Add Hash#key

mrbgems/mruby-hash-ext/mrblib/hash.rb

@@ -160,4 +160,24 @@ class Hash
     end
     self
   end
+
+  ##
+  #  call-seq:
+  #     hsh.key(value)    -> key
+  #
+  #  Returns the key of an occurrence of a given value. If the value is
+  #  not found, returns <code>nil</code>.
+  #
+  #     h = { "a" => 100, "b" => 200, "c" => 300, "d" => 300 }
+  #     h.key(200)   #=> "b"
+  #     h.key(300)   #=> "c"
+  #     h.key(999)   #=> nil
+  #
+
+  def key(val)
+    self.each do |k, v|
+      return k if v == val
+    end
+    nil
+  end
 end

mrbgems/mruby-hash-ext/test/hash.rb

@@ -102,3 +102,12 @@ assert("Hash#keep_if") do
   h = { 1 => 2, 3 => 4, 5 => 6 }
   assert_equal({ 1 => 2, 3=> 4, 5 =>6} , h.keep_if { true })
 end
+
+assert("Hash#key") do
+  h = { "a" => 100, "b" => 200, "c" => 300, "d" => 300, nil => 'nil', 'nil' => nil }
+  assert_equal "b", h.key(200)
+  assert_equal "c", h.key(300)
+  assert_nil h.key(999)
+  assert_nil h.key('nil')
+  assert_equal 'nil', h.key(nil)
+end