feat: Add Memoizable module

3 files changed, 71 insertions, 0 deletions

diff --git a/CHANGELOG.md b/CHANGELOG.md
index f788ccf..12cd96c 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,5 +1,7 @@
 ## [Unreleased]
 
+- feat: Add Memoizable module
+
 ## [0.1.0] - 2021-12-22
 
 - Initial release
diff --git a/lib/straw.rb b/lib/straw.rb
index 1a7bad2..5ca7d12 100644
--- a/lib/straw.rb
+++ b/lib/straw.rb
@@ -4,4 +4,24 @@ require_relative "straw/version"
 
 module Straw
   class Error < StandardError; end
+
+  module Memoizable
+    def memoize(key)
+      if memoized?(key)
+        instance_variable_get(var_for(key))
+      else
+        instance_variable_set(var_for(key), yield)
+      end
+    end
+
+    def memoized?(key)
+      instance_variable_defined?(var_for(key))
+    end
+
+    private
+
+    def var_for(key)
+      "@#{key}"
+    end
+  end
 end
diff --git a/test/test_memoizable.rb b/test/test_memoizable.rb
new file mode 100644
index 0000000..05b5be6
--- /dev/null
+++ b/test/test_memoizable.rb
@@ -0,0 +1,49 @@
+# frozen_string_literal: true
+
+require "test_helper"
+
+describe ::Straw::Memoizable do
+  def setup
+    item_clazz = Class.new do
+      include ::Straw::Memoizable
+
+      attr_reader :calls
+
+      def initialize
+        @calls = Hash.new { |h, k| h[k] = 0 }
+      end
+
+      def value(item)
+        memoize(:value) do
+          @calls[:value] += 1
+          item
+        end
+      end
+    end
+    @subject = item_clazz.new
+  end
+
+  describe "#memoize" do
+    describe "when the result has not been memoized" do
+      it "computes the result only once" do
+        instance = Object.new
+        2.times { _(@subject.value(instance)).must_equal instance }
+
+        _(@subject.calls[:value]).must_equal 1
+      end
+
+      it "caches a nil value to prevent duplicate calls" do
+        2.times { assert_nil(@subject.value(nil)) }
+
+        _(@subject.calls[:value]).must_equal 1
+      end
+
+      it "caches a false value to prevent duplicate calls" do
+        2.times { _(@subject.value(false)).must_equal false }
+
+        _(@subject.calls[:value]).must_equal 1
+      end
+    end
+  end
+end
+