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// 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    byteReader
        n    uint64
        bits uint
        err  error
}

// bitReader needs to read bytes from an io.Reader. We attempt to convert the
// given io.Reader to this interface and, if it doesn't already fit, we wrap in
// a bufio.Reader.
type byteReader interface {
        ReadByte() (byte, error)
}

func newBitReader(r io.Reader) bitReader {
        byter, ok := r.(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) Err() error {
        return br.err
}