internal/pkg: decompress zstd streams

This uses internal/zstd until compress/zstd becomes available.

Signed-off-by: Ophestra <cat@gensokyo.uk>

internal/zstd/window.go

@@ -0,0 +1,94 @@
+// 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) {
+	b := w.data[:0]
+	if cap(b) < size {
+		b = make([]byte, 0, size)
+	}
+	w.data = b
+	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]...)
+	}
+}