module Test.QuickCheck.DynamicLogic.CanGenerate (canGenerate) where

import System.IO.Unsafe
import Test.QuickCheck

-- | @canGenerate prob g p@
--   returns @False@ if we are sure @Prob(g generates x satisfying p) >= prob@
--   otherwise @True@ (and we know such an x can be generated).
canGenerate :: Double -> Gen a -> (a -> Bool) -> Bool
canGenerate prob g p = unsafePerformIO $ tryToGenerate 1
 where
  tryToGenerate luck
    | luck < eps = return False
    | otherwise = do
        x <- generate g
        if p x
          then return True
          else tryToGenerate (luck * (1 - prob))

  -- Our confidence level is 1-eps
  eps = 1.0e-9