Initial data commit

Task/Permutations/Nim/permutations-3.nim

@@ -0,0 +1,31 @@
+import intsets
+from math import fac
+block:
+  # test all permutations of length from 0 to 9
+  for l in 0..9:
+
+    # prepare data
+    var xs = newSeq[int](l)
+    for i in 0..<l: xs[i] = i
+    var s = initIntSet()
+
+    for cs in inplacePermutations(xs):
+
+      # each permutation must be of length l
+      assert len(cs) == l
+
+      # each permutation must contain digits from 0 to l-1 exactly once
+      var ds = newSeq[bool](l)
+      for c in cs:
+        assert not ds[c]
+        ds[c] = true
+
+      # generate a unique number for each permutation
+      var h = 0
+      for e in cs:
+        h = l * h + e
+      assert not s.contains(h)
+      s.incl(h)
+
+    # check exactly l! unique number of permutations
+    assert len(s) == fac(l)