perf: flatten concat_source when constructing it (#78)

Port of https://github.com/webpack/webpack-sources/blob/9f98066311d53a153fdc7c633422a1d086528027/lib/ConcatSource.js#L18-L27

related #72

Co-authored-by: Cong-Cong Pan <[email protected]>

src/concat_source.rs

@@ -54,7 +54,7 @@ use crate::{
 ///   .unwrap()
 /// );
 /// ```
-#[derive(Debug, Default, Clone, Eq)]
+#[derive(Debug, Default, Clone)]
 pub struct ConcatSource {
   children: Vec<BoxSource>,
 }
@@ -66,26 +66,46 @@ impl ConcatSource {
     T: Source + 'static,
     S: IntoIterator<Item = T>,
   {
-    Self {
-      children: sources.into_iter().map(|s| SourceExt::boxed(s)).collect(),
-    }
+    // Flatten the children
+    let children = sources
+      .into_iter()
+      .flat_map(
+        |source| match source.as_any().downcast_ref::<ConcatSource>() {
+          // This clone is cheap because `BoxSource` is `Arc`ed for each child.
+          Some(concat_source) => concat_source.children.clone(),
+          None => {
+            vec![SourceExt::boxed(source)]
+          }
+        },
+      )
+      .collect();
+    Self { children }
+  }
+
+  fn children(&self) -> &Vec<BoxSource> {
+    &self.children
   }
 
   /// Add a [Source] to concat.
-  pub fn add<S: Source + 'static>(&mut self, item: S) {
-    self.children.push(SourceExt::boxed(item));
+  pub fn add<S: Source + 'static>(&mut self, source: S) {
+    if let Some(concat_source) = source.as_any().downcast_ref::<ConcatSource>()
+    {
+      self.children.extend(concat_source.children.clone());
+    } else {
+      self.children.push(SourceExt::boxed(source));
+    }
   }
 }
 
 impl Source for ConcatSource {
   fn source(&self) -> Cow<str> {
-    let all = self.children.iter().map(|child| child.source()).collect();
+    let all = self.children().iter().map(|child| child.source()).collect();
     Cow::Owned(all)
   }
 
   fn buffer(&self) -> Cow<[u8]> {
     let all = self
-      .children
+      .children()
       .iter()
       .map(|child| child.buffer())
       .collect::<Vec<_>>()
@@ -94,15 +114,15 @@ impl Source for ConcatSource {
   }
 
   fn size(&self) -> usize {
-    self.children.iter().map(|child| child.size()).sum()
+    self.children().iter().map(|child| child.size()).sum()
   }
 
   fn map(&self, options: &MapOptions) -> Option<SourceMap> {
     get_map(self, options)
   }
 
   fn to_writer(&self, writer: &mut dyn std::io::Write) -> std::io::Result<()> {
-    for child in &self.children {
+    for child in self.children() {
       child.to_writer(writer)?;
     }
     Ok(())
@@ -112,17 +132,18 @@ impl Source for ConcatSource {
 impl Hash for ConcatSource {
   fn hash<H: Hasher>(&self, state: &mut H) {
     "ConcatSource".hash(state);
-    for child in self.children.iter() {
+    for child in self.children().iter() {
       child.hash(state);
     }
   }
 }
 
 impl PartialEq for ConcatSource {
   fn eq(&self, other: &Self) -> bool {
-    self.children == other.children
+    self.children() == other.children()
   }
 }
+impl Eq for ConcatSource {}
 
 impl StreamChunks for ConcatSource {
   fn stream_chunks(
@@ -132,16 +153,16 @@ impl StreamChunks for ConcatSource {
     on_source: OnSource,
     on_name: OnName,
   ) -> crate::helpers::GeneratedInfo {
-    if self.children.len() == 1 {
-      return self.children[0]
+    if self.children().len() == 1 {
+      return self.children()[0]
         .stream_chunks(options, on_chunk, on_source, on_name);
     }
     let mut current_line_offset = 0;
     let mut current_column_offset = 0;
     let mut source_mapping: HashMap<String, u32> = HashMap::default();
     let mut name_mapping: HashMap<String, u32> = HashMap::default();
     let mut need_to_cloas_mapping = false;
-    for item in &self.children {
+    for item in self.children() {
       let source_index_mapping: RefCell<HashMap<u32, u32>> =
         RefCell::new(HashMap::default());
       let name_index_mapping: RefCell<HashMap<u32, u32>> =