Use a table lookup for escaping unprintable bytes (#846)

msg.go

@@ -416,10 +416,7 @@ Loop:
 					s = append(s, '\\', b)
 				default:
 					if b < ' ' || b > '~' { // unprintable, use \DDD
-						var buf [3]byte
-						bufs := strconv.AppendInt(buf[:0], int64(b), 10)
-						s = append(s, '\\', '0', '0', '0')
-						copy(s[len(s)-len(bufs):], bufs)
+						s = append(s, escapeByte(b)...)
 					} else {
 						s = append(s, b)
 					}

msg_helpers.go

@@ -275,7 +275,7 @@ func unpackString(msg []byte, off int) (string, int, error) {
 			s.WriteByte('\\')
 			s.WriteByte(b)
 		case b < ' ' || b > '~': // unprintable
-			writeEscapedByte(&s, b)
+			s.WriteString(escapeByte(b))
 		default:
 			s.WriteByte(b)
 		}

types.go

@@ -460,7 +460,7 @@ func sprintTxtOctet(s string) string {
 		case b == '.':
 			dst.WriteByte('.')
 		case b < ' ' || b > '~':
-			writeEscapedByte(&dst, b)
+			dst.WriteString(escapeByte(b))
 		default:
 			dst.WriteByte(b)
 		}
@@ -508,20 +508,44 @@ func writeTXTStringByte(s *strings.Builder, b byte) {
 		s.WriteByte('\\')
 		s.WriteByte(b)
 	case b < ' ' || b > '~':
-		writeEscapedByte(s, b)
+		s.WriteString(escapeByte(b))
 	default:
 		s.WriteByte(b)
 	}
 }
 
-func writeEscapedByte(s *strings.Builder, b byte) {
-	var buf [3]byte
-	bufs := strconv.AppendInt(buf[:0], int64(b), 10)
-	s.WriteByte('\\')
-	for i := len(bufs); i < 3; i++ {
-		s.WriteByte('0')
+const (
+	escapedByteSmall = "" +
+		`\000\001\002\003\004\005\006\007\008\009` +
+		`\010\011\012\013\014\015\016\017\018\019` +
+		`\020\021\022\023\024\025\026\027\028\029` +
+		`\030\031`
+	escapedByteLarge = `\127\128\129` +
+		`\130\131\132\133\134\135\136\137\138\139` +
+		`\140\141\142\143\144\145\146\147\148\149` +
+		`\150\151\152\153\154\155\156\157\158\159` +
+		`\160\161\162\163\164\165\166\167\168\169` +
+		`\170\171\172\173\174\175\176\177\178\179` +
+		`\180\181\182\183\184\185\186\187\188\189` +
+		`\190\191\192\193\194\195\196\197\198\199` +
+		`\200\201\202\203\204\205\206\207\208\209` +
+		`\210\211\212\213\214\215\216\217\218\219` +
+		`\220\221\222\223\224\225\226\227\228\229` +
+		`\230\231\232\233\234\235\236\237\238\239` +
+		`\240\241\242\243\244\245\246\247\248\249` +
+		`\250\251\252\253\254\255`
+)
+
+// escapeByte returns the \DDD escaping of b which must
+// satisfy b < ' ' || b > '~'.
+func escapeByte(b byte) string {
+	if b < ' ' {
+		return escapedByteSmall[b*4 : b*4+4]
 	}
-	s.Write(bufs)
+
+	b -= '~' + 1
+	// The cast here is needed as b*4 may overflow byte.
+	return escapedByteLarge[int(b)*4 : int(b)*4+4]
 }
 
 func nextByte(s string, offset int) (byte, int) {