## source code

```{-  The Computer Language Benchmarks Game
https://salsa.debian.org/benchmarksgame-team/benchmarksgame/
contributed by Miha Vučkovič
-}

import System.Environment
import Control.Applicative

flop (2:x1:t) = x1:2:t
flop (3:x1:x2:t) = x2:x1:3:t
flop (4:x1:x2:x3:t) = x3:x2:x1:4:t
flop (5:x1:x2:x3:x4:t) = x4:x3:x2:x1:5:t
flop (6:x1:x2:x3:x4:x5:t) = x5:x4:x3:x2:x1:6:t
flop (7:x1:x2:x3:x4:x5:x6:t) = x6:x5:x4:x3:x2:x1:7:t

flop lst@(h:_) = r where
(t, r) = flop' h (lst, t)
flop' 0 (t, r) = (t, r)
flop' n ((h:t), r) = flop' (n-1) (t, h:r)

flopS (1:_) = 0
flopS lst = 1 + flopS (flop lst)

rotate n (h:t) = rotate' (n-1) t where
rotate' 0 l = h:l
rotate' n (f:t) = f:(rotate' (n-1) t)

checksum i f
| mod i 2 == 0 = f
| True = -f

pfold r [] = r
pfold (ac, af) ((c, f):t)  = seq sc \$ seq sf \$ pfold (sc, sf) t where
sc = ac+c
sf = max af f

permut n = foldr perm [[1..n]] [2..n] where
perm x lst = concat [take x \$ iterate (rotate x) l | l <- lst]

main = do
let (chksm, mflops) = pfold (0,0) \$ map (\(i, p) -> let flops = flopS p in (checksum i flops, flops)) \$ zip [0..] (permut n)
putStrLn \$ (show chksm) ++ "\nPfannkuchen(" ++ (show n) ++ ") = " ++ (show \$ mflops)
```

## notes, command-line, and program output

```NOTES:
The Glorious Glasgow Haskell Compilation System,
version 8.10.1

Sat, 24 Oct 2020 23:50:21 GMT

MAKE:
mv fannkuchredux.ghc fannkuchredux.hs
/opt/src/ghc-8.10.1/bin/ghc --make -fllvm -O2 -XBangPatterns -threaded -rtsopts -XScopedTypeVariables fannkuchredux.hs -o fannkuchredux.ghc_run
[1 of 1] Compiling Main             ( fannkuchredux.hs, fannkuchredux.o )

fannkuchredux.hs:17:1: warning: [-Wtabs]
Tab character found here, and in six further locations.
|
17 |         (t, r) = flop' h (lst, t)
| ^^^^^^^^

<no location info>: error:
Warning: Couldn't figure out LLVM version!
Make sure you have installed LLVM 9