# Forward referencing is one of those painful problems in programming - if
# you try to use something before it's defined, trouble ensues. Using the
# ForwardReferencing module and the ForwardReference class can let you
# gracefully recover from problems caused by forward references in data and
# processing.

require 'eymiha'

# The ForwardReferencing module can be mixed into a class to allow it to
# capture and resolve ForwardReferences.
module ForwardReferencing

  # An array containing the set of unresolved forward references.
  attr_reader :forward_references

  # To be called from the initializer of the includer, this sets up the forward
  # reference capture and resolution mechanisms.
  def start_forward_referencing
    @forward_references = []
    @had_forward_reference_resolution = false
    @forward_reference_resolver = nil
  end

  # To be called when a section of code that could contain a forward reference
  # is entered. The method returns a newly created ForwardReference with the
  # given dependency that can be jumped to during resolution.
  def create_forward_reference dependency=nil
    forward_reference = ForwardReference.new dependency
    @forward_references << forward_reference
    forward_reference
  end

  # To be called when a section of code that could contain a forward reference
  # has successfully been reached. It is used to remove the ForwardReference
  # that was created at the start of the section, and asserts that a
  # resolution was made.
  def remove_forward_reference forward_reference=nil
    @forward_references.delete forward_reference if
      (forward_reference.kind_of? ForwardReference)
    @had_forward_reference_resolution = true
  end

  # To be called to try to resolve any unresolved ForwardReferences by jumping
  # to each in turn and retrying the code that caused it. This method repeats
  # until nothing more is resolved. At that point unresolved forward reference
  # may still exist, to be possibly resolved by another call to this method
  # downstream.
  def resolve_forward_references
    forward_references = @forward_references
    @forward_references = []
    @had_forward_reference_resolution = false
    if forward_references.size > 0
      @forward_reference_resolver ||= callcc {|cont| cont} while
        (@forward_reference_resolver == nil)
      forward_reference = forward_references.shift
      forward_reference.continuation.call if forward_reference != nil
    end
    @forward_reference_resolver = nil
    resolve_forward_references if @had_forward_reference_resolution
  end

  # To be called at the end of a section of code that could contain a forward
  # reference, it will continue during normal processing and jump back to the
  # resolve_forward_references method during resolution.
  def continue_forward_reference_resolution
    @forward_reference_resolver.call if @forward_reference_resolver
  end

  # Returns a hash of dependencies to arrays of the ForwardReferences that
  # have them as dependencies.
  def forward_reference_dependencies
    dependencies = {}
    @forward_references.each { |forward_reference|
      dependency = forward_reference.dependency
      forward_references = dependencies[dependency]
      dependencies[dependency] = [] if forward_references == nil
      dependencies[dependency] << forward_reference
    }
    dependencies
  end
  
  # Returns a string indicating the current state of ForwardReferencing.
  def forward_references_to_s
    "#{self_name}  #{forward_references_remaining} unresolved"
  end

  # Returns the number of unresolved forward references.
  def forward_references_remaining
    @forward_references.size
  end

end


# A ForwardReferencer is simply a class-wrapper for the ForwardReferencing
# module. method have been shortened there is reduced potential for conflict
# from inheritence than from inclusion or extension.
class ForwardReferencer

  understands ForwardReferencing

  alias initialize   start_forward_referencing
  alias create       create_forward_reference
  alias remove       remove_forward_reference
  alias resolve      resolve_forward_references
  alias continue     continue_forward_reference_resolution
  alias dependencies forward_reference_dependencies
  alias to_s         forward_references_to_s
  alias remaining    forward_references_remaining
  
end


# A ForwardReference holds a continuation and a dependency, the where and
# the why of forward referencing.
class ForwardReference

  # Holds the place to jump back to for attempting to resolve a forward
  # reference.
  attr_reader :continuation
  
  # Holds an arbitrary object that indicates why the forward reference
  # occurred.
  attr_accessor :dependency

  # Returns a new instance with a valid continuation and the given dependency.
  def initialize(dependency)
    @continuation = nil
    @continuation = callcc{|cont| cont} while (@continuation == nil)
    @dependency = dependency
  end

  # Returns a string indicating the current state of the ForwardReference.
  def to_s
    "#{self_name}  dependency #{dependency}  #{continuation}"
  end

end