Optimize Hash#transform_{keys,values}

src/hash.cr

@@ -1690,8 +1690,18 @@ class Hash(K, V)
  # hash.transform_keys { |key, value| key.to_s * value } # => {"a" => 1, "bb" => 2, "ccc" => 3}
  # ```
  def transform_keys(& : K, V -> K2) : Hash(K2, V) forall K2
- each_with_object({} of K2 => V) do |(key, value), memo|
- memo[yield(key, value)] = value
+ if size <= 4
+ # Building up an empty hash is faster than a pre-allocated one as long as
+ # the size is below 5.
+ each_with_object({} of K2 => V) do |(key, value), memo|
+ memo[yield(key, value)] = value
+ end
+ else
+ hash = Hash(K2, V).new(initial_capacity: size)
+ each do |key, value|
+ hash[yield(key, value)] = value
+ end
+ hash
  end
  end
 
@@ -1705,8 +1715,18 @@ class Hash(K, V)
  # hash.transform_values { |value, key| "#{key}#{value}" } # => {:a => "a1", :b => "b2", :c => "c3"}
  # ```
  def transform_values(& : V, K -> V2) : Hash(K, V2) forall V2
- each_with_object({} of K => V2) do |(key, value), memo|
- memo[key] = yield(value, key)
+ if size <= 4
+ # Building up an empty hash is faster than a pre-allocated one as long as
+ # the size is below 5.
+ each_with_object({} of K => V2) do |(key, value), memo|
+ memo[key] = yield(value, key)
+ end
+ else
+ hash = Hash(K, V2).new(initial_capacity: size)
+ each do |key, value|
+ hash[key] = yield(value, key)
+ end
+ hash
  end
  end