source code

```{-  The Computer Language Benchmarks Game

https://salsa.debian.org/benchmarksgame-team/benchmarksgame/

contributed by Miha Vučkovič

-}

import System.Environment
import qualified Data.Vector.Unboxed as V
import qualified Data.Vector.Unboxed.Mutable as MV
import qualified Data.Vector.Generic.Mutable as GMV
import Control.Parallel
import Control.Parallel.Strategies
import Data.STRef

flopST v flopCount = do
if h == 1 then return flopCount
else do
GMV.reverse \$ MV.unsafeSlice 0 h v
flopST v (flopCount + 1)

countFlops :: V.Vector Int -> Int
countFlops v = runST \$ do
mv <- V.thaw v
flopST mv 0

permut n = foldr permConcat [V.fromList [1..n]] [5..n]

permConcat x lst = concat [take x \$ iterate (rotate x) l | l <- lst]

perm :: Int -> V.Vector Int -> [V.Vector Int]
perm x l = take x \$ iterate (rotate x) l

rotate :: Int -> V.Vector Int -> V.Vector Int
rotate 1 xs = xs
rotate n v = runST \$ do
mv <- V.thaw v
veryUnsafeV <- V.unsafeThaw v
MV.unsafeCopy (MV.unsafeSlice 0 (n-1) mv) (MV.unsafeSlice 1 (n-1) veryUnsafeV)
MV.unsafeWrite mv (n-1) h
V.unsafeFreeze mv

calculateMaxAndChecksum :: [Int] -> (Int, Int)
calculateMaxAndChecksum = go 0 0
where go !m !c [] = (m, c)
go !m !c [x] = (max m x, c + x)
go !m !c (x0:x1:xs) = go (max3 m x0 x1) (c + x0 - x1) xs
max3 !a !b !c = max c (max a b)

niceParallelMap :: NFData b => Int -> (a -> b) -> [a] -> [b]
niceParallelMap bufferSize f xs = runEval \$ parBuffer bufferSize rdeepseq \$ map f xs

main :: IO ()
main = do
let permutations = permut n
flopCounts = concat \$ niceParallelMap 50
(map countFlops . concatMap (perm 2) . concatMap (perm 3) . perm 4) permutations
(mflops, checksum) = calculateMaxAndChecksum flopCounts
putStrLn \$ show checksum ++ "\nPfannkuchen(" ++ show n ++ ") = " ++ show mflops
```

notes, command-line, and program output

```NOTES:
Compilation System,
version 9.4.4
LLVM version: 13.0.1

Mon, 30 Jan 2023 20:02:36 GMT

MAKE:
mv fannkuchredux.ghc-2.ghc fannkuchredux.ghc-2.hs
~/.ghcup/ghc/9.4.4/bin/ghc --make -fllvm -O2 -XBangPatterns -threaded -rtsopts -XScopedTypeVariables fannkuchredux.ghc-2.hs -o fannkuchredux.ghc-2.ghc_run