simple_linked_list.rb

Element = Struct.new :datum, :next do
  def tail?
    !self.next
  end
end

class SimpleLinkedList
  attr_reader :size, :head

  def self.from_a values
    new.tap do |ll|
      Array(values).reverse.each { |n| ll.push n }
    end
  end

  def initialize
    @size = 0
  end

  def empty?
    !@head
  end

  def push val
    @size += 1
    @head = Element.new val, @head
  end

  def peek
    @head && @head.datum
  end

  def pop
    return unless @head
    @head.datum.tap do
      @head = @head.next
      @size -= 1
    end
  end

  def to_a
    reduce([]) { |acc, n| acc.push n }
  end

  def reverse
    reduce(SimpleLinkedList.new) { |acc, n| acc.push n }
  end

  # N.B. not using this for gainz, but ...
  # def size
  #   reduce(0) { |acc, _| acc += 1 }
  # end

  def reduce acc
    node = @head
    while node
      yield(acc, node.datum)
      node = node.next
    end
    acc
  end
end