improve JS string.join speed (#468)

5 files changed, 55 insertions, 29 deletions

diff --git a/CHANGELOG.md b/CHANGELOG.md
index 8d74a39..fe10b24 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -2,6 +2,7 @@
 
 ## v0.29.2 - 2023-06-21
 
+- improve `string.join` and `string.concat` performance on JavaScript target.
 - The `result` module gains the `try_recover` function.
 - The `string` module gains the `byte_size` function.
 
diff --git a/gleam.toml b/gleam.toml
index 44a866e..53ece59 100644
--- a/gleam.toml
+++ b/gleam.toml
@@ -9,10 +9,6 @@ links = [
   { title = "Sponsor", href = "https://github.com/sponsors/lpil" },
 ]
 
-[dependencies]
-
-[dev-dependencies]
-
 [javascript.deno]
 allow_read = [
   "./",
diff --git a/src/gleam/string.gleam b/src/gleam/string.gleam
index b20e726..2e6f28d 100644
--- a/src/gleam/string.gleam
+++ b/src/gleam/string.gleam
@@ -562,9 +562,20 @@ pub fn repeat(string: String, times times: Int) -> String {
 /// ```
 ///
 pub fn join(strings: List(String), with separator: String) -> String {
-  strings
-  |> list.intersperse(with: separator)
-  |> concat
+  do_join(strings, separator)
+}
+
+if erlang {
+  fn do_join(strings: List(String), separator: String) -> String {
+    strings
+    |> list.intersperse(with: separator)
+    |> concat
+  }
+}
+
+if javascript {
+  external fn do_join(strings: List(String), string: String) -> String =
+    "../gleam_stdlib.mjs" "join"
 }
 
 /// Pads a `String` on the left until it has at least given number of graphemes.
@@ -717,8 +728,8 @@ if javascript {
     "../gleam_stdlib.mjs" "trim_right"
 }
 
-/// Splits a non-empty `String` into its head and tail. This lets you
-/// pattern match on `String`s exactly as you would with lists.
+/// Splits a non-empty `String` into its first element (head) and rest (tail).
+/// This lets you pattern match on `String`s exactly as you would with lists.
 ///
 /// ## Examples
 ///
diff --git a/src/gleam/string_builder.gleam b/src/gleam/string_builder.gleam
index 6137910..bb88455 100644
--- a/src/gleam/string_builder.gleam
+++ b/src/gleam/string_builder.gleam
@@ -89,7 +89,7 @@ if erlang {
 
 if javascript {
   external fn do_from_strings(List(String)) -> StringBuilder =
-    "../gleam_stdlib.mjs" "join"
+    "../gleam_stdlib.mjs" "concat"
 }
 
 /// Joins a list of builders into a single builder.
@@ -107,7 +107,7 @@ if erlang {
 
 if javascript {
   external fn do_concat(List(StringBuilder)) -> StringBuilder =
-    "../gleam_stdlib.mjs" "join"
+    "../gleam_stdlib.mjs" "concat"
 }
 
 /// Converts a string into a builder.
diff --git a/src/gleam_stdlib.mjs b/src/gleam_stdlib.mjs
index 4fd06d9..6af9aff 100644
--- a/src/gleam_stdlib.mjs
+++ b/src/gleam_stdlib.mjs
@@ -196,8 +196,26 @@ export function split(xs, pattern) {
   return List.fromArray(xs.split(pattern));
 }
 
-export function join(xs) {
-  return xs.toArray().join("");
+export function join(xs, separator) {
+  let result = "";
+  let i = 0;
+  for (const x of xs) {
+    if (i == 0) {
+      result = x
+    } else {
+      result = result + separator + x;
+    }
+    i++;
+  }
+  return result;
+}
+
+export function concat(xs) {
+  let result = "";
+  for (const x of xs) {
+    result = result + x;
+  }
+  return result;
 }
 
 export function length(data) {
@@ -497,14 +515,14 @@ function uint6ToB64(nUint6) {
   return nUint6 < 26
     ? nUint6 + 65
     : nUint6 < 52
-    ? nUint6 + 71
-    : nUint6 < 62
-    ? nUint6 - 4
-    : nUint6 === 62
-    ? 43
-    : nUint6 === 63
-    ? 47
-    : 65;
+      ? nUint6 + 71
+      : nUint6 < 62
+        ? nUint6 - 4
+        : nUint6 === 62
+          ? 43
+          : nUint6 === 63
+            ? 47
+            : 65;
 }
 
 // From https://developer.mozilla.org/en-US/docs/Glossary/Base64#Solution_2_%E2%80%93_rewrite_the_DOMs_atob()_and_btoa()_using_JavaScript's_TypedArrays_and_UTF-8
@@ -512,14 +530,14 @@ function b64ToUint6(nChr) {
   return nChr > 64 && nChr < 91
     ? nChr - 65
     : nChr > 96 && nChr < 123
-    ? nChr - 71
-    : nChr > 47 && nChr < 58
-    ? nChr + 4
-    : nChr === 43
-    ? 62
-    : nChr === 47
-    ? 63
-    : 0;
+      ? nChr - 71
+      : nChr > 47 && nChr < 58
+        ? nChr + 4
+        : nChr === 43
+          ? 62
+          : nChr === 47
+            ? 63
+            : 0;
 }
 
 // From https://developer.mozilla.org/en-US/docs/Glossary/Base64#Solution_2_%E2%80%93_rewrite_the_DOMs_atob()_and_btoa()_using_JavaScript's_TypedArrays_and_UTF-8