1 files changed, 90 insertions, 0 deletions
diff --git a/src/compress/bzip2/bit_reader.go b/src/compress/bzip2/bit_reader.go
new file mode 100644
index 000000000..32d1036ae
--- /dev/null
+++ b/src/compress/bzip2/bit_reader.go
@@ -0,0 +1,90 @@
+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package bzip2
+
+import (
+	"bufio"
+	"io"
+)
+
+// bitReader wraps an io.Reader and provides the ability to read values,
+// bit-by-bit, from it. Its Read* methods don't return the usual error
+// because the error handling was verbose. Instead, any error is kept and can
+// be checked afterwards.
+type bitReader struct {
+	r    io.ByteReader
+	n    uint64
+	bits uint
+	err  error
+}
+
+// newBitReader returns a new bitReader reading from r. If r is not
+// already an io.ByteReader, it will be converted via a bufio.Reader.
+func newBitReader(r io.Reader) bitReader {
+	byter, ok := r.(io.ByteReader)
+	if !ok {
+		byter = bufio.NewReader(r)
+	}
+	return bitReader{r: byter}
+}
+
+// ReadBits64 reads the given number of bits and returns them in the
+// least-significant part of a uint64. In the event of an error, it returns 0
+// and the error can be obtained by calling Err().
+func (br *bitReader) ReadBits64(bits uint) (n uint64) {
+	for bits > br.bits {
+		b, err := br.r.ReadByte()
+		if err == io.EOF {
+			err = io.ErrUnexpectedEOF
+		}
+		if err != nil {
+			br.err = err
+			return 0
+		}
+		br.n <<= 8
+		br.n |= uint64(b)
+		br.bits += 8
+	}
+
+	// br.n looks like this (assuming that br.bits = 14 and bits = 6):
+	// Bit: 111111
+	//      5432109876543210
+	//
+	//         (6 bits, the desired output)
+	//        |-----|
+	//        V     V
+	//      0101101101001110
+	//        ^            ^
+	//        |------------|
+	//           br.bits (num valid bits)
+	//
+	// This the next line right shifts the desired bits into the
+	// least-significant places and masks off anything above.
+	n = (br.n >> (br.bits - bits)) & ((1 << bits) - 1)
+	br.bits -= bits
+	return
+}
+
+func (br *bitReader) ReadBits(bits uint) (n int) {
+	n64 := br.ReadBits64(bits)
+	return int(n64)
+}
+
+func (br *bitReader) ReadBit() bool {
+	n := br.ReadBits(1)
+	return n != 0
+}
+
+func (br *bitReader) TryReadBit() (bit byte, ok bool) {
+	if br.bits > 0 {
+		br.bits--
+		return byte(br.n>>br.bits) & 1, true
+	}
+	return 0, false
+}
+
+func (br *bitReader) Err() error {
+	return br.err
+}