ReadFrom trashes the old instace

[omerfirmak]

ReadFrom creates a new bitset and overrides the previous one when it does not encounter any error during the process.

bitset/bitset.go

Lines 949 to 968 in d53c44e

	newset := New(uint(length))
	if uint64(newset.length) != length {
	return 0, errors.New("unmarshalling error: type mismatch")
	}
	var item [8]byte
	nWords := wordsNeeded(uint(length))
	reader := bufio.NewReader(io.LimitReader(stream, 8*int64(nWords)))
	for i := 0; i < nWords; i++ {
	if _, err := io.ReadFull(reader, item[:]); err != nil {
	if err == io.EOF {
	err = io.ErrUnexpectedEOF
	}
	return 0, err
	}
	newset.set[i] = binaryOrder.Uint64(item[:])
	}
	*b = *newset

For applications that constantly marshal/unmarshal bitsets, this is a huge problem. What ReadFrom should do instead is try to reuse the existing bitset and the underlying []uint64 it has. It should avoid creating any one-time temporary heap buffers as well.

To be honest I don't know why it works like that but the fact that ReadFrom does not modify the existing instance if it encounters an error makes this a little tricky.

[lemire]

I agree. Let us fix that in the next release.