require 'forwardable'
require 'compo/composite'

module Compo
  # Implementation of Composite that stores its children in an Array.
  #
  # IDs for items entering a ListComposite must be numeric, and will change if
  # an item with an ID less than the item in question is deleted or inserted.
  # This means the ID function for objects in a ListComposite may report
  # different values at different times.
  #
  # Adding an object at an occupied ID moves the occupant and those at
  # successive IDs up by one.
  class ArrayComposite
    include Composite
    extend Forwardable

    # Initialises an ArrayComposite
    #
    # @api  public
    # @example  Initializes an ArrayComposite.
    #   comp = ArrayComposite.new
    def initialize
      @children = []
    end

    # Returns the ArrayComposite's children, as a Hash
    #
    # @api  public
    # @example  Gets the children of an empty ArrayComposite.
    #   comp.children
    #   #=> {}
    # @example  Gets the children of a populated ArrayComposite.
    #   comp.children
    #   #=> {0: :first, 1: :second}
    #
    # @return [Hash]  The Hash mapping the IDs of children to their values.
    def children
      Hash[(0...@children.size).zip(@children)]
    end

    private

    def_delegator :@children, :delete, :remove!
    def_delegator :@children, :delete_at, :remove_id!

    # Inserts a child into the ArrayComposite with the given ID
    #
    # You probably want to use #add instead.
    #
    # @api  private
    #
    # @param id [Object]  The ID under which the child is to be added.
    # @param child [Object]  The child to add to the ArrayComposite.
    #
    # @return [Object]  The newly added child, or nil if the ID was invalid.
    def add!(id, child)
      valid_id?(id) ? do_insert(id, child) : nil
    end

    # Checks to see if the given ID is valid
    #
    # A valid ID for ArrayComposites is a number between 0 and the current
    # size of the children list.
    #
    # @api  private
    #
    # @param id [Object]  The candidate ID.
    #
    # @return [Boolean]  True if the ID is valid; false if not.
    def valid_id?(id)
      id.is_a?(Numeric) && (0..@children.size).cover?(id)
    end

    # Actually performs the insertion of an item into the array
    #
    # @api  private
    #
    # @param id [Numeric]  The index into the array at which the child is to be
    #   inserted.
    # @param child [Object]  The object to insert into the children array.
    #
    # @return [Object]  The inserted child.
    def do_insert(id, child)
      @children.insert(id, child)
      child
    end

    # Creates an ID function for the given child
    #
    # The returned proc is O(n), as it checks the child array at each call to
    # find the current ID of the child.
    #
    # @api  private
    #
    # @param child [Object]  The child whose ID is to be returned by the proc.
    #
    # @return [Proc]  A proc returning the child's ID.
    def id_function(object)
      proc { @children.index(object) }
    end
  end
end