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139 lines
3.1 KiB
Go
139 lines
3.1 KiB
Go
package bit
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import (
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"bufio"
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"bytes"
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"io"
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)
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// bit.Reader allows for bit-level reading of arbitrary source data. This is
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// based on the bit reader found in the standard library's bzip2 package.
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// https://golang.org/src/compress/bzip2/bit_reader.go
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type Reader struct {
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src io.ByteReader // source of data
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n uint64 // bit buffer
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bits uint // number of valid bits in n
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err error // stored error
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}
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// NewReader creates a new bit.Reader for any arbitrary reader.
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func NewReader(r io.Reader) *Reader {
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br, ok := r.(io.ByteReader)
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if !ok {
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br = bufio.NewReader(r)
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}
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return &Reader{src: br}
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}
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// NewByteReader creates a bit.Reader for a static slice of bytes. It's just
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// using a bytes.Reader internally.
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func NewBytesReader(b []byte) *Reader {
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return NewReader(bytes.NewReader(b))
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}
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// ReadBits reads the given number of bits and returns them in the
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// least-significant part of a uint64.
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func (r *Reader) ReadBits(bits uint) (n uint64) {
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for bits > r.bits {
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b, err := r.src.ReadByte()
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if err != nil {
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r.err = err
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return 0
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}
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r.n |= uint64(b) << r.bits
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r.bits += 8
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}
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n = r.n & (1<<bits - 1)
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r.n >>= bits
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r.bits -= bits
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return
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}
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// discards up to bits bits. returns a bool indicating wheter any errors occured.
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func (r *Reader) DiscardBits(n int) {
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r.ReadBits(uint(n))
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}
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// ReadByte reads a single byte, regardless of alignment.
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func (r *Reader) ReadByte() (byte, error) {
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if r.bits == 0 {
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return r.src.ReadByte()
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}
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b := byte(r.ReadBits(8))
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if err := r.Err(); err != nil {
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return 0, err
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}
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return b, nil
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}
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// Read reads like an io.Reader, taking care of alignment internally.
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func (r *Reader) Read(buf []byte) (int, error) {
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for i := 0; i < len(buf); i++ {
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b, err := r.ReadByte()
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if err != nil {
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return 0, err
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}
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buf[i] = b
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}
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return len(buf), nil
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}
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// discards N byte of data on the reader or until EOF
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func (r *Reader) DiscardBytes(n int) {
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for i := 0; i < n; i++ {
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_, err := r.ReadByte()
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if err != nil {
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r.err = err
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return
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}
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}
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}
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// ReadUbitVar reads a prefixed uint value. A prefix is 2 bits wide, followed
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// by the 4 least-significant bits, then a variable number of most-significant
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// bits based on the prefix.
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//
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// 00 - 4
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// 01 - 8
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// 10 - 12 (why 12? this really baffles me)
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// 11 - 32
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func (r *Reader) ReadUBitVar() uint64 {
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switch b := r.ReadBits(6); b >> 4 {
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case 0:
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return b & 0xf
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case 1:
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return b&0xf | r.ReadBits(4)<<4
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case 2:
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return b&0xf | r.ReadBits(8)<<4
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case 3:
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return b&0xf | r.ReadBits(28)<<4
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default:
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panic("not reached")
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}
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}
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// ReadVarInt reads a variable length int value as a uint64. This is the binary
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// representation used by Protobuf. Each byte contributes 7 bits to the value
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// in little-endian order. The most-significant bit of each byte represents a
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// continuation bit.
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func (r *Reader) ReadVarInt() uint64 {
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var (
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x uint64
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b uint64
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shift uint
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)
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for ; shift < 64; shift += 7 {
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b = r.ReadBits(8)
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if r.Err() != nil {
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return 0
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}
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x |= b & 0x7f << shift
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if b&0x80 == 0 {
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return x
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}
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}
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return x
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}
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func (r *Reader) Err() error { return r.err }
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