window.go

     1  // Copyright 2023 The Go Authors. All rights reserved.
     2  // Use of this source code is governed by a BSD-style
     3  // license that can be found in the LICENSE file.
     4  
     5  package zstd
     6  
     7  // window stores up to size bytes of data.
     8  // It is implemented as a circular buffer:
     9  // sequential save calls append to the data slice until
    10  // its length reaches configured size and after that,
    11  // save calls overwrite previously saved data at off
    12  // and update off such that it always points at
    13  // the byte stored before others.
    14  type window struct {
    15  	size int
    16  	data []byte
    17  	off  int
    18  }
    19  
    20  // reset clears stored data and configures window size.
    21  func (w *window) reset(size int) {
    22  	w.data = w.data[:0]
    23  	w.off = 0
    24  	w.size = size
    25  }
    26  
    27  // len returns the number of stored bytes.
    28  func (w *window) len() uint32 {
    29  	return uint32(len(w.data))
    30  }
    31  
    32  // save stores up to size last bytes from the buf.
    33  func (w *window) save(buf []byte) {
    34  	if w.size == 0 {
    35  		return
    36  	}
    37  	if len(buf) == 0 {
    38  		return
    39  	}
    40  
    41  	if len(buf) >= w.size {
    42  		from := len(buf) - w.size
    43  		w.data = append(w.data[:0], buf[from:]...)
    44  		w.off = 0
    45  		return
    46  	}
    47  
    48  	// Update off to point to the oldest remaining byte.
    49  	free := w.size - len(w.data)
    50  	if free == 0 {
    51  		n := copy(w.data[w.off:], buf)
    52  		if n == len(buf) {
    53  			w.off += n
    54  		} else {
    55  			w.off = copy(w.data, buf[n:])
    56  		}
    57  	} else {
    58  		if free >= len(buf) {
    59  			w.data = append(w.data, buf...)
    60  		} else {
    61  			w.data = append(w.data, buf[:free]...)
    62  			w.off = copy(w.data, buf[free:])
    63  		}
    64  	}
    65  }
    66  
    67  // appendTo appends stored bytes between from and to indices to the buf.
    68  // Index from must be less or equal to index to and to must be less or equal to w.len().
    69  func (w *window) appendTo(buf []byte, from, to uint32) []byte {
    70  	dataLen := uint32(len(w.data))
    71  	from += uint32(w.off)
    72  	to += uint32(w.off)
    73  
    74  	wrap := false
    75  	if from > dataLen {
    76  		from -= dataLen
    77  		wrap = !wrap
    78  	}
    79  	if to > dataLen {
    80  		to -= dataLen
    81  		wrap = !wrap
    82  	}
    83  
    84  	if wrap {
    85  		buf = append(buf, w.data[from:]...)
    86  		return append(buf, w.data[:to]...)
    87  	} else {
    88  		return append(buf, w.data[from:to]...)
    89  	}
    90  }
    91
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