Just use a single 1024-byte buffer.

DanielRosenwasser · web-flow · commit f850a85e0f8a · 2021-05-11T01:32:16.000Z
diff --git a/src/compiler/emitter.ts b/src/compiler/emitter.ts
@@ -532,14 +532,12 @@ namespace ts {
             const bundle = sourceFileOrBundle.kind === SyntaxKind.Bundle ? sourceFileOrBundle : undefined;
             const sourceFile = sourceFileOrBundle.kind === SyntaxKind.SourceFile ? sourceFileOrBundle : undefined;
             const sourceFiles = bundle ? bundle.sourceFiles : [sourceFile!];
-            const guessedLength = sourceFile?.text.length ?? reduceLeft(sourceFiles, (acc, file) => acc + file.text.length, 0);
 
             let sourceMapGenerator: SourceMapGenerator | undefined;
             if (shouldEmitSourceMaps(mapOptions, sourceFileOrBundle)) {
                 sourceMapGenerator = createSourceMapGenerator(
                     host,
                     getBaseFileName(normalizeSlashes(jsFilePath)),
-                    guessedLength,
                     getSourceRoot(mapOptions),
                     getSourceMapDirectory(mapOptions, jsFilePath, sourceFile),
                     mapOptions);
diff --git a/src/compiler/sourcemap.ts b/src/compiler/sourcemap.ts
@@ -4,9 +4,7 @@ namespace ts {
         extendedDiagnostics?: boolean;
     }
 
-    let mappingsBuffer: Uint8Array;
-
-    export function createSourceMapGenerator(host: EmitHost, file: string, guessedInputLength: number, sourceRoot: string, sourcesDirectoryPath: string, generatorOptions: SourceMapGeneratorOptions): SourceMapGenerator {
+    export function createSourceMapGenerator(host: EmitHost, file: string, sourceRoot: string, sourcesDirectoryPath: string, generatorOptions: SourceMapGeneratorOptions): SourceMapGenerator {
         const { enter, exit } = generatorOptions.extendedDiagnostics
             ? performance.createTimer("Source Map", "beforeSourcemap", "afterSourcemap")
             : performance.nullTimer;
@@ -19,18 +17,15 @@ namespace ts {
 
         const names: string[] = [];
         let nameToNameIndexMap: ESMap<string, number> | undefined;
-        mappingsBuffer ||= new Uint8Array(guessedInputLength + 1 >> 1);
-        let lastMappings: string | undefined;
-        let mappingsPos = 0;
+        let mappings = "";
+        let mappingsBuffer = new Uint8Array(1024);
+        let mappingsBufferPos = 0;
         function setMapping(charCode: number) {
-            // resize to 1.5 * length
-            if (mappingsPos >= mappingsBuffer.length) {
-                const oldLength = mappingsBuffer.length + 1;
-                const replacementBuffer = new Uint8Array(oldLength + ((oldLength + 1) >> 1));
-                replacementBuffer.set(mappingsBuffer);
-                mappingsBuffer = replacementBuffer;
+            if (mappingsBufferPos >= mappingsBuffer.length) {
+                mappings += String.fromCharCode.apply(undefined, mappingsBuffer);
+                mappingsBufferPos = 0;
             }
-            mappingsBuffer[mappingsPos++] = charCode;
+            mappingsBuffer[mappingsBufferPos++] = charCode;
         }
 
         function base64VLQFormatEncode(inValue: number) {
@@ -302,17 +297,16 @@ namespace ts {
             exit();
         }
 
-        function serializeMappings(len: number): string {
-            let mappings = "";
-            for (let i = 0; i < len; i += 1024) {
-                mappings += String.fromCharCode.apply(undefined, mappingsBuffer.subarray(i, Math.min(len, i + 1024)));
+        function flushMappings(): void {
+            if (mappingsBufferPos > 0) {
+                mappings += String.fromCharCode.apply(undefined, mappingsBuffer.subarray(0, mappingsBufferPos));
+                mappingsBufferPos = 0;
             }
-            return mappings;
         }
 
         function toJSON(): RawSourceMap {
             commitPendingMapping();
-            const mappings = (lastMappings ??= serializeMappings(mappingsPos));
+            flushMappings();
             return {
                 version: 3,
                 file,
