Potential workaround for poor zstd performance

Performance is still not great, but better

internal/data/fullreader.go

@@ -40,6 +40,7 @@ type outDat struct {
 func (r FullReader) process(index int, offset int64, out chan outDat) {
 	var err error
 	var dat []byte
+	var rdr io.ReadCloser
 	size := realSize(r.sizes[index])
 	if size == 0 {
 		out <- outDat{
@@ -49,12 +50,24 @@ func (r FullReader) process(index int, offset int64, out chan outDat) {
 		}
 		return
 	}
-	rdr := io.LimitReader(toreader.NewReader(r.r, offset), int64(size))
+	// rdr := io.LimitReader(toreader.NewReader(r.r, offset), int64(size))
 	if size == r.sizes[index] {
-		rdr, err = r.d.Reader(rdr)
-	}
-	if err == nil {
-		dat, err = io.ReadAll(rdr)
+		//Special workaround for zstd for increased performancce.
+		if zstd, ok := r.d.(*decompress.Zstd); ok {
+			dat = make([]byte, size)
+			_, err = r.r.ReadAt(dat, offset)
+			if err == nil {
+				dat, err = zstd.Decode(dat)
+			}
+		} else {
+			rdr, err = r.d.Reader(io.LimitReader(toreader.NewReader(r.r, offset), int64(size)))
+			if err == nil {
+				dat, err = io.ReadAll(rdr)
+			}
+		}
+	} else {
+		dat = make([]byte, size)
+		_, err = r.r.ReadAt(dat, offset)
 	}
 	out <- outDat{
 		i:    index,

internal/data/reader.go

@@ -12,17 +12,20 @@ type Reader struct {
 	cur        io.Reader
 	fragRdr    io.Reader
 	d          decompress.Decompressor
+	comRdr     io.Reader
 	blockSizes []uint32
 	blockSize  uint32
+	resetable  bool
 }
 
 func NewReader(r io.Reader, d decompress.Decompressor, blockSizes []uint32, blockSize uint32) *Reader {
-	var out Reader
-	out.d = d
-	out.master = r
-	out.blockSizes = blockSizes
-	out.blockSize = blockSize
-	return &out
+	return &Reader{
+		d:          d,
+		master:     r,
+		blockSizes: blockSizes,
+		blockSize:  blockSize,
+		resetable:  true,
+	}
 }
 
 func (r *Reader) AddFragment(rdr io.Reader) {
@@ -50,7 +53,19 @@ func (r *Reader) advance() (err error) {
 		} else {
 			r.cur = io.LimitReader(r.master, int64(size))
 			if size == r.blockSizes[0] {
-				r.cur, err = r.d.Reader(r.cur)
+				if r.d.Resetable() {
+					if r.comRdr == nil {
+						r.cur, err = r.d.Reader(r.cur)
+						if err != nil {
+							return
+						}
+					} else {
+						err = r.d.Reset(r.comRdr, r.cur)
+						r.cur = r.comRdr
+					}
+				} else {
+					r.cur, err = r.d.Reader(r.cur)
+				}
 			}
 		}
 	}