/
w.go
108 lines (92 loc) · 1.72 KB
/
w.go
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package v2
import (
"encoding/binary"
"errors"
"hash/crc32"
"io"
"unsafe"
)
const (
blockSize = 64
headerSize = int(unsafe.Sizeof(header{}))
uint16SZ = 2
uint32SZ = 4
uint64SZ = 8
)
type header struct {
kind uint16
crc32 uint32
size uint64
}
func writeHeader(buf []byte, hdr *header) (int, error) {
if len(buf) < headerSize {
return 0, io.ErrShortWrite
}
var n int
binary.LittleEndian.PutUint16(buf[n:n+uint16SZ], hdr.kind)
n += uint16SZ
binary.LittleEndian.PutUint32(buf[n:n+uint32SZ], hdr.crc32)
n += uint32SZ
binary.LittleEndian.PutUint64(buf[n:n+uint64SZ], hdr.size)
n += uint64SZ
return n, nil
}
type Writer struct {
buf []byte
n int
w io.Writer
}
func NewWriter(w io.Writer, size int) *Writer {
if size%blockSize != 0 {
size = (size + blockSize - 1) &^ (blockSize - 1)
}
return &Writer{
buf: make([]byte, size),
n: 0,
w: w,
}
}
var ErrDataOverflow = errors.New("data overflow--too big")
func (w *Writer) undoWrite(n int) {
if n > 0 && n < w.n {
copy(w.buf[0:w.n-n], w.buf[n:w.n])
}
w.n -= n
}
func (w *Writer) Write(p []byte) (int, error) {
// error checking
if len(p)+headerSize > len(w.buf[w.n:]) {
return 0, ErrDataOverflow
}
var nn int
// write header
n, err := writeHeader(w.buf[w.n:], &header{
kind: 0xff,
crc32: crc32.ChecksumIEEE(p),
size: uint64(len(p)),
})
if err != nil {
w.undoWrite(n)
return n, err
}
// update offsets
w.n += n
nn += n
// write data to buffer
n = copy(w.buf[w.n:], p)
w.n += n
nn += n
// take up some slack
if w.n%blockSize != 0 {
w.n = (w.n + blockSize - 1) &^ (blockSize - 1)
}
// persist
x, err := w.w.Write(w.buf[:w.n])
if err != nil {
w.undoWrite(x)
return x, err
}
// reset buffer
w.n = 0
return x, nil
}