Add Hash#transform_keys!

spec/std/hash_spec.cr

@@ -923,6 +923,29 @@ describe "Hash" do
     h2.should eq({"1a" => "a", "2b" => "b", "3c" => "c"})
   end
 
+  describe "transform_keys!" do
+    it "transforms keys in place" do
+      h = {1 => "a", 2 => "b", 3 => "c"}
+
+      h.transform_keys! { |x| x + 1 }.should be(h)
+      h.should eq({2 => "a", 3 => "b", 4 => "c"})
+    end
+
+    it "does nothing when empty hash" do
+      h = {} of Int32 => String
+
+      h.transform_keys! { |x| x + 1 }
+      h.should be_empty
+    end
+
+    it "transforms keys with values included" do
+      h = {"1" => "a", "2" => "b", "3" => "c"}
+
+      h.transform_keys! { |k, v| "#{k}#{v}" }
+      h.should eq({"1a" => "a", "2b" => "b", "3c" => "c"})
+    end
+  end
+
   it "transforms values" do
     h1 = {"a" => 1, "b" => 2, "c" => 3}
 

src/hash.cr

@@ -1755,6 +1755,24 @@ class Hash(K, V)
     end
   end
 
+  # Destructively transforms all keys using a block. Same as transform_keys but modifies in place.
+  # The block cannot change a type of keys.
+  # The block yields the key and value.
+  #
+  # ```
+  # hash = {"a" => 1, "b" => 2, "c" => 3}
+  # hash.transform_keys! { |key| key.upcase }
+  # hash # => {"A" => 1, "B" => 2, "C" => 3}
+  # hash.transform_keys! { |key, value| key * value }
+  # hash # => {"a" => 1, "bb" => 2, "ccc" => 3}
+  # ```
+  def transform_keys!(&block : K, V -> K) : self
+    transformed_hash = self.transform_keys(&block)
+    initialize_dup_entries(transformed_hash) # we need only to copy the buffer
+    initialize_copy_non_entries_vars(transformed_hash)
+    self
+  end
+
   # Returns a new hash with the results of running block once for every value.
   # The block can change a type of values.
   # The block yields the value and key.