module Grape
  module Validations
    module Types
      # Instances of this class may be passed to
      # +Virtus::Attribute.build+ as the +:coercer+
      # option for custom types that do not otherwise
      # satisfy the requirements for +Virtus::Attribute::coerce+
      # and +Virtus::Attribute::value_coerced?+ to work
      # as expected.
      #
      # Subclasses of +Virtus::Attribute+ or +Axiom::Types::Type+
      # (or for which an axiom type can be inferred, i.e. the
      # primitives, +Date+, +Time+, etc.) do not need any such
      # coercer to be passed with them.
      #
      # Coercion
      # --------
      #
      # This class will detect type classes that implement
      # a class-level +parse+ method. The method should accept one
      # +String+ argument and should return the value coerced to
      # the appropriate type. The method may raise an exception if
      # there are any problems parsing the string.
      #
      # Alternately an optional +method+ may be supplied (see the
      # +coerce_with+ option of {Grape::Dsl::Parameters#requires}).
      # This may be any class or object implementing +parse+ or +call+,
      # with the same contract as described above.
      #
      # Type Checking
      # -------------
      #
      # Calls to +value_coerced?+ will consult this class to check
      # that the coerced value produced above is in fact of the
      # expected type. By default this class performs a basic check
      # against the type supplied, but this behaviour will be
      # overridden if the class implements a class-level
      # +coerced?+ or +parsed?+ method. This method
      # will receive a single parameter that is the coerced value
      # and should return +true+ iff the value meets type expectations.
      # Arbitrary assertions may be made here but the grape validation
      # system should be preferred.
      #
      # Alternately a proc or other object responding to +call+ may be
      # supplied in place of a type. This should implement the same
      # contract as +coerced?+, and must be supplied with a coercion
      # +method+.
      class CustomTypeCoercer
        # Uses +Virtus::Attribute.build+ to build a new
        # attribute that makes use of this class for
        # coercion and type validation logic.
        #
        # @return [Virtus::Attribute]
        def self.build(type, method = nil)
          Virtus::Attribute.build(type, coercer: new(type, method))
        end

        # A new coercer for the given type specification
        # and coercion method.
        #
        # @param type [Class,#coerced?,#parsed?,#call?]
        #   specifier for the target type. See class docs.
        # @param method [#parse,#call]
        #   optional coercion method. See class docs.
        def initialize(type, method = nil)
          coercion_method = infer_coercion_method type, method

          @method = enforce_symbolized_keys type, coercion_method

          @type_check = infer_type_check(type)
        end

        # This method is called from somewhere within
        # +Virtus::Attribute::coerce+ in order to coerce
        # the given value.
        #
        # @param value [String] value to be coerced, in grape
        #   this should always be a string.
        # @return [Object] the coerced result
        def call(value)
          @method.call value
        end

        # This method is called from somewhere within
        # +Virtus::Attribute::value_coerced?+ in order to
        # assert that the value has been coerced successfully.
        #
        # @param _primitive [Axiom::Types::Type] primitive type
        #   for the coercion as detected by axiom-types' inference
        #   system. For custom types this is typically not much use
        #   (i.e. it is +Axiom::Types::Object+) unless special
        #   inference rules have been declared for the type.
        # @param value [Object] a coerced result returned from {#call}
        # @return [true,false] whether or not the coerced value
        #   satisfies type requirements.
        def success?(_primitive, value)
          @type_check.call value
        end

        private

        # Determine the coercion method we're expected to use
        # based on the parameters given.
        #
        # @param type see #new
        # @param method see #new
        # @return [#call] coercion method
        def infer_coercion_method(type, method)
          if method
            if method.respond_to? :parse
              method.method :parse
            else
              method
            end
          else
            # Try to use parse() declared on the target type.
            # This may raise an exception, but we are out of ideas anyway.
            type.method :parse
          end
        end

        # Determine how the type validity of a coerced
        # value should be decided.
        #
        # @param type see #new
        # @return [#call] a procedure which accepts a single parameter
        #   and returns +true+ if the passed object is of the correct type.
        def infer_type_check(type)
          # First check for special class methods
          if type.respond_to? :coerced?
            type.method :coerced?
          elsif type.respond_to? :parsed?
            type.method :parsed?
          elsif type.respond_to? :call
            # Arbitrary proc passed for type validation.
            # Note that this will fail unless a method is also
            # passed, or if the type also implements a parse() method.
            type
          elsif type.is_a?(Enumerable)
            ->(value) { value.all? { |item| item.is_a? type[0] } }
          else
            # By default, do a simple type check
            ->(value) { value.is_a? type }
          end
        end

        # Enforce symbolized keys for complex types
        # by wrapping the coercion method such that
        # any Hash objects in the immediate heirarchy
        # are passed through +Hashie.symbolize_keys!+.
        # This helps common libs such as JSON to work easily.
        #
        # @param type see #new
        # @param method see #infer_coercion_method
        # @return [#call] +method+ wrapped in an additional
        #   key-conversion step, or just returns +method+
        #   itself if no conversion is deemed to be
        #   necessary.
        def enforce_symbolized_keys(type, method)
          # Collections have all values processed individually
          if type == Array || type == Set
            lambda do |val|
              method.call(val).tap do |new_value|
                new_value.each do |item|
                  Hashie.symbolize_keys!(item) if item.is_a? Hash
                end
              end
            end

          # Hash objects are processed directly
          elsif type == Hash
            lambda do |val|
              Hashie.symbolize_keys! method.call(val)
            end

          # Simple types are not processed.
          # This includes Array<primitive> types.
          else
            method
          end
        end
      end
    end
  end
end