Improve performance of string.repeat

CHANGELOG.md

@@ -1,5 +1,8 @@
 # Changelog
 
+- The performance of the `string.repeat` function has been improved. It now runs
+  in loglinear time.
+
 ## v0.62.1 - 2025-08-07
 
 - `string.inspect` now shows Erlang atoms as `atom.create("value")`, to match

src/gleam/string.gleam

@@ -406,7 +406,7 @@ fn concat_loop(strings: List(String), accumulator: String) -> String {
 
 /// Creates a new `String` by repeating a `String` a given number of times.
 ///
-/// This function runs in linear time.
+/// This function runs in loglinear time.
 ///
 /// ## Examples
 ///
@@ -416,13 +416,26 @@ fn concat_loop(strings: List(String), accumulator: String) -> String {
 /// ```
 ///
 pub fn repeat(string: String, times times: Int) -> String {
-  repeat_loop(string, times, "")
+  case times <= 0 {
+    True -> ""
+    False -> repeat_loop(string, times, string, "")
+  }
 }
 
-fn repeat_loop(string: String, times: Int, acc: String) -> String {
+fn repeat_loop(
+  string: String,
+  times: Int,
+  doubling_acc: String,
+  acc: String,
+) -> String {
+  let acc = case times % 2 {
+    0 -> acc
+    _ -> acc <> doubling_acc
+  }
+  let times = times / 2
   case times <= 0 {
     True -> acc
-    False -> repeat_loop(string, times - 1, acc <> string)
+    False -> repeat_loop(string, times, doubling_acc <> doubling_acc, acc)
   }
 }
 

test/gleam/string_test.gleam

@@ -106,8 +106,14 @@ pub fn concat_emoji_test() {
 }
 
 pub fn repeat_test() {
+  assert string.repeat("hi", times: 1) == "hi"
+
+  assert string.repeat("hi", times: 2) == "hihi"
+
   assert string.repeat("hi", times: 3) == "hihihi"
 
+  assert string.repeat("a", times: 10_001) |> string.length == 10_001
+
   assert string.repeat("hi", 0) == ""
 
   assert string.repeat("hi", -1) == ""