// randomNumber returns a uniform random value in [0, max).
func randomNumber(rand io.Reader, max *big.Int) (n *big.Int, err os.Error) {
k := (max.Len() + 7) / 8
// r is the number of bits in the used in the most significant byte of
// max.
r := uint(max.Len() % 8)
if r == 0 {
r = 8
}
bytes := make([]byte, k)
n = new(big.Int)
for {
_, err = io.ReadFull(rand, bytes)
if err != nil {
return
}
// Clear bits in the first byte to increase the probability
// that the candidate is < max.
bytes[0] &= uint8(int(1<<r) - 1)
n.SetBytes(bytes)
if n.Cmp(max) < 0 {
return
}
}
return
}