class Kramdown::Utils::LRUCache

entry is added and the cache is full, the first entry is removed.
and re-inserting a key-value pair on access moves the key to the last position. When an
The cache relies on the fact that Ruby’s Hash class maintains insertion order. So deleting
A simple least recently used (LRU) cache.

def [](key)

Returns the stored value for +key+ or +nil+ if no value was stored under the key. 
def [](key)
  (val = @cache.delete(key)).nil? ? nil : @cache[key] = val
end

def []=(key, value)

Stores the +value+ under the +key+. 
def []=(key, value)
  @cache.delete(key)
  @cache[key] = value
  @cache.shift if @cache.length > @size
end

def initialize(size)

Creates a new LRUCache that can hold +size+ entries. 
def initialize(size)
  @size = size
  @cache = {}
end