Speed up source map emit for large single-line object literals

tsc/internal/compiler/emit_test.go

@@ -16,7 +16,7 @@ import (
 // generateLongLineTS generates TypeScript source code that produces a single very long line.
 // This simulates generated code (e.g., from code generators) that has no line breaks,
 // which triggers O(n²) behavior in source map generation due to
-// GetECMALineAndUTF16CharacterOfPosition scanning from line start for each position.
+// the emit writer scanning from line start for each generated position.
 func generateLongLineTS(numProperties int) string {
 	// Build a large object literal all on one line, with no line breaks.
 	var b strings.Builder

tsc/internal/printer/textwriter.go

@@ -20,6 +20,8 @@ type textWriter struct {
 	lineStart               bool
 	lineCount               int
 	linePos                 int
+	columnPos               int
+	column                  core.UTF16Offset
 	hasTrailingCommentState bool
 }
 
@@ -41,9 +43,10 @@ func (w *textWriter) GetColumn() core.UTF16Offset {
 	if w.lineStart {
 		return core.UTF16Offset(w.indent * w.indentSize)
 	}
-	// Count UTF-16 code units from the last line start.
-	// For ASCII-only output (the common case), this equals the byte count.
-	return core.UTF16Len(w.builder.String()[w.linePos:])
+	textPos := w.builder.Len()
+	w.column += core.UTF16Len(w.builder.String()[w.columnPos:textPos])
+	w.columnPos = textPos
+	return w.column
 }
 
 func (w *textWriter) GetIndent() int {
@@ -107,6 +110,8 @@ func (w *textWriter) updateLineCountAndPosFor(s string) {
 		w.lineCount += count - 1
 		curLen := w.builder.Len()
 		w.linePos = curLen - len(s) + int(lastLineStart)
+		w.columnPos = w.linePos
+		w.column = 0
 		w.lineStart = (w.linePos - curLen) == 0
 		return
 	}
@@ -163,6 +168,8 @@ func (w *textWriter) writeLineRaw() {
 	w.lastWritten = w.newLine
 	w.lineCount++
 	w.linePos = w.builder.Len()
+	w.columnPos = w.linePos
+	w.column = 0
 	w.lineStart = true
 	w.hasTrailingCommentState = false
 }

tsc/internal/printer/textwriter_test.go

@@ -0,0 +1,38 @@
+package printer
+
+import (
+	"testing"
+
+	"github.com/microsoft/TypeScript/tsc/internal/core"
+	"gotest.tools/v3/assert"
+)
+
+func TestTextWriterCachesUTF16Column(t *testing.T) {
+	t.Parallel()
+
+	w := NewTextWriter("\n", 4).(*textWriter)
+	w.Write("a𝟘")
+	assert.Equal(t, w.GetColumn(), core.UTF16Offset(3))
+	assert.Equal(t, w.columnPos, w.builder.Len())
+
+	w.Write("𝟙b")
+	assert.Equal(t, w.GetColumn(), core.UTF16Offset(6))
+	assert.Equal(t, w.columnPos, w.builder.Len())
+}
+
+func TestTextWriterResetsCachedColumnAfterNewLine(t *testing.T) {
+	t.Parallel()
+
+	w := NewTextWriter("\n", 4).(*textWriter)
+	w.Write("previous line")
+	w.GetColumn()
+	w.RawWrite("\n𝟘")
+	assert.Equal(t, w.GetColumn(), core.UTF16Offset(2))
+	assert.Equal(t, w.columnPos, w.builder.Len())
+
+	w.WriteLine()
+	w.IncreaseIndent()
+	assert.Equal(t, w.GetColumn(), core.UTF16Offset(4))
+	w.Write("x")
+	assert.Equal(t, w.GetColumn(), core.UTF16Offset(5))
+}