chunks: drop shift, remove branch

This removes the need for the shift value and just calculates
everything on the fly through count.
Removes a branch condition.

chunks/bstream.go

@@ -4,11 +4,8 @@ import "io"
 
 // bstream is a stream of bits
 type bstream struct {
-	// the data stream
-	stream []byte
-
-	count uint8 // how many bits are valid in current byte
-	shift uint8 // pos of next bit in current byte
+	stream []byte // the data stream
+	count  uint8  // how many bits are valid in current byte
 }
 
 func newBReader(b []byte) *bstream {
@@ -83,32 +80,22 @@ func (b *bstream) writeBits(u uint64, nbits int) {
 	}
 }
 
-func (b *bstream) headByte() byte {
-	return b.stream[0] << b.shift
-}
-
-func (b *bstream) advance() {
-	b.stream = b.stream[1:]
-	b.shift = 0
-}
-
 func (b *bstream) readBit() (bit, error) {
 	if len(b.stream) == 0 {
 		return false, io.EOF
 	}
 
 	if b.count == 0 {
-		b.advance()
-		// did we just run out of stuff to read?
+		b.stream = b.stream[1:]
+
 		if len(b.stream) == 0 {
 			return false, io.EOF
 		}
 		b.count = 8
 	}
 
-	d := b.headByte() & 0x80
+	d := (b.stream[0] << (8 - b.count)) & 0x80
 	b.count--
-	b.shift++
 	return d != 0, nil
 }
 
@@ -118,22 +105,21 @@ func (b *bstream) readByte() (byte, error) {
 	}
 
 	if b.count == 0 {
-		b.advance()
+		b.stream = b.stream[1:]
 
 		if len(b.stream) == 0 {
 			return 0, io.EOF
 		}
-
-		b.count = 8
+		return b.stream[0], nil
 	}
 
 	if b.count == 8 {
 		b.count = 0
-		return b.headByte(), nil
+		return b.stream[0], nil
 	}
 
-	byt := b.headByte()
-	b.advance()
+	byt := b.stream[0] << (8 - b.count)
+	b.stream = b.stream[1:]
 
 	if len(b.stream) == 0 {
 		return 0, io.EOF
@@ -141,7 +127,6 @@ func (b *bstream) readByte() (byte, error) {
 
 	// We just advanced the stream and can assume the shift to be 0.
 	byt |= b.stream[0] >> b.count
-	b.shift = 8 - b.count
 
 	return byt, nil
 }
@@ -164,9 +149,9 @@ func (b *bstream) readBits(nbits int) (uint64, error) {
 	}
 
 	if nbits > int(b.count) {
-		u = (u << uint(b.count)) | uint64(b.headByte()>>(8-b.count))
+		u = (u << uint(b.count)) | uint64((b.stream[0]<<(8-b.count))>>(8-b.count))
 		nbits -= int(b.count)
-		b.advance()
+		b.stream = b.stream[1:]
 
 		if len(b.stream) == 0 {
 			return 0, io.EOF
@@ -174,8 +159,7 @@ func (b *bstream) readBits(nbits int) (uint64, error) {
 		b.count = 8
 	}
 
-	u = (u << uint(nbits)) | uint64(b.headByte()>>(8-uint(nbits)))
-	b.shift = b.shift + uint8(nbits)
+	u = (u << uint(nbits)) | uint64((b.stream[0]<<(8-b.count))>>(8-uint(nbits)))
 	b.count -= uint8(nbits)
 	return u, nil
 }