Ruby port of the SIEVE caching algorithm using a ring instead of a linked list

SIEVE.rb

# See https://cachemon.github.io/SIEVE-website/blog/2023/12/17/sieve-is-simpler-than-lru/
# for a description and the Python version
# *LARGELY UNTESTED*

Node = Struct.new(:value, :visited, :prev, :next)
    
class SieveCache
  def initialize(capacity)
    @capacity = capacity
    @cache = {}
    @hand = @head = Node.new
    @head.next = @head
    @head.prev = @head
    # Yes, this is a dirty trick.
    def @head.visited = true
  end

  def add_to_head(node)
    node.prev = @head
    node.next = @head.next
    @head.next.prev = node
    @head.next = node
  end

  def remove_node(node)
    node.prev.next = node.next
    node.next.prev = node.prev
  end

  def evict
    while @hand.visited
      @hand.visited = false
      @hand = @hand.prev
    end
    @cache.delete(@hand.value)
    @hand = remove_node(@hand)
  end

  def access x
    if @cache[x]
      @cache[x].visited = true
      return
    end
    evict if @cache.size == @capacity
    add_to_head(@cache[x] = Node[x])
  end

  def show_cache
    current = @head
    until current.next == @head do
      current = current.next
      print "#{current.value} (Visited: #{current.visited ? "True" : "False"})"
      print " -> " if current.next != @head
    end
    puts
  end
end

# Example usage
cache = SieveCache.new(3)
cache.access('A')
cache.access('B')
cache.access('C')
cache.access('D')
cache.show_cache()
cache.access('C')
cache.access('E')
cache.show_cache()

SIEVE2.rb

# See https://cachemon.github.io/SIEVE-website/blog/2023/12/17/sieve-is-simpler-than-lru/
# for a description and the Python version
# *LARGELY UNTESTED*
#
# Single linked list version

Node = Struct.new(:value, :visited, :next)
    
class SieveCache
  def initialize(capacity)
    @capacity = capacity
    @cache = {}
    @prev = @hand = @head = Node.new
    @head.next = @head
    # Yes, this is a dirty trick.
    def @head.visited = true
  end

  def add_to_head(node)
    node.next = @head.next
    @head.next = node
  end

  def evict
    while @hand.visited
      @hand.visited = false
      @prev = @hand
      @hand = @hand.next
    end
    @cache.delete(@hand.value)
    @hand = @prev.next = @prev.next.next
  end

  def access x
    if @cache[x]
      @cache[x].visited = true
      return
    end
    evict if @cache.size == @capacity
    add_to_head(@cache[x] = Node[x])
  end

  def show_cache
    current = @head
    until current.next == @head do
      current = current.next
      print "#{current.value} (Visited: #{current.visited ? "True" : "False"})"
      print " -> " if current.next != @head
    end
    puts
  end
end

# Example usage
cache = SieveCache.new(3)
cache.access('A')
cache.access('B')
cache.access('C')
cache.access('D')
cache.show_cache()
cache.access('C')
cache.access('E')
cache.show_cache()