Optimize external source maps without full cache

Author: JoostK

src/compiler/emitter.ts

@@ -862,6 +862,8 @@ namespace ts {
         let sourceMapGenerator: SourceMapGenerator | undefined;
         let sourceMapSource: SourceMapSource;
         let sourceMapSourceIndex = -1;
+        let mostRecentlyAddedSourceMapSource: SourceMapSource;
+        let mostRecentlyAddedSourceMapSourceIndex = -1;
 
         // Comments
         let containerPos = -1;
@@ -5337,9 +5339,10 @@ namespace ts {
         function emitSourcePos(source: SourceMapSource, pos: number) {
             if (source !== sourceMapSource) {
                 const savedSourceMapSource = sourceMapSource;
+                const savedSourceMapSourceIndex = sourceMapSourceIndex;
                 setSourceMapSource(source);
                 emitPos(pos);
-                setSourceMapSource(savedSourceMapSource);
+                resetSourceMapSource(savedSourceMapSource, savedSourceMapSourceIndex);
             }
             else {
                 emitPos(pos);
@@ -5386,6 +5389,13 @@ namespace ts {
 
             sourceMapSource = source;
 
+            if (source === mostRecentlyAddedSourceMapSource) {
+                // Fast path for when the new source map is the most recently added, in which case
+                // we use its captured index without going through the source map generator.
+                sourceMapSourceIndex = mostRecentlyAddedSourceMapSourceIndex;
+                return;
+            }
+
             if (isJsonSourceMapSource(source)) {
                 return;
             }
@@ -5394,6 +5404,14 @@ namespace ts {
             if (printerOptions.inlineSources) {
                 sourceMapGenerator!.setSourceContent(sourceMapSourceIndex, source.text);
             }
+
+            mostRecentlyAddedSourceMapSource = source;
+            mostRecentlyAddedSourceMapSourceIndex = sourceMapSourceIndex;
+        }
+
+        function resetSourceMapSource(source: SourceMapSource, sourceIndex: number) {
+            sourceMapSource = source;
+            sourceMapSourceIndex = sourceIndex;
         }
 
         function isJsonSourceMapSource(sourceFile: SourceMapSource) {

src/compiler/sourcemap.ts

@@ -13,7 +13,6 @@ namespace ts {
         const rawSources: string[] = [];
         const sources: string[] = [];
         const sourceToSourceIndexMap = new Map<string, number>();
-        const fileNameToSourceIndexMap = new Map<string, number>();
         let sourcesContent: (string | null)[] | undefined;
 
         const names: string[] = [];
@@ -52,10 +51,6 @@ namespace ts {
 
         function addSource(fileName: string) {
             enter();
-            const sourceIndexByFileName = fileNameToSourceIndexMap.get(fileName);
-            if (sourceIndexByFileName !== undefined) {
-                return sourceIndexByFileName;
-            }
             const source = getRelativePathToDirectoryOrUrl(sourcesDirectoryPath,
                 fileName,
                 host.getCurrentDirectory(),
@@ -69,7 +64,6 @@ namespace ts {
                 rawSources.push(fileName);
                 sourceToSourceIndexMap.set(source, sourceIndex);
             }
-            fileNameToSourceIndexMap.set(fileName, sourceIndex);
             exit();
             return sourceIndex;
         }