squeeze the last ms of performance out of the rewriter

rewriter/src/lib.rs

@@ -1,7 +1,8 @@
 pub mod rewrite;
 
-use std::str::FromStr;
+use std::str::{from_utf8, FromStr};
 
+use js_sys::Uint8Array;
 use rewrite::rewrite;
 use url::Url;
 use wasm_bindgen::prelude::*;
@@ -13,12 +14,12 @@ extern "C" {
 }
 
 #[wasm_bindgen]
-pub fn rewrite_js(js: &str, url: &str) -> String {
+pub fn rewrite_js(js: &str, url: &str) -> Vec<u8> {
     rewrite(js, Url::from_str(url).unwrap())
 }
 
 #[wasm_bindgen]
-pub fn rewrite_js_from_arraybuffer(js: &[u8], url: &str) -> String {
+pub fn rewrite_js_from_arraybuffer(js: &[u8], url: &str) -> Vec<u8> {
     // technically slower than the c++ string conversion but it will create *less copies*
 
     let js = unsafe { std::str::from_utf8_unchecked(js) };

rewriter/src/main.rs

@@ -1,5 +1,9 @@
 #![allow(clippy::print_stdout)]
-use std::{env, path::Path, str::FromStr};
+use std::{
+    env,
+    path::Path,
+    str::{from_utf8, FromStr},
+};
 
 use oxc_allocator::Allocator;
 use oxc_ast::{
@@ -28,10 +32,15 @@ fn main() -> std::io::Result<()> {
 
     println!(
         "{}",
-        rewrite(
-            &source_text,
-            Url::from_str("https://google.com/glorngle/si.js").unwrap()
+        from_utf8(
+            rewrite(
+                &source_text,
+                Url::from_str("https://google.com/glorngle/si.js").unwrap()
+            )
+            .as_slice()
         )
+        .unwrap()
+        .to_string()
     );
 
     Ok(())

rewriter/src/rewrite.rs

@@ -184,7 +184,7 @@ impl Rewriter {
     }
 }
 
-pub fn rewrite(js: &str, url: Url) -> String {
+pub fn rewrite(js: &str, url: Url) -> Vec<u8> {
     let allocator = Allocator::default();
     let source_type = SourceType::default();
     let ret = Parser::new(&allocator, &js, source_type).parse();
@@ -241,10 +241,8 @@ pub fn rewrite(js: &str, url: Url) -> String {
         }
     }
 
-    let mut buffer = String::new();
-    // pre-allocate the space we need. should make copies faster
     let size_estimate = (original_len as i32 + difference) as usize;
-    buffer.reserve(size_estimate);
+    let mut buffer: Vec<u8> = Vec::with_capacity(size_estimate);
 
     let mut offset = 0;
     for change in ast_pass.jschanges {
@@ -254,8 +252,9 @@ pub fn rewrite(js: &str, url: Url) -> String {
                 let start = span.start as usize;
                 let end = span.end as usize;
 
-                buffer.push_str(&js[offset..start]);
-                buffer.push_str(&text);
+                buffer.extend_from_slice(unsafe { js.slice_unchecked(offset, start) }.as_bytes());
+
+                buffer.extend_from_slice(text.as_bytes());
                 offset = end;
 
                 // offset = (offset as i64 + (text.len() as i64 - len as i64)) as usize;
@@ -281,7 +280,7 @@ pub fn rewrite(js: &str, url: Url) -> String {
             _ => {}
         }
     }
-    buffer.push_str(&js[offset..]);
+    buffer.extend_from_slice(js[offset..].as_bytes());
 
     return buffer;
 }