// build 2^w keys, each having a unique value in the first w bits
func (s *XLSuite) makePermutedKeys(rng *xr.PRNG, w uint) (
fields []int, // FOR DEBUGGING ONLY
keys [][]byte) {
fieldCount := (1 << w) - 1 // we don't want the zero value
fields = rng.Perm(fieldCount) // so 2^w distinct values
for i := 0; i < len(fields); i++ {
fields[i] += 1
}
keyLen := uint((int(w)*fieldCount + 7) / 8) // in bytes, rounded up
keyCount := uint(fieldCount)
keys = make([][]byte, keyCount)
for i := uint(0); i < keyCount; i++ {
key := make([]byte, keyLen) // all zeroes
if i != uint(0) {
copy(key, keys[i-1])
}
// OR the field into the appropriate byte(s) of the key
bitOffset := w * i
whichByte := bitOffset / uint(8)
whichBit := bitOffset % uint(8)
// lower half of the field
key[whichByte] |= byte(fields[i] << whichBit)
if whichBit+w >= 8 {
key[whichByte+1] |= byte(fields[i] >> (8 - whichBit))
}
keys[i] = key
}
return
}