From 729830a934fd1a581319e9e08c7d82bd0062d89c Mon Sep 17 00:00:00 2001
From: JoostK <joost.koehoorn@gmail.com>
Date: Wed, 19 Aug 2020 09:58:56 +0200
Subject: [PATCH] Optimize external source maps without full cache

---
 src/compiler/emitter.ts | 20 +++++++++++++++++++-
 1 file changed, 19 insertions(+), 1 deletion(-)

diff --git a/src/compiler/emitter.ts b/src/compiler/emitter.ts
index f976c38ea6af8..e273a40511967 100644
--- a/src/compiler/emitter.ts
+++ b/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) {