feat: improve null decoding

dec.go

@@ -246,6 +246,22 @@ func (d *Decoder) read() error {
 	return nil
 }
 
+func (d *Decoder) readAtLeast(min int) error {
+	if d.reader == nil {
+		d.head = d.tail
+		return io.ErrUnexpectedEOF
+	}
+
+	n, err := io.ReadAtLeast(d.reader, d.buf, min)
+	if err != nil {
+		return err
+	}
+
+	d.head = 0
+	d.tail = n
+	return nil
+}
+
 func (d *Decoder) unread() { d.head-- }
 
 // limit maximum depth of nesting, as allowed by https://tools.ietf.org/html/rfc7159#section-9

dec_skip.go

@@ -2,17 +2,40 @@ package jx
 
 import (
 	"io"
+	"math/bits"
 
 	"github.com/go-faster/errors"
 )
 
 // Null reads a json object as null and
 // returns whether it's a null or not.
 func (d *Decoder) Null() error {
-	if err := d.consume('n'); err != nil {
+	const encodedNull = 'n' | 'u'<<8 | 'l'<<16 | 'l'<<24
+
+	if buf := d.buf[d.head:d.tail]; len(buf) >= 4 {
+		c := uint32(buf[0]) | uint32(buf[1])<<8 | uint32(buf[2])<<16 | uint32(buf[3])<<24
+		if mask := c ^ encodedNull; mask != 0 {
+			idx := bits.TrailingZeros32(mask) / 8
+			return badToken(buf[idx])
+		}
+		d.head += 4
+		return nil
+	}
+
+	var buf [4]byte
+	n := copy(buf[:], d.buf[d.head:d.tail])
+	if err := d.readAtLeast(4 - n); err != nil {
 		return err
 	}
-	return d.skipThreeBytes('u', 'l', 'l') // null
+	copy(buf[n:], d.buf[d.head:d.tail])
+
+	c := uint32(buf[0]) | uint32(buf[1])<<8 | uint32(buf[2])<<16 | uint32(buf[3])<<24
+	if mask := c ^ encodedNull; mask != 0 {
+		idx := bits.TrailingZeros32(mask) / 8
+		return badToken(buf[idx])
+	}
+	d.head += 4
+	return nil
 }
 
 // Bool reads a json object as Bool

null_test.go

@@ -44,3 +44,25 @@ func Test_decode_null_skip(t *testing.T) {
 		t.FailNow()
 	}
 }
+
+func TestNullError(t *testing.T) {
+	a := require.New(t)
+	var (
+		b     = [4]byte{'n', 'u', 'l', 'l'}
+		valid = b
+	)
+	for i := range b {
+		// Reset buffer.
+		b = valid
+		for c := byte(0); c < 255; c++ {
+			// Skip expected value.
+			if valid[i] == c {
+				continue
+			}
+			b[i] = c
+			var token badTokenErr
+			a.ErrorAs(DecodeBytes(b[:]).Null(), &token)
+			a.Equalf(c, token.Token, "%c != %c (%q)", c, token.Token, b)
+		}
+	}
+}