Extract casts to their own module

Simplifies ModelAttribute class; means that it doesn't add a 'cast' method to
classes that extend ModelAttribute.

lib/model_attribute.rb

@@ -1,5 +1,5 @@
 require "model_attribute/version"
-require "model_attribute/json"
+require "model_attribute/casts"
 require "model_attribute/errors"
 require "time"
 
@@ -56,7 +56,7 @@ def write_attribute(name, value, type = nil)
       type ||= self.class.instance_variable_get('@attribute_types')[name]
       raise InvalidAttributeNameError.new(name) unless type
 
-      value = cast(value, type)
+      value = Casts.cast(value, type)
       return if value == read_attribute(name)
 
       if changes.has_key? name
@@ -151,52 +151,5 @@ def inspect
       end.join(', ')
       "#<#{self.class} #{attribute_string}>"
     end
-
-    def cast(value, type)
-      return nil if value.nil?
-
-      case type
-      when :integer
-        int = Integer(value)
-        float = Float(value)
-        raise "Can't cast #{value.inspect} to an integer without loss of precision" unless int == float
-        int
-      when :boolean
-        if !!value == value
-          value
-        elsif value == 't'
-          true
-        elsif value == 'f'
-          false
-        else
-          raise "Can't cast #{value.inspect} to boolean"
-        end
-      when :time
-        case value
-        when Time
-          value
-        when Date, DateTime
-          value.to_time
-        when Integer
-          # Assume milliseconds since epoch.
-          Time.at(value / 1000.0)
-        when Numeric
-          # Numeric, but not an integer. Assume seconds since epoch.
-          Time.at(value)
-        else
-          Time.parse(value)
-        end
-      when :string
-        String(value)
-      when :json
-        if Json.valid?(value)
-          value
-        else
-          raise "JSON only supports nil, numeric, string, boolean and arrays and hashes of those."
-        end
-      else
-        raise UnsupportedTypeError.new(type)
-      end
-    end
   end
 end

lib/model_attribute/casts.rb

@@ -0,0 +1,74 @@
+module ModelAttribute
+  module Casts
+    class << self
+      def cast(value, type)
+        return nil if value.nil?
+
+        case type
+        when :integer
+          int = Integer(value)
+          float = Float(value)
+          raise "Can't cast #{value.inspect} to an integer without loss of precision" unless int == float
+          int
+        when :boolean
+          if !!value == value
+            value
+          elsif value == 't'
+            true
+          elsif value == 'f'
+            false
+          else
+            raise "Can't cast #{value.inspect} to boolean"
+          end
+        when :time
+          case value
+          when Time
+            value
+          when Date, DateTime
+            value.to_time
+          when Integer
+            # Assume milliseconds since epoch.
+            Time.at(value / 1000.0)
+          when Numeric
+            # Numeric, but not an integer. Assume seconds since epoch.
+            Time.at(value)
+          else
+            Time.parse(value)
+          end
+        when :string
+          String(value)
+        when :json
+          if valid_json?(value)
+            value
+          else
+            raise "JSON only supports nil, numeric, string, boolean and arrays and hashes of those."
+          end
+        else
+          raise UnsupportedTypeError.new(type)
+        end
+      end
+
+      private
+
+      def valid_json?(value)
+        (value == nil         ||
+         value == true        ||
+         value == false       ||
+         value.is_a?(Numeric) ||
+         value.is_a?(String)  ||
+         (value.is_a?(Array) && valid_json_array?(value)) ||
+         (value.is_a?(Hash)  && valid_json_hash?(value) ))
+      end
+
+      def valid_json_array?(array)
+        array.all? { |value| valid_json?(value) }
+      end
+
+      def valid_json_hash?(hash)
+        hash.all? do |key, value|
+          key.is_a?(String) && valid_json?(value)
+        end
+      end
+    end
+  end
+end