September 2017 Update

Task/Permutations-by-swapping/Phix/permutations-by-swapping.phix

@@ -0,0 +1,23 @@
+function spermutations(integer p, integer i)
+-- generate the i'th permutation of [1..p]:
+-- first obtain the appropriate permutation of [1..p-1],
+-- then insert p/move it down k(=0..p-1) places from the end.
+    integer k = mod(i-1,2*p)
+    if k>=p then k=2*p-1-k  end if
+    sequence res
+    integer parity
+    if p>1 then
+        {res,parity} = spermutations(p-1,floor((i-1)/p)+1)
+        res = res[1..length(res)-k]&p&res[length(res)-k+1..$]
+    else
+        res = {1}
+    end if
+    return {res,iff(and_bits(i,1)?1:-1)}
+end function
+
+for p=1 to 4 do
+    printf(1,"==%d==\n",p)
+    for i=1 to factorial(p) do
+        ?{i,spermutations(p,i)}
+    end for
+end for