var fr fileReader
		switch maker := v.maker.(type) {
		case makeReg:
			r := testNonEmptyReader{strings.NewReader(maker.str)}
			fr = &regFileReader{r, maker.size}
		case makeSparse:
			if !validateSparseEntries(maker.spd, maker.size) {
				t.Fatalf("invalid sparse map: %v", maker.spd)
			}
			sph := invertSparseEntries(maker.spd, maker.size)
			r := testNonEmptyReader{strings.NewReader(maker.makeReg.str)}