1 files changed, 101 insertions, 0 deletions
diff --git a/internal/flatex/window_decoder.go b/internal/flatex/window_decoder.go
new file mode 100644
index 00000000..492c6a96
--- /dev/null
+++ b/internal/flatex/window_decoder.go
@@ -0,0 +1,101 @@
+package flatex
+
+// windowDecoder implements the sliding window used in decompression.
+type windowDecoder struct {
+	hist []byte
+
+	wrPos int
+	rdPos int
+	full  bool
+}
+
+func (wd *windowDecoder) init(size int) {
+	*wd = windowDecoder{hist: wd.hist}
+
+	if cap(wd.hist) < size {
+		wd.hist = make([]byte, size)
+	}
+	wd.hist = wd.hist[:size]
+
+	wd.wrPos = 0
+	wd.rdPos = 0
+	wd.full = false
+}
+
+func (wd *windowDecoder) histSize() int {
+	if wd.full {
+		return len(wd.hist)
+	}
+	return wd.wrPos
+}
+
+func (wd *windowDecoder) availRead() int {
+	return wd.wrPos - wd.rdPos
+}
+
+func (wd *windowDecoder) availWrite() int {
+	return len(wd.hist) - wd.wrPos
+}
+
+func (wd *windowDecoder) writeSlice() []byte {
+	return wd.hist[wd.wrPos:]
+}
+
+func (wd *windowDecoder) writeMark(cnt int) {
+	wd.wrPos += cnt
+}
+
+func (wd *windowDecoder) writeByte(c byte) {
+	wd.hist[wd.wrPos] = c
+	wd.wrPos++
+}
+
+func (wd *windowDecoder) writeCopy(dist, length int) int {
+	dstBase := wd.wrPos
+	dstPos := dstBase
+	srcPos := dstPos - dist
+	endPos := dstPos + length
+	if endPos > len(wd.hist) {
+		endPos = len(wd.hist)
+	}
+
+	if srcPos < 0 {
+		srcPos += len(wd.hist)
+		dstPos += copy(wd.hist[dstPos:endPos], wd.hist[srcPos:])
+		srcPos = 0
+	}
+
+	for dstPos < endPos {
+		dstPos += copy(wd.hist[dstPos:endPos], wd.hist[srcPos:dstPos])
+	}
+
+	wd.wrPos = dstPos
+	return dstPos - dstBase
+}
+
+func (wd *windowDecoder) tryWriteCopy(dist, length int) int {
+	dstPos := wd.wrPos
+	endPos := dstPos + length
+	if dstPos < dist || endPos > len(wd.hist) {
+		return 0
+	}
+	dstBase := dstPos
+	srcPos := dstPos - dist
+
+	for dstPos < endPos {
+		dstPos += copy(wd.hist[dstPos:endPos], wd.hist[srcPos:dstPos])
+	}
+
+	wd.wrPos = dstPos
+	return dstPos - dstBase
+}
+
+func (wd *windowDecoder) readFlush() []byte {
+	toRead := wd.hist[wd.rdPos:wd.wrPos]
+	wd.rdPos = wd.wrPos
+	if wd.wrPos == len(wd.hist) {
+		wd.wrPos, wd.rdPos = 0, 0
+		wd.full = true
+	}
+	return toRead
+}