From 718b989eb42c5ca8a0718b0cfc9d6a7132fd9e9f Mon Sep 17 00:00:00 2001
From: David Zbarsky <dzbarsky@gmail.com>
Date: Sat, 9 Aug 2025 21:29:07 -0400
Subject: [PATCH] Reduce string copies and allocations when emitting inlined
 sourcemaps

---
 internal/compiler/emitter.go    |  6 +-----
 internal/sourcemap/generator.go | 23 +++++++++++++++++++----
 2 files changed, 20 insertions(+), 9 deletions(-)

diff --git a/internal/compiler/emitter.go b/internal/compiler/emitter.go
index ab0cbf0f65..00ec371408 100644
--- a/internal/compiler/emitter.go
+++ b/internal/compiler/emitter.go
@@ -1,8 +1,6 @@
 package compiler
 
 import (
-	"encoding/base64"
-
 	"github.com/microsoft/typescript-go/internal/ast"
 	"github.com/microsoft/typescript-go/internal/binder"
 	"github.com/microsoft/typescript-go/internal/core"
@@ -335,9 +333,7 @@ func (e *emitter) getSourceMapDirectory(mapOptions *core.CompilerOptions, filePa
 func (e *emitter) getSourceMappingURL(mapOptions *core.CompilerOptions, sourceMapGenerator *sourcemap.Generator, filePath string, sourceMapFilePath string, sourceFile *ast.SourceFile) string {
 	if mapOptions.InlineSourceMap.IsTrue() {
 		// Encode the sourceMap into the sourceMap url
-		sourceMapText := sourceMapGenerator.String()
-		base64SourceMapText := base64.StdEncoding.EncodeToString([]byte(sourceMapText))
-		return "data:application/json;base64," + base64SourceMapText
+		return sourceMapGenerator.Base64DataURL()
 	}
 
 	sourceMapFile := tspath.GetBaseFileName(tspath.NormalizeSlashes(sourceMapFilePath))
diff --git a/internal/sourcemap/generator.go b/internal/sourcemap/generator.go
index ea4edab607..d680f9b86d 100644
--- a/internal/sourcemap/generator.go
+++ b/internal/sourcemap/generator.go
@@ -1,11 +1,13 @@
 package sourcemap
 
 import (
+	"encoding/base64"
+	"encoding/json"
 	"errors"
 	"slices"
 	"strings"
+	"unsafe"
 
-	"github.com/go-json-experiment/json"
 	"github.com/microsoft/typescript-go/internal/tspath"
 )
 
@@ -333,13 +335,26 @@ func (gen *Generator) RawSourceMap() *RawSourceMap {
 	}
 }
 
-// Gets the string representation of the source map
-func (gen *Generator) String() string {
+func (gen *Generator) bytes() []byte {
 	buf, err := json.Marshal(gen.RawSourceMap())
 	if err != nil {
 		panic(err.Error())
 	}
-	return string(buf)
+	return buf
+}
+
+// Gets the string representation of the source map
+func (gen *Generator) String() string {
+	return string(gen.bytes())
+}
+
+func (gen *Generator) Base64DataURL() string {
+	const prefix = "data:application/json;base64,"
+	data := gen.bytes()
+	buf := make([]byte, len(prefix)+base64.StdEncoding.EncodedLen(len(data)))
+	copy(buf, prefix)
+	base64.StdEncoding.Encode(buf[len(prefix):], data)
+	return unsafe.String(unsafe.SliceData(buf), len(buf))
 }
 
 func base64FormatEncode(value int) rune {