # :yields: |key, value|
      @signed.each_pair(&block)
      @insecure.each_pair(&block)
    end

    # Lookup a value by its key.
    #
    # === Parameters
    # key(String):: the key
    #
    # === Return
    # value(Object):: The value associated with +key+, or nil if +key+ is not present
    def [](key)
      @signed[key] || @insecure[key]
    end

    # Set a value in the global session hash. If the supplied key is denoted as
    # secure by the global session schema, causes a new signature to be computed
    # when the session is next serialized.
    #
    # === Parameters
    # key(String):: The key to set
    # value(Object):: The value to set
    #
    # === Return
    # value(Object):: Always returns the value that was set
    #
    # ===Raise
    # InvalidSession:: if the session has been invalidated (and therefore can't be written to)
    # ArgumentError:: if the configuration doesn't define the specified key as part of the global session
    # NoAuthority:: if the specified key is secure and the local node is not an authority
    # UnserializableType:: if the specified value can't be serialized as JSON
    def []=(key, value)
      raise InvalidSession unless valid?

      #Ensure that the value is serializable (will raise if not)
      canonicalize(value)

      if @schema_signed.include?(key)
        authority_check
        @signed[key]  = value
        @dirty_secure = true
      elsif @schema_insecure.include?(key)
        @insecure[key] = value
        @dirty_insecure = true
      else
        raise ArgumentError, "Attribute '#{key}' is not specified in global session configuration"
      end

      return value
    end

    # Invalidate this session by reporting its UUID to the Directory.
    #
    # === Return
    # unknown(Object):: Returns whatever the Directory returns
    def invalidate!
      @directory.report_invalid_session(@id, @expired_at)
    end

    # Renews this global session, changing its expiry timestamp into the future.
    # Causes a new signature will be computed when the session is next serialized.
    #
    # === Return
    # true:: Always returns true
    def renew!
      authority_check
      @expired_at = @configuration['timeout'].to_i.minutes.from_now.utc
      @dirty_secure = true
    end

    # Return the SHA1 hash of the most recently-computed RSA signature of this session.
    # This isn't really intended for the end user; it exists so the Web framework integration
    # code can optimize request speed by caching the most recently verified signature in the
    # local session and avoid re-verifying it on every request.
    #
    # === Return
    # digest(String):: SHA1 hex-digest of most-recently-computed signature
    def signature_digest
      @signature ? digest(@signature) : nil
    end

    private

    def authority_check # :nodoc:
      unless @directory.local_authority_name
        raise NoAuthority, 'Cannot change secure session attributes; we are not an authority'
      end      
    end

    def canonical_digest(input) # :nodoc:
      canonical = Encoding::JSON.dump(canonicalize(input))
      return digest(canonical)
    end

    def digest(input) # :nodoc:
      return Digest::SHA1.new().update(input).hexdigest
    end

    def canonicalize(input) # :nodoc:
      case input
        when Hash
          output = Array.new
          ordered_keys = input.keys.sort
          ordered_keys.each do |key|
            output << [ canonicalize(key), canonicalize(input[key]) ]
          end
        when Array
          output = input.collect { |x| canonicalize(x) }
        when Numeric, String, NilClass
          output = input
        else
          raise UnserializableType, "Objects of type #{input.class.name} cannot be serialized in the global session"
      end

      return output
    end

    def load_from_cookie(cookie, valid_signature_digest) # :nodoc:
      begin
        zbin = Encoding::Base64Cookie.load(cookie)
        json = Zlib::Inflate.inflate(zbin)
        hash = Encoding::JSON.load(json)
      rescue Exception => e
        mc = MalformedCookie.new("Caused by #{e.class.name}: #{e.message}")
        mc.set_backtrace(e.backtrace)
        raise mc
      end

      id         = hash['id']
      authority  = hash['a']
      created_at = Time.at(hash['tc'].to_i).utc
      expired_at = Time.at(hash['te'].to_i).utc
      signed     = hash['ds']
      insecure   = hash.delete('dx')
      signature  = hash.delete('s')

      unless valid_signature_digest == digest(signature)
        #Check signature
        expected = canonical_digest(hash)
        signer   = @directory.authorities[authority]
        raise SecurityError, "Unknown signing authority #{authority}" unless signer
        got      = signer.public_decrypt(Encoding::Base64Cookie.load(signature))
        unless (got == expected)
          raise SecurityError, "Signature mismatch on global session cookie; tampering suspected"
        end
      end
      
      #Check trust in signing authority
      unless @directory.trusted_authority?(authority)
        raise SecurityError, "Global sessions signed by #{authority} are not trusted"
      end

      #Check expiration
      unless expired_at > Time.now.utc
        raise ExpiredSession, "Session expired at #{expired_at}"        
      end

      #Check other validity (delegate to directory)
      unless @directory.valid_session?(id, expired_at)
        raise InvalidSession, "Global session has been invalidated"
      end

      #If all validation stuff passed, assign our instance variables.
      @id         = id
      @authority  = authority
      @created_at = created_at
      @expired_at = expired_at
      @signed     = signed
      @insecure   = insecure
      @signature  = signature
      @cookie     = cookie
    end

    def create_from_scratch # :nodoc:
      authority_check

      @signed          = {}
      @insecure        = {}
      @created_at      = Time.now.utc
      @authority       = @directory.local_authority_name

      if defined?(::UUIDTools) # UUIDTools v2
        @id = ::UUIDTools::UUID.timestamp_create.to_s
      elsif defined?(::UUID)   # UUIDTools v1
        @id = ::UUID.timestamp_create.to_s
      else
        raise TypeError, "Neither UUIDTools nor UUID defined; unsupported UUIDTools version?"
      end

      renew!
    end

    def create_invalid # :nodoc:
      @id         = nil
      @created_at = Time.now.utc
      @expired_at = created_at
      @signed     = {}
      @insecure   = {}
      @authority  = nil
    end
  end  
end