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Task/Permutations/Nim/permutations.nim

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+# iterative Boothroyd method
+iterator permutations[T](ys: openarray[T]): seq[T] =
+  var
+    d = 1
+    c = newSeq[int](ys.len)
+    xs = newSeq[T](ys.len)
+
+  for i, y in ys: xs[i] = y
+  yield xs
+
+  block outer:
+    while true:
+      while d > 1:
+        dec d
+        c[d] = 0
+      while c[d] >= d:
+        inc d
+        if d >= ys.len: break outer
+
+      let i = if (d and 1) == 1: c[d] else: 0
+      swap xs[i], xs[d]
+      yield xs
+      inc c[d]
+
+var x = @[1,2,3]
+
+for i in permutations(x):
+  echo i