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buffile.go
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buffile.go
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// package buffile helps batch write by writes to the tempobary in-memory
// buffer under the assumption that many writes come in to the part of
// single file frequently.
package buffile
import (
"io"
"os"
)
type fileLike interface {
io.ReaderAt
io.WriterAt
io.Closer
}
// BufferedFile abstracts a file with block-sized buffer to group
// writes that are likely consective. This object does not provide
// any mean of concurrency guarantee. Also note that the underlying
// file is assumed to have the content already written on disk, and
// the behavior on writing beyond the file size is not defined.
type BufferedFile struct {
fp fileLike
blockSize int
buffer []byte
bufferOffset int64
}
const defaultBlockSize = 32 * 1024
func New(filePath string) (*BufferedFile, error) {
fp, err := os.OpenFile(filePath, os.O_RDWR, 0700)
if err != nil {
return nil, err
}
blockSize := defaultBlockSize
return &BufferedFile{
fp: fp,
blockSize: blockSize,
}, nil
}
func (f *BufferedFile) Close() error {
f.writeBuffer()
return f.fp.Close()
}
func (f *BufferedFile) readBuffer(offset int64, size int) error {
// we always read from block boundary
readOffset := offset - offset%int64(f.blockSize)
// read size is block lower + offset residual + actual size
readSize := int(offset)%f.blockSize + size
// align to block size
readSize = readSize + f.blockSize
readSize -= readSize % f.blockSize
// len(nil slice) is 0
if len(f.buffer) < readSize {
f.buffer = make([]byte, readSize)
}
if n, err := f.fp.ReadAt(f.buffer, readOffset); err != nil {
if err == io.EOF {
// read short is fine at the end of file
f.buffer = f.buffer[:n]
} else {
return err
}
}
f.bufferOffset = readOffset
return nil
}
func (f *BufferedFile) writeBuffer() error {
if f.buffer != nil {
if _, err := f.fp.WriteAt(f.buffer, f.bufferOffset); err != nil {
return err
}
}
return nil
}
func (f *BufferedFile) ensureBuffer(data []byte, offset int64) error {
if f.buffer == nil {
return f.readBuffer(offset, len(data))
}
bufferLower := f.bufferOffset
bufferUpper := f.bufferOffset + int64(len(f.buffer))
if offset < bufferLower || offset+int64(len(data)) > bufferUpper {
if err := f.writeBuffer(); err != nil {
return err
}
if err := f.readBuffer(offset, len(data)); err != nil {
return err
}
}
return nil
}
// WriteAt writes the data at offset from the beginning of the file. Upon the
// return from this call, the data does not reach to disk yet. Make sure to close
// BufferedFile f to write the data on disk.
func (f *BufferedFile) WriteAt(data []byte, offset int64) (int, error) {
if err := f.ensureBuffer(data, offset); err != nil {
return 0, err
}
writePos := offset - f.bufferOffset
n := copy(f.buffer[writePos:], data)
return n, nil
}