130 lines
3.3 KiB
Go
130 lines
3.3 KiB
Go
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package wsflate
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import (
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"fmt"
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"io"
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)
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var (
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compressionTail = [4]byte{
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0, 0, 0xff, 0xff,
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}
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compressionReadTail = [9]byte{
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0, 0, 0xff, 0xff,
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1,
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0, 0, 0xff, 0xff,
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}
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)
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// Compressor is an interface holding deflate compression implementation.
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type Compressor interface {
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io.Writer
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Flush() error
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}
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// WriteResetter is an optional interface that Compressor can implement.
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type WriteResetter interface {
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Reset(io.Writer)
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}
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// Writer implements compression for an io.Writer object using Compressor.
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// Essentially Writer is a thin wrapper around Compressor interface to meet
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// PMCE specs.
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//
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// After all data has been written client should call Flush() method.
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// If any error occurs after writing to or flushing a Writer, all subsequent
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// calls to Write(), Flush() or Close() will return the error.
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//
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// Writer might be reused for different io.Writer objects after its Reset()
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// method has been called.
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type Writer struct {
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// NOTE: Writer uses compressor constructor function instead of field to
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// reach these goals:
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// 1. To shrink Compressor interface and make it easier to be implemented.
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// 2. If used as a field (and argument to the NewWriter()), Compressor object
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// will probably be initialized twice - first time to pass into Writer, and
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// second time during Writer initialization (which does Reset() internally).
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// 3. To get rid of wrappers if Reset() would be a part of Compressor.
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// E.g. non conformant implementations would have to provide it somehow,
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// probably making a wrapper with the same constructor function.
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// 4. To make Reader and Writer API the same. That is, there is no Reset()
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// method for flate.Reader already, so we need to provide it as a wrapper
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// (see point #3), or drop the Reader.Reset() method.
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dest io.Writer
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ctor func(io.Writer) Compressor
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c Compressor
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cbuf cbuf
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err error
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}
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// NewWriter returns a new Writer.
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func NewWriter(w io.Writer, ctor func(io.Writer) Compressor) *Writer {
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// NOTE: NewWriter() is chosen against structure with exported fields here
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// due its Reset() method, which in case of structure, would change
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// exported field.
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ret := &Writer{
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dest: w,
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ctor: ctor,
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}
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ret.Reset(w)
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return ret
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}
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// Reset resets Writer to compress data into dest.
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// Any not flushed data will be lost.
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func (w *Writer) Reset(dest io.Writer) {
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w.err = nil
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w.cbuf.reset(dest)
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if x, ok := w.c.(WriteResetter); ok {
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x.Reset(&w.cbuf)
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} else {
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w.c = w.ctor(&w.cbuf)
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}
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}
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// Write implements io.Writer.
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func (w *Writer) Write(p []byte) (n int, err error) {
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if w.err != nil {
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return 0, w.err
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}
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n, w.err = w.c.Write(p)
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return n, w.err
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}
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// Flush writes any pending data into w.Dest.
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func (w *Writer) Flush() error {
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if w.err != nil {
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return w.err
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}
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w.err = w.c.Flush()
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w.checkTail()
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return w.err
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}
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// Close closes Writer and a Compressor instance used under the hood (if it
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// implements io.Closer interface).
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func (w *Writer) Close() error {
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if w.err != nil {
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return w.err
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}
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if c, ok := w.c.(io.Closer); ok {
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w.err = c.Close()
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}
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w.checkTail()
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return w.err
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}
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// Err returns an error happened during any operation.
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func (w *Writer) Err() error {
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return w.err
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}
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func (w *Writer) checkTail() {
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if w.err == nil && w.cbuf.buf != compressionTail {
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w.err = fmt.Errorf(
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"wsflate: bad compressor: unexpected stream tail: %#x vs %#x",
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w.cbuf.buf, compressionTail,
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)
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}
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}
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