Class: Elephas::Cache

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# File 'lib/elephas/cache.rb', line 14

class Cache
  attr_accessor :backend
  attr_accessor :prefix

  # Initialize the cache.
  #
  # @param backend [Backends::Base] The backend to use. By default uses an Hash backend.
  def initialize(backend = nil)
    @backend = backend || Elephas::Backends::Hash.new
    @prefix = "elephas-#{::Elephas::Version::STRING}-cache"
  end

  # This is the main method of the framework.
  #
  # It tries reading a key from the cache.
  #
  # If it doesn't find it, it uses the provided block (which receives options as argument) to compute its value and then store it into the cache for later usages.
  #
  # ```ruby
  # cache = Elephas::Cache.new(Elephas::Backends::Hash.new)
  #
  # value = cache.use("KEY") do |options|
  #   "VALUE"
  # end
  #
  # value
  # # => "VALUE"
  #
  # value = cache.use("KEY") do |options|
  #   "ANOTHER VALUE"
  # end
  #
  # value
  # # => "VALUE"
  # ```
  #
  # @param key [String] The key to lookup.
  # @param options [Hash] A list of options for managing this key.
  # @param block [Proc] An optional block to run to compute the value for the key if nothing is found.
  # @return [Object|Entry] The found or newly-set value associated to the key.
  # @see .setup_options
  def use(key, options = {}, &block)
    rv = nil

    # Get options
    options = setup_options(options, key)

    # Check if the storage has the value (if we don't have to skip the cache)
    rv = choose_backend(options).read(options[:hash]) if options[:force] == false && options[:ttl] > 0
    rv = compute_value(options, &block) if rv.nil? && block # Try to compute the value from the block

    # Return value
    options[:as_entry] ? rv : rv.value.dup
  end

  # Reads a value from the cache.
  #
  # @param key [String] The key to lookup.
  # @param backend [Backends::Base|NilClass] The backend to use. Defaults to the current backend.
  # @return [Object|NilClass] The read value or `nil`.
  def read(key, backend = nil)
    choose_backend({backend: backend}).read(key)
  end

  # Writes a value to the cache.
  #
  # @param key [String] The key to associate the value with.
  # @param value [Object] The value to write. Setting a value to `nil` **doesn't** mean *deleting* the value.
  # @param options [Hash] A list of options for writing.
  # @see .setup_options
  # @return [Object] The value itself.
  def write(key, value, options = {})
    choose_backend(options).write(key, value, setup_options(options, key))
  end

  # Deletes a value from the cache.
  #
  # @param key [String] The key to delete.
  # @param backend [Backends::Base|NilClass] The backend to use. Defaults to the current backend.
  # @return [Boolean] `true` if the key was in the cache, `false` otherwise.
  def delete(key, backend = nil)
    choose_backend({backend: backend}).delete(key)
  end

  # Checks if a key exists in the cache.
  #
  # @param key [String] The key to lookup.
  # @param backend [Backends::Base|NilClass] The backend to use. Defaults to the current backend.
  # @return [Boolean] `true` if the key is in the cache, `false` otherwise.
  def exists?(key, backend = nil)
    choose_backend({backend: backend}).exists?(key)
  end

  # Setups options for use into the framework.
  # Valid options are:
  #
  #   * **:ttl**: The TTL (time to live, in milliseconds) of the entry. It means how long will the value stay in cache. Setting it to 0 or less means never cache the entry.
  #   * **:force**: Setting it to `true` will always skip the cache.
  #   * **:key**: The key associated to this value. **You should never set this option directly.**
  #   * **:prefix**: The prefix used in cache. This is used to avoid conflicts with other caching frameworks.
  #   * **:complete_key**: The complete key used for computing the hash. By default is concatenation of `:key` and `:prefix` options.
  #   * **:hash**: The hash used to store the key in the cache. Should be unique
  #   * **:as_entry**: In `Elephas::Cache.use`, setting this to `true` will return the entire `Entry` object rather than the value only.
  #
  # @param options [Object] An initial setup.
  # @param key [String] The key to associate to this options.
  # @return [Hash] An options hash.
  def setup_options(options, key)
    options = {ttl: 1.hour * 1000, force: false, as_entry: false}.merge(options.ensure_hash({}))

    # Sanitize options.
    options = sanitize_options(options, key)

    # Wrap the final key to ensure we don't have colliding namespaces.
    options[:complete_key] ||= "#{options[:prefix]}[#{options[:key]}]"

    # Compute the hash key used for referencing this value.
    options[:hash] ||= ::Elephas::Entry.hashify_key(options[:complete_key])

    options
  end

  private
    # Computes a new value and saves it to the cache.
    #
    # @param options [Hash] A list of options for managing the value.
    # @param block [Proc] The block to run to compute the value.
    # @return [Object|Entry] The new value.
    def compute_value(options, &block)
      rv = block.call(options)
      rv = ::Elephas::Entry.ensure(rv, options[:complete_key], options) # Make sure is an entry
      write(rv.hash, rv, options) if !rv.value.nil? && options[:ttl] > 0 # We have a value and we have to store it
      rv
    end

    # Sanitizes options for safe usage.
    #
    # @param options [Object] An initial setup.
    # @param key [String] The key to associate to this options.
    # @return [Hash] An options hash.
    def sanitize_options(options, key)
      options[:key] ||= key
      options[:ttl] == options[:ttl].blank? ? 1.hour * 1000 : [options[:ttl].to_integer, 0].max
      options[:force] = options[:force].to_boolean
      options[:prefix] = options[:prefix].present? ? options[:prefix] : prefix

      options
    end

    # Choose a backend to use.
    #
    # @param options [Backends::Base|Hash] The backend to use. Defaults to the current backend.
    def choose_backend(options)
      backend = options.ensure_hash({}).symbolize_keys[:backend]
      backend.is_a?(Elephas::Backends::Base) ? backend : self.backend
    end
end