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// Copyright 2023 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 zstd
// window stores up to size bytes of data.
// It is implemented as a circular buffer:
// sequential save calls append to the data slice until
// its length reaches configured size and after that,
// save calls overwrite previously saved data at off
// and update off such that it always points at
// the byte stored before others.
type window struct {
size int
data []byte
off int
}
// reset clears stored data and configures window size.
func (w *window) reset(size int) {
w.data = w.data[:0]
w.off = 0
w.size = size
}
// len returns the number of stored bytes.
func (w *window) len() uint32 {
return uint32(len(w.data))
}
// save stores up to size last bytes from the buf.
func (w *window) save(buf []byte) {
if w.size == 0 {
return
}
if len(buf) == 0 {
return
}
if len(buf) >= w.size {
from := len(buf) - w.size
w.data = append(w.data[:0], buf[from:]...)
w.off = 0
return
}
// Update off to point to the oldest remaining byte.
free := w.size - len(w.data)
if free == 0 {
n := copy(w.data[w.off:], buf)
if n == len(buf) {
w.off += n
} else {
w.off = copy(w.data, buf[n:])
}
} else {
if free >= len(buf) {
w.data = append(w.data, buf...)
} else {
w.data = append(w.data, buf[:free]...)
w.off = copy(w.data, buf[free:])
}
}
}
// appendTo appends stored bytes between from and to indices to the buf.
// Index from must be less or equal to index to and to must be less or equal to w.len().
func (w *window) appendTo(buf []byte, from, to uint32) []byte {
dataLen := uint32(len(w.data))
from += uint32(w.off)
to += uint32(w.off)
wrap := false
if from > dataLen {
from -= dataLen
wrap = !wrap
}
if to > dataLen {
to -= dataLen
wrap = !wrap
}
if wrap {
buf = append(buf, w.data[from:]...)
return append(buf, w.data[:to]...)
} else {
return append(buf, w.data[from:to]...)
}
}
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