Add Set#map! (#16271)

Author: andrykonchin

spec/std/set_spec.cr

@@ -426,6 +426,22 @@ describe "Set" do
     end
   end
 
+  describe "#map!" do
+    it "replaces elements with the block's return values" do
+      set = Set{1, 2, 3}
+      set.map! { |n| n * -1 }.should be(set)
+      set.should eq(Set{-1, -2, -3})
+    end
+
+    it "exhibits reference semantic" do
+      set = Set{1, 2, 3}
+      copy = set
+
+      set.map! { |n| n * -1 }
+      set.should eq(copy)
+    end
+  end
+
   describe "#select!" do
     it "keeps only elements that evaluate to true" do
       set = Set{1, 2, 3}

src/set.cr

@@ -494,6 +494,15 @@ struct Set(T)
     @hash.same?(other.@hash)
   end
 
+  # Replaces every element of the set with a value returned from the block, and
+  # returns `self`.
+  def map!(& : T -> T) : self
+    hash = @hash.transform_keys { |k, _| yield(k) }
+    @hash.clear
+    @hash.merge!(hash)
+    self
+  end
+
   # Deletes every element of the set for which the block is falsey, and returns
   # `self`.
   def select!(& : T ->) : self