module Crypto.Number.Generate
( generateMax
, generateBetween
, generateOfSize
) where
import Crypto.Number.Basic
import Crypto.Number.Serialize
import Crypto.Random.API
import qualified Data.ByteString as B
import Data.Bits ((.|.), (.&.), shiftL)
generateMax :: CPRG g => g -> Integer -> (Integer, g)
generateMax rng m
| m < 1 = error "generateMax: m must be >= 1"
| m == 1 = (0,rng)
| otherwise =
let (tentativeResult, rng') =
withRandomBytes rng (lengthBytes m) $ \bs ->
let lengthBits = (log2 (m1) + 1)
mask = if lengthBits `mod` 8 == 0
then 0xff
else (1 `shiftL` (lengthBits `mod` 8)) 1 in
os2ip $ snd $ B.mapAccumL (\acc w -> (0xff, w .&. acc))
mask bs in
if tentativeResult < m
then (tentativeResult, rng')
else generateMax rng' m
generateBetween :: CPRG g => g -> Integer -> Integer -> (Integer, g)
generateBetween rng low high = (low + v, rng')
where (v, rng') = generateMax rng (high low + 1)
generateOfSize :: CPRG g => g -> Int -> (Integer, g)
generateOfSize rng bits = withRandomBytes rng (bits `div` 8) $ \bs ->
os2ip $ snd $ B.mapAccumL (\acc w -> (0, w .|. acc)) 0xc0 bs