Data commit

Task/Permutations-by-swapping/F-Sharp/permutations-by-swapping-1.fs

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+(*Implement Johnson-Trotter algorithm
+  Nigel Galloway January 24th 2017*)
+module Ring
+let PlainChanges (N:'n[]) = seq{
+  let gn  = [|for n in N -> 1|]
+  let ni  = [|for n in N -> 0|]
+  let gel = Array.length(N)-1
+  yield N
+  let rec _Ni g e l = seq{
+    match (l,g) with
+    |_ when l<0   -> gn.[g] <- -gn.[g]; yield! _Ni (g-1) e (ni.[g-1] + gn.[g-1])
+    |(1,0)        -> ()
+    |_ when l=g+1 -> gn.[g] <- -gn.[g]; yield! _Ni (g-1) (e+1) (ni.[g-1] + gn.[g-1])
+    |_ -> let n = N.[g-ni.[g]+e];
+          N.[g-ni.[g]+e] <- N.[g-l+e]; N.[g-l+e] <- n; yield N
+          ni.[g] <- l; yield! _Ni gel 0 (ni.[gel] + gn.[gel])}
+  yield! _Ni gel 0 1
+}

Task/Permutations-by-swapping/F-Sharp/permutations-by-swapping-2.fs

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+for n in Ring.PlainChanges [|1;2;3;4|] do printfn "%A" n