perf: eliminate allocations in scanner and formatter

big.go

@@ -1406,7 +1406,8 @@ func (x *Big) Scale() int {
 
 // Scan implements fmt.Scanner.
 func (z *Big) Scan(state fmt.ScanState, verb rune) error {
-	return z.scan(byteReader{state})
+	scanner := byteScanner{r: byteReader{state}}
+	return z.scan(&scanner)
 }
 
 // Set sets z to x and returns z.
@@ -1665,7 +1666,8 @@ var Regexp = regexp.MustCompile(`(?i)(([+-]?(\d+\.\d*|\.?\d+)([eE][+-]?\d+)?)|(i
 // SetString will only return (nil, false) if a library error
 // occurs. In general, it safe to ignore the bool result.
 func (z *Big) SetString(s string) (*Big, bool) {
-	if err := z.scan(strings.NewReader(s)); err != nil {
+	scanner := byteScanner{s: s}
+	if err := z.scan(&scanner); err != nil {
 		return nil, false
 	}
 	return z, true
@@ -1766,7 +1768,8 @@ func (z *Big) UnmarshalJSON(data []byte) error {
 
 // UnmarshalText implements encoding.TextUnmarshaler.
 func (z *Big) UnmarshalText(data []byte) error {
-	return z.scan(bytes.NewReader(data))
+	scanner := byteScanner{b: data}
+	return z.scan(&scanner)
 }
 
 // validate ensures x's internal state is correct. There's no need for it to
@@ -1815,4 +1818,4 @@ func (x *Big) validate() {
 	default:
 		panic(fmt.Sprintf("invalid form %s", x.form))
 	}
-}
+}

bytes_scanner.go

@@ -0,0 +1,41 @@
+package decimal
+
+import "io"
+
+type byteScanner struct {
+	r io.ByteScanner
+	s string
+	b []byte
+	i int
+}
+
+func (s *byteScanner) ReadByte() (byte, error) {
+	if s.r != nil {
+		return s.r.ReadByte()
+	}
+	if s.b != nil {
+		if s.i >= len(s.b) {
+			return 0, io.EOF
+		}
+		c := s.b[s.i]
+		s.i++
+		return c, nil
+	}
+	if s.i >= len(s.s) {
+		return 0, io.EOF
+	}
+	c := s.s[s.i]
+	s.i++
+	return c, nil
+}
+
+func (s *byteScanner) UnreadByte() error {
+	if s.r != nil {
+		return s.r.UnreadByte()
+	}
+	if s.i <= 0 {
+		return io.EOF
+	}
+	s.i--
+	return nil
+}

format.go

@@ -154,6 +154,18 @@ func (f *formatter) Write(p []byte) (n int, err error) {
 
 var sciE = [2]byte{GDA: 'E', Go: 'e'}
 
+func (f *formatter) writeZeros(n int) {
+	const zeros = "0000000000000000000000000000000000000000000000000000000000000000" // 64
+	for n > 0 {
+		chunk := n
+		if chunk > len(zeros) {
+			chunk = len(zeros)
+		}
+		f.WriteString(zeros[:chunk])
+		n -= chunk
+	}
+}
+
 func (f *formatter) format(x *Big, format format, e byte) {
 	if x == nil {
 		f.WriteString("<nil>")
@@ -186,7 +198,7 @@ func (f *formatter) format(x *Big, format format, e byte) {
 			f.WriteByte('0')
 		} else {
 			f.WriteString("0.")
-			io.CopyN(f, zeroReader{}, int64(f.width))
+			f.writeZeros(f.width)
 		}
 		return
 	}
@@ -237,7 +249,7 @@ func (f *formatter) format(x *Big, format format, e byte) {
 		// No decimal places, write b and fill with zeros.
 		if format == plain && exp > 0 {
 			f.Write(b)
-			io.CopyN(f, zeroReader{}, int64(exp))
+			f.writeZeros(exp)
 			return
 		}
 	}
@@ -282,7 +294,7 @@ func (f *formatter) formatPlain(b []byte, exp int) {
 	// log10(b) < scale, so before p "0s" and before b: 0.00000123456
 	default:
 		f.WriteString(zeroRadix)
-		io.CopyN(f, zeroReader{}, -int64(radix))
+		f.writeZeros(-radix)
 
 		end := len(b)
 		if f.prec < end {

scan.go

@@ -9,7 +9,7 @@ import (
 	"github.com/ericlagergren/decimal/internal/c"
 )
 
-func (z *Big) scan(r io.ByteScanner) error {
+func (z *Big) scan(r *byteScanner) error {
 	if debug {
 		defer func() { z.validate() }()
 	}
@@ -97,7 +97,7 @@ func (z *Big) scan(r io.ByteScanner) error {
 	return nil
 }
 
-func scanSign(r io.ByteScanner) (bool, error) {
+func scanSign(r *byteScanner) (bool, error) {
 	ch, err := r.ReadByte()
 	if err != nil {
 		return false, err
@@ -112,7 +112,7 @@ func scanSign(r io.ByteScanner) (bool, error) {
 	}
 }
 
-func (z *Big) scanForm(r io.ByteScanner) (form, error) {
+func (z *Big) scanForm(r *byteScanner) (form, error) {
 	ch, err := r.ReadByte()
 	if err != nil {
 		return 0, err
@@ -186,7 +186,7 @@ func (z *Big) scanForm(r io.ByteScanner) (form, error) {
 	return qnan, nil
 }
 
-func (z *Big) scanInfinity(r io.ByteScanner) (form, error) {
+func (z *Big) scanInfinity(r *byteScanner) (form, error) {
 	const (
 		s = "infinity"
 	)
@@ -208,7 +208,7 @@ func (z *Big) scanInfinity(r io.ByteScanner) (form, error) {
 	return inf, nil
 }
 
-func (z *Big) scanMant(r io.ByteScanner) (err error) {
+func (z *Big) scanMant(r *byteScanner) (err error) {
 	buf := make([]byte, 0, 20)
 	seenDot := false
 	dot := 0
@@ -282,7 +282,7 @@ Loop:
 	return nil
 }
 
-func (z *Big) scanExponent(r io.ByteScanner) error {
+func (z *Big) scanExponent(r *byteScanner) error {
 	ch, err := r.ReadByte()
 	if err != nil {
 		if err == io.EOF {
@@ -375,4 +375,4 @@ func (r byteReader) ReadByte() (byte, error) {
 
 func (r byteReader) UnreadByte() error {
 	return r.UnreadRune()
-}
+}