)

// blockPadding computes the number of bytes needed to pad offset up to the
// nearest block edge where 0 <= n < blockSize.
func blockPadding(offset int64) (n int64) {
	return -offset & (blockSize - 1)
}

var zeroBlock block

type block [blockSize]byte

// Convert block to any number of formats.
func (b *block) toV7() *headerV7       { return (*headerV7)(b) }

// offsets and lengths that are multiples of blockSize.
func alignSparseEntries(src []sparseEntry, size int64) []sparseEntry {
	dst := src[:0]
	for _, s := range src {
		pos, end := s.Offset, s.endOffset()
		pos += blockPadding(+pos) // Round-up to nearest blockSize
		if end != size {
			end -= blockPadding(-end) // Round-down to nearest blockSize
		}
		if pos < end {

	}, {
		input: makeInput(FormatGNU, "",
			makeSparseStrings(sparseDatas{{0, 1}, {2, 1}, {4, 1}, {6, 1}, {8, 1}, {10, 1}})...)[:blockSize],
		wantErr: io.ErrUnexpectedEOF,
	}, {
		input: makeInput(FormatGNU, "",
			makeSparseStrings(sparseDatas{{0, 1}, {2, 1}, {4, 1}, {6, 1}, {8, 1}, {10, 1}})...)[:3*blockSize/2],
		wantErr: io.ErrUnexpectedEOF,
	}, {
		input: makeInput(FormatGNU, "",

					testReadFrom{fileOps{
						int64(1e10 - blockSize),
						strings.Repeat("\x00", blockSize-100) + strings.Repeat("0123456789", 10),
						int64(1e10 - blockSize),
						strings.Repeat("\x00", blockSize-100) + strings.Repeat("0123456789", 10),
						int64(1e10 - blockSize),
						strings.Repeat("\x00", blockSize-100) + strings.Repeat("0123456789", 10),
						int64(1e10 - blockSize),