Optimise string.pad_left, string.pad_right. Optimise string.slice on JS.

src/gleam/string.gleam

@@ -1,7 +1,7 @@
 //// Strings in Gleam are UTF-8 binaries. They can be written in your code as
 //// text surrounded by `"double quotes"`.
 
-import gleam/iterator.{type Iterator}
+import gleam/iterator
 import gleam/list
 import gleam/option.{type Option, None, Some}
 import gleam/order
@@ -222,13 +222,8 @@ pub fn slice(from string: String, at_index idx: Int, length len: Int) -> String
 }
 
 @external(erlang, "gleam_stdlib", "slice")
-fn do_slice(string: String, idx: Int, len: Int) -> String {
- string
- |> to_graphemes
- |> list.drop(idx)
- |> list.take(len)
- |> concat
-}
+@external(javascript, "../gleam_stdlib.mjs", "string_slice")
+fn do_slice(string: String, idx: Int, len: Int) -> String
 
 /// Drops contents of the first `String` that occur before the second `String`.
 /// If the `from` string does not contain the `before` string, `from` is returned unchanged.
@@ -489,13 +484,18 @@ fn do_join(strings: List(String), separator: String) -> String {
 /// // -> "121"
 /// ```
 ///
-pub fn pad_left(string: String, to desired_length: Int, with pad_string: String) {
+pub fn pad_left(
+ string: String,
+ to desired_length: Int,
+ with pad_string: String,
+) -> String {
  let current_length = length(string)
  let to_pad_length = desired_length - current_length
- padding(to_pad_length, pad_string)
- |> iterator.append(iterator.single(string))
- |> iterator.to_list
- |> concat
+
+ case to_pad_length <= 0 {
+ True -> string
+ False -> padding(to_pad_length, pad_string) <> string
+ }
 }
 
 /// Pads a `String` on the right until it has a given length.
@@ -521,22 +521,22 @@ pub fn pad_right(
  string: String,
  to desired_length: Int,
  with pad_string: String,
-) {
+) -> String {
  let current_length = length(string)
  let to_pad_length = desired_length - current_length
- iterator.single(string)
- |> iterator.append(padding(to_pad_length, pad_string))
- |> iterator.to_list
- |> concat
+
+ case to_pad_length <= 0 {
+ True -> string
+ False -> string <> padding(to_pad_length, pad_string)
+ }
 }
 
-fn padding(size: Int, pad_string: String) -> Iterator(String) {
- let pad_length = length(pad_string)
- let num_pads = size / pad_length
- let extra = size % pad_length
- iterator.repeat(pad_string)
- |> iterator.take(num_pads)
- |> iterator.append(iterator.single(slice(pad_string, 0, extra)))
+fn padding(size: Int, pad_string: String) -> String {
+ let pad_string_length = length(pad_string)
+ let num_pads = size / pad_string_length
+ let extra = size % pad_string_length
+
+ repeat(pad_string, num_pads) <> slice(pad_string, 0, extra)
 }
 
 /// Removes whitespace on both sides of a `String`.

src/gleam_stdlib.mjs

@@ -223,6 +223,34 @@ export function length(data) {
  return data.length;
 }
 
+export function string_slice(string, idx, len) {
+ if (len <= 0 || idx >= string.length) {
+ return "";
+ }
+
+ const iterator = graphemes_iterator(string);
+ if (iterator) {
+ while (idx-- > 0) {
+ iterator.next();
+ }
+
+ let result = "";
+
+ while (len-- > 0) {
+ const v = iterator.next().value;
+ if (v === undefined) {
+ break;
+ }
+
+ result += v.segment;
+ }
+
+ return result;
+ } else {
+ return string.match(/./gsu).slice(idx, idx + len).join("");
+ }
+}
+
 export function crop_string(string, substring) {
  return string.substring(string.indexOf(substring));
 }