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 :: forall a. Double -> Gen a -> (a -> Bool) -> Bool
canGenerate Double
prob Gen a
g a -> Bool
p = IO Bool -> Bool
forall a. IO a -> a
unsafePerformIO (IO Bool -> Bool) -> IO Bool -> Bool
forall a b. (a -> b) -> a -> b
$ Double -> IO Bool
tryToGenerate Double
1
  where
    tryToGenerate :: Double -> IO Bool
tryToGenerate Double
luck
      | Double
luck Double -> Double -> Bool
forall a. Ord a => a -> a -> Bool
< Double
eps = Bool -> IO Bool
forall a. a -> IO a
forall (m :: * -> *) a. Monad m => a -> m a
return Bool
False
      | Bool
otherwise = do
          a
x <- Gen a -> IO a
forall a. Gen a -> IO a
generate Gen a
g
          if a -> Bool
p a
x
            then Bool -> IO Bool
forall a. a -> IO a
forall (m :: * -> *) a. Monad m => a -> m a
return Bool
True
            else Double -> IO Bool
tryToGenerate (Double
luck Double -> Double -> Double
forall a. Num a => a -> a -> a
* (Double
1 Double -> Double -> Double
forall a. Num a => a -> a -> a
- Double
prob))

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