utf8: use lookup table to validate ASCII (#654)

parser_test.go

@@ -1,6 +1,8 @@
 package toml
 
 import (
+	"strconv"
+	"strings"
 	"testing"
 
 	"github.com/pelletier/go-toml/v2/internal/ast"
@@ -371,6 +373,23 @@ func BenchmarkParseBasicStringWithUnicode(b *testing.B) {
 	})
 }
 
+func BenchmarkParseBasicStringsEasy(b *testing.B) {
+	p := &parser{}
+
+	for _, size := range []int{1, 4, 8, 16, 21} {
+		b.Run(strconv.Itoa(size), func(b *testing.B) {
+			input := []byte(`"` + strings.Repeat("A", size) + `"`)
+
+			b.ReportAllocs()
+			b.SetBytes(int64(len(input)))
+
+			for i := 0; i < b.N; i++ {
+				p.parseBasicString(input)
+			}
+		})
+	}
+}
+
 func TestParser_AST_DateTimes(t *testing.T) {
 	examples := []struct {
 		desc  string

utf8.go

@@ -140,8 +140,45 @@ func utf8ValidNext(p []byte) int {
 	return size
 }
 
+var invalidAsciiTable = [256]bool{
+	0x00: true,
+	0x01: true,
+	0x02: true,
+	0x03: true,
+	0x04: true,
+	0x05: true,
+	0x06: true,
+	0x07: true,
+	0x08: true,
+	// 0x09 TAB
+	// 0x0A LF
+	0x0B: true,
+	0x0C: true,
+	// 0x0D CR
+	0x0E: true,
+	0x0F: true,
+	0x10: true,
+	0x11: true,
+	0x12: true,
+	0x13: true,
+	0x14: true,
+	0x15: true,
+	0x16: true,
+	0x17: true,
+	0x18: true,
+	0x19: true,
+	0x1A: true,
+	0x1B: true,
+	0x1C: true,
+	0x1D: true,
+	0x1E: true,
+	0x1F: true,
+	// 0x20 - 0x7E Printable ASCII characters
+	0x7F: true,
+}
+
 func invalidAscii(b byte) bool {
-	return b <= 0x08 || (b > 0x0A && b < 0x0D) || (b > 0x0D && b <= 0x1F) || b == 0x7F
+	return invalidAsciiTable[b]
 }
 
 // acceptRange gives the range of valid values for the second byte in a UTF-8