blockOffset                int // Offset in bytes to start of block.
	blocks                     int // 0 = one block.
	objSize, partSize          int
	wantMD5                    string
}

func readAndMap(files []string, partNum, blockNum int) (*mappedData, error) {
	var m mappedData
	sort.Strings(files)
	m.parityData = make(map[int]map[int][]byte)
	for _, file := range files {
		meta, err := os.ReadFile(file + ".json")