cache.rb

module IdentityCache
  BACKEND_KEY = 'idc:backend:key'
  
  def with_memoization(&block)
    Thread.current[IdentityCache::BACKEND_KEY] = MemoizedDecorator.new(IdentityCache.cache_backend)
    yield
  ensure
    Thread.current[IdentityCache::BACKEND_KEY] = nil
  end
  
  def fetch(key, &block)
    
    if (entry = CacheEntry.find(key)).nil?
      entry.value = yield
      entry.save
    end
    entry.value
  end
  
  def delete(key)
    CacheEntry.delete(key)
  end
  
  class CacheEntry
    DELETED = 'D'
    
    def initialize(key, value, cas)
      @key, @value. @cas = key, value, cas
    end

    def nil?
      @value.nil? || @value == DELETED
    end

    def value
      @value == IdentityCache::CACHED_NIL ? nil : @value
    end

    def value=(v)
      @value = v.nil? ? IdentityCache::CACHED_NIL : v
    end

    def save
      if @cas.nil?
        backend.add(@key, @value)
      else
        backend.cas(@key, @value, @cas)
      end
    end

    def self.find(key)
      value, cas = backend.get(key)
      CacheEntry.new(key, value, cas)
    end
    
    def self.delete(key)
      backend.replace(key, DELETED, ttl: 1)
    end
    
    def self.backend
      Thread.current[IdentityCache::BACKEND_KEY] || IdentityCache.cache_backend
    end
  end
  
end