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# frozen_string_literal: true
module Rake
# Polylithic linked list structure used to implement several data
# structures in Rake.
class LinkedList
include Enumerable
attr_reader :head, :tail
# Polymorphically add a new element to the head of a list. The
# type of head node will be the same list type as the tail.
def conj(item)
self.class.cons(item, self)
end
# Is the list empty?
# .make guards against a list being empty making any instantiated LinkedList
# object not empty by default
# You should consider overriding this method if you implement your own .make method
def empty?
false
end
# Lists are structurally equivalent.
def ==(other)
current = self
while !current.empty? && !other.empty?
return false if current.head != other.head
current = current.tail
other = other.tail
end
current.empty? && other.empty?
end
# Convert to string: LL(item, item...)
def to_s
items = map(&:to_s).join(", ")
"LL(#{items})"
end
# Same as +to_s+, but with inspected items.
def inspect
items = map(&:inspect).join(", ")
"LL(#{items})"
end
# For each item in the list.
def each
current = self
while !current.empty?
yield(current.head)
current = current.tail
end
self
end
# Make a list out of the given arguments. This method is
# polymorphic
def self.make(*args)
# return an EmptyLinkedList if there are no arguments
return empty if !args || args.empty?
# build a LinkedList by starting at the tail and iterating
# through each argument
# inject takes an EmptyLinkedList to start
args.reverse.inject(empty) do |list, item|
list = cons(item, list)
list # return the newly created list for each item in the block
end
end
# Cons a new head onto the tail list.
def self.cons(head, tail)
new(head, tail)
end
# The standard empty list class for the given LinkedList class.
def self.empty
self::EMPTY
end
protected
def initialize(head, tail=EMPTY)
@head = head
@tail = tail
end
# Represent an empty list, using the Null Object Pattern.
#
# When inheriting from the LinkedList class, you should implement
# a type specific Empty class as well. Make sure you set the class
# instance variable @parent to the associated list class (this
# allows conj, cons and make to work polymorphically).
class EmptyLinkedList < LinkedList
@parent = LinkedList
def initialize
end
def empty?
true
end
def self.cons(head, tail)
@parent.cons(head, tail)
end
end
EMPTY = EmptyLinkedList.new
end
end