From 4b90da471370a85239456b6341d0ae0e3cf054fa Mon Sep 17 00:00:00 2001
From: David Sherret <dsherret@users.noreply.github.com>
Date: Sun, 29 Dec 2024 18:10:12 -0500
Subject: [PATCH] perf: pre-allocate text change string capacity (#289)

---
 Cargo.lock          | 14 ++++++++++--
 Cargo.toml          |  1 +
 src/text_changes.rs | 52 ++++++++++++++++++++++-----------------------
 3 files changed, 39 insertions(+), 28 deletions(-)

diff --git a/Cargo.lock b/Cargo.lock
index cd9766a..dca2948 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -237,6 +237,7 @@ dependencies = [
  "serde",
  "serde_json",
  "sourcemap",
+ "string_capacity",
  "swc_atoms",
  "swc_bundler",
  "swc_common",
@@ -479,9 +480,9 @@ dependencies = [
 
 [[package]]
 name = "itoa"
-version = "1.0.11"
+version = "1.0.14"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "49f1f14873335454500d59611f1cf4a4b0f786f9ac11f4312a78e4cf2566695b"
+checksum = "d75a2a4b1b190afb6f5425f10f6a8f959d2ea0b9c2b1d79553551850539e4674"
 
 [[package]]
 name = "lazy_static"
@@ -963,6 +964,15 @@ version = "1.1.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "a2eb9349b6444b326872e140eb1cf5e7c522154d69e7a0ffb0fb81c06b37543f"
 
+[[package]]
+name = "string_capacity"
+version = "0.1.4"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "5d66c912ea6a5e9d1b1cc1be9c72325a5502f0b868578a7dd0fa97bb710626c2"
+dependencies = [
+ "itoa",
+]
+
 [[package]]
 name = "string_enum"
 version = "0.4.4"
diff --git a/Cargo.toml b/Cargo.toml
index a0daa2c..13925b0 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -77,6 +77,7 @@ swc_visit = { version = "=0.6.2", optional = true }
 swc_visit_macros = { version = "=0.5.13", optional = true }
 # just for error handling
 sourcemap = { version = "9.0.0", optional = true }
+string_capacity = "0.1.4"
 thiserror = "2"
 
 [dev-dependencies]
diff --git a/src/text_changes.rs b/src/text_changes.rs
index 2b389f4..796bbbd 100644
--- a/src/text_changes.rs
+++ b/src/text_changes.rs
@@ -3,6 +3,8 @@
 use std::cmp::Ordering;
 use std::ops::Range;
 
+use string_capacity::StringBuilder;
+
 #[derive(Clone, Debug)]
 pub struct TextChange {
   /// Range start to end byte index.
@@ -30,34 +32,32 @@ pub fn apply_text_changes(
     ordering => ordering,
   });
 
-  let mut last_index = 0;
-  let mut final_text = String::new();
-
-  for (i, change) in changes.iter().enumerate() {
-    if change.range.start > change.range.end {
-      panic!(
-        "Text change had start index {} greater than end index {}.\n\n{:?}",
-        change.range.start,
-        change.range.end,
-        &changes[0..i + 1],
-      )
-    }
-    if change.range.start < last_index {
-      panic!("Text changes were overlapping. Past index was {}, but new change had index {}.\n\n{:?}", last_index, change.range.start, &changes[0..i + 1]);
-    } else if change.range.start > last_index && last_index < source.len() {
-      final_text.push_str(
-        &source[last_index..std::cmp::min(source.len(), change.range.start)],
-      );
+  StringBuilder::build(|builder| {
+    let mut last_index = 0;
+    for (i, change) in changes.iter().enumerate() {
+      if change.range.start > change.range.end {
+        panic!(
+          "Text change had start index {} greater than end index {}.\n\n{:?}",
+          change.range.start,
+          change.range.end,
+          &changes[0..i + 1],
+        )
+      }
+      if change.range.start < last_index {
+        panic!("Text changes were overlapping. Past index was {}, but new change had index {}.\n\n{:?}", last_index, change.range.start, &changes[0..i + 1]);
+      } else if change.range.start > last_index && last_index < source.len() {
+        builder.append(
+          &source[last_index..std::cmp::min(source.len(), change.range.start)],
+        );
+      }
+      builder.append(&change.new_text);
+      last_index = change.range.end;
     }
-    final_text.push_str(&change.new_text);
-    last_index = change.range.end;
-  }
 
-  if last_index < source.len() {
-    final_text.push_str(&source[last_index..]);
-  }
-
-  final_text
+    if last_index < source.len() {
+      builder.append(&source[last_index..]);
+    }
+  }).unwrap()
 }
 
 #[cfg(test)]