Add JSONLogic.uses_data to provide a means to find missing variables

lib/json_logic.rb

@@ -16,6 +16,22 @@ def self.apply(logic, data)
     Operation.perform(operator, values, data || {})
   end
 
+  def self.uses_data(logic)
+    collection = []
+
+    operator, values = logic.first
+    values = [values] unless values.is_a?(Array)
+    if operator == 'var'
+      collection.append(values[0])
+    else
+      values.each { |val|
+        collection.concat(uses_data(val))
+      }
+    end
+
+    return collection.uniq
+  end
+
   def self.filter(logic, data)
     data.select { |d| apply(logic, d) }
   end

test/json_logic_test.rb

@@ -98,4 +98,37 @@ def test_in_with_variable
       { "x" => "foo", "y" => "bar" }
     )
   end
+
+  def test_uses_data
+    assert_equal ["x", "y"], JSONLogic.uses_data(
+      {
+        "in" => [
+          {"var" => "x"},
+          {"var" => "y"},
+        ]
+      }
+    )
+  end
+
+  def test_uses_data_missing
+    vars = JSONLogic.uses_data(
+      {
+        "in" => [
+          {"var" => "x"},
+          {"var" => "y"},
+        ]
+      }
+    )
+
+    provided_data_missing_y = {
+      x: 3,
+    }
+
+    provided_data_missing_x = {
+      y: 4,
+    }
+
+    assert_equal ["y"], JSONLogic.apply({"missing": [vars]}, provided_data_missing_y)
+    assert_equal ["x"], JSONLogic.apply({"missing": [vars]}, provided_data_missing_x)
+  end
 end