restore byteReader.window()

The unary version was added to try to improve the performance of
parseString, but 7 compiler versions on, it doens't seem to pull its
weight. Make the code match the version on the slides for consistency.

reader.go

@@ -17,8 +17,8 @@ func (b *byteReader) release(n int) {
 
 // window returns the current window.
 // The window is invalidated by calls to release or extend.
-func (b *byteReader) window(offset int) []byte {
-	return b.data[b.offset+offset:]
+func (b *byteReader) window() []byte {
+	return b.data[b.offset:]
 }
 
 // tuning constants for byteReader.extend.

reader_test.go

@@ -29,7 +29,7 @@ func BenchmarkCountWhitespace(b *testing.B) {
 
 func countWhitespace(br *byteReader) int {
 	n := 0
-	w := br.window(0)
+	w := br.window()
 	for {
 		for _, c := range w {
 			if whitespace[c] {
@@ -40,6 +40,6 @@ func countWhitespace(br *byteReader) int {
 		if br.extend() == 0 {
 			return n
 		}
-		w = br.window(0)
+		w = br.window()
 	}
 }

scanner.go

@@ -61,7 +61,7 @@ var whitespace = [256]bool{
 //	-, 0-9 A number
 func (s *Scanner) Next() []byte {
 	s.br.release(s.pos)
-	w := s.br.window(0)
+	w := s.br.window()
 loop:
 	for pos, c := range w {
 		// strip any leading whitespace.
@@ -92,7 +92,7 @@ loop:
 			// ensure the number is correct.
 			s.pos = s.parseNumber(c)
 		}
-		return s.br.window(0)[:s.pos]
+		return s.br.window()[:s.pos]
 	}
 
 	// it's all whitespace, ignore it
@@ -103,13 +103,13 @@ loop:
 		// eof
 		return nil
 	}
-	w = s.br.window(0)
+	w = s.br.window()
 	goto loop
 }
 
 func validateToken(br *byteReader, expected string) int {
 	for {
-		w := br.window(0)
+		w := br.window()
 		n := len(expected)
 		if len(w) >= n {
 			if string(w[:n]) != expected {
@@ -131,7 +131,7 @@ func validateToken(br *byteReader, expected string) int {
 // " before the end of the byteReader.
 func (s *Scanner) parseString() int {
 	escaped := false
-	w := s.br.window(1)
+	w := s.br.window()[1:]
 	pos := 0
 	for {
 		for _, c := range w {
@@ -152,7 +152,7 @@ func (s *Scanner) parseString() int {
 			// EOF.
 			return 0
 		}
-		w = s.br.window(pos + 1)
+		w = s.br.window()[pos+1:]
 	}
 }
 
@@ -169,14 +169,14 @@ func (s *Scanner) parseNumber(c byte) int {
 	)
 
 	pos := 0
-	w := s.br.window(0)
+	w := s.br.window()
 	// int vs uint8 costs 10% on canada.json
 	var state uint8 = begin
 
 	// handle the case that the first character is a hyphen
 	if c == '-' {
 		pos++
-		w = s.br.window(1)
+		w = s.br.window()[1:] // TODO - can we subslice w here?
 	}
 
 	for {
@@ -256,7 +256,7 @@ func (s *Scanner) parseNumber(c byte) int {
 				return 0
 			}
 		}
-		w = s.br.window(pos)
+		w = s.br.window()[pos:]
 	}
 }
 

scanner_test.go

@@ -155,7 +155,7 @@ func BenchmarkParseNumber(b *testing.B) {
 					},
 				}
 				scanner.br.extend()
-				n := scanner.parseNumber(scanner.br.window(0)[0])
+				n := scanner.parseNumber(scanner.br.window()[0])
 				if n != len(tc) {
 					b.Fatalf("expected: %v, got: %v", len(tc), n)
 				}