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------------------------------------------------------------------------------
--- Library for pseudo-random number generation in Curry.
--- 
--- This library provides operations for generating pseudo-random
--- number sequences.
--- For any given seed, the sequences generated by the operations
--- in this module should be **identical** to the sequences
--- generated by the `java.util.Random package`.
--- 
--- The algorithm is taken from 
--- <http://en.wikipedia.org/wiki/Random_number_generation>.
--- There is an assumption that all operations are implicitly
--- executed mod 2^32 (unsigned 32-bit integers) !!!
--- GHC computes between -2^29 and 2^29-1,  thus the sequence
--- is NOT as random as one would like.
---
---     m_w = <choose-initializer>;    /* must not be zero */
---     m_z = <choose-initializer>;    /* must not be zero */
---      
---     uint get_random()
---     {
---         m_z = 36969 * (m_z & 65535) + (m_z >> 16);
---         m_w = 18000 * (m_w & 65535) + (m_w >> 16);
---         return (m_z << 16) + m_w;  /* 32-bit result */
---     }
---
--- @author Sergio Antoy (with extensions by Michael Hanus)
--- @version February 2016
--- @category algorithm
------------------------------------------------------------------------------

module Random
  ( nextInt, nextIntRange, nextBoolean, getRandomSeed
  , shuffle
  ) where

import Integer ( abs )
import System  ( getCPUTime )
import Time

zfact :: Int
zfact = 36969

wfact :: Int
wfact = 18000

two16 :: Int
two16 = 65536

large :: Int
large = 536870911 -- 2^29 - 1
------------------------------------------------------------------
--                       Public Operations
------------------------------------------------------------------

--- Returns a sequence of pseudorandom, integer values.
--- 
--- @param seed - The seed of the random sequence.

nextInt :: Int -> [Int]
nextInt seed =
  let ns = if seed == 0 then 1 else seed
      next2 mw mz =
          let mza = zfact * (mz `mod` two16) + (mz * two16)
              mwa = wfact * (mw `mod` two16) + (mw * two16)
              tmp = (mza `div` two16 + mwa)
              res = if tmp < 0 then tmp+large else tmp
          in res : next2 mwa mza
  in next2 ns ns

--- Returns a pseudorandom sequence of values
--- between 0 (inclusive) and the specified value (exclusive).
---
--- @param seed - The seed of the random sequence.
--- @param n - The bound on the random number to be returned. 
---            Must be positive.

nextIntRange :: Int -> Int -> [Int]
nextIntRange seed n | n>0
    = map (`mod` n) (nextInt seed)

--- Returns a pseudorandom sequence of boolean values. 
---
--- @param seed - The seed of the random sequence.

nextBoolean :: Int -> [Bool]
nextBoolean seed = map (/= 0) (nextInt seed)


--- Returns a time-dependent integer number as a seed for really random numbers.
--- Should only be used as a seed for pseudorandom number sequence
--- and not as a random number since the precision is limited to milliseconds

getRandomSeed :: IO Int
getRandomSeed =
  getClockTime >>= \time ->
  getCPUTime >>= \msecs ->
  let (CalendarTime y mo d h m s _) = toUTCTime time
   in return ((y+mo+d+h+(m+1)*(s+1)*(msecs+1)) `mod` two16)

--- Computes a random permutation of the given list.
---
--- @param rnd random seed
--- @param l lists to shuffle
--- @return shuffled list
---
shuffle :: Int -> [a] -> [a]
shuffle rnd xs = shuffleWithLen (nextInt rnd) (length xs) xs

shuffleWithLen :: [Int] -> Int -> [a] -> [a]
shuffleWithLen [] _ _ =
  error "Internal error in Random.shuffleWithLen"
shuffleWithLen (r:rs) len xs
  | len == 0  = []
  | otherwise = z : shuffleWithLen rs (len-1) (ys++zs)
 where
  (ys,z:zs) = splitAt (abs r `mod` len) xs

{-     Simple tests and examples

testInt = take 20 (nextInt 0)

testIntRange = take 120 (nextIntRange 0 6)

testBoolean = take 20 (nextBoolean 0)

reallyRandom = do seed <- getRandomSeed
                  putStrLn (show (take 20 (nextIntRange seed 100)))
-}