perf: restore preformatted blocks in one pass

textplain_test.go

@@ -1,6 +1,7 @@
 package textplain_test
 
 import (
+	"strconv"
 	"strings"
 	"testing"
 
@@ -1060,6 +1061,19 @@ func TestNestedLists(t *testing.T) {
 	)
 }
 
+func TestManyPreformattedBlocks(t *testing.T) {
+	var body, expect []string
+	for i := range 50 {
+		n := strconv.Itoa(i)
+		body = append(body, "<pre>block "+n+"\n  indented</pre>")
+		expect = append(expect, "block "+n+"\n  indented")
+	}
+
+	result, err := textplain.Convert(strings.Join(body, ""))
+	require.NoError(t, err)
+	assert.Equal(t, strings.Join(expect, "\n\n"), result)
+}
+
 func TestPreformatted(t *testing.T) {
 	runTestCases(t,
 		testCase{

tree.go

@@ -72,9 +72,7 @@ func Convert(document string, opts ...Option) (string, error) {
 
 	wrapped := WordWrap(strings.TrimSpace(text), cv.opts.lineLength)
 
-	for _, block := range preformatted {
-		wrapped = strings.Replace(wrapped, prePlaceholder, block, 1)
-	}
+	wrapped = restorePre(wrapped, preformatted)
 
 	return applyQuotes(strings.ReplaceAll(wrapped, indentMark, "  ")) + cv.footnotes(), nil
 }
@@ -408,6 +406,38 @@ func extractPre(text string) ([]string, string) {
 	return blocks, out.String()
 }
 
+// restorePre puts each preformatted block back in place of its placeholder, in
+// one pass
+func restorePre(text string, blocks []string) string {
+	if len(blocks) == 0 {
+		return text
+	}
+
+	size := len(text)
+	for _, block := range blocks {
+		size += len(block)
+	}
+
+	var out strings.Builder
+
+	out.Grow(size)
+
+	for _, block := range blocks {
+		before, after, found := strings.Cut(text, prePlaceholder)
+		if !found {
+			break
+		}
+
+		out.WriteString(before)
+		out.WriteString(block)
+		text = after
+	}
+
+	out.WriteString(text)
+
+	return out.String()
+}
+
 // textOf collects the raw text of a subtree, keeping whitespace as written
 func textOf(n *html.Node) string {
 	var (