September Morn Update

Task/Permutations/Nim/permutations-2.nim

@@ -0,0 +1,38 @@
+# nim implementation of the (very fast) Go example
+# http://rosettacode.org/wiki/Permutations#Go
+# implementing a recursive https://en.wikipedia.org/wiki/Steinhaus–Johnson–Trotter_algorithm
+
+proc perm( s: openArray[int], emit: proc(emit:openArray[int]) ) =
+    var s = @s
+    if s.len == 0:
+        emit(s)
+        return
+
+    var rc : proc(np: int)
+    rc = proc(np: int) =
+
+        if np == 1:
+            emit(s)
+            return
+
+        var
+            np1 = np - 1
+            pp = s.len - np1
+
+        rc(np1) # recurs prior swaps
+
+        for i in countDown(pp, 1):
+            swap s[i], s[i-1]
+            rc(np1) # recurs swap
+
+        let w = s[0]
+        s[0..<pp] = s[1..pp]
+        s[pp] = w
+
+    rc(s.len)
+
+var se = @[0, 1, 2, 3] #, 4, 5, 6, 7, 8, 9, 10]
+
+perm(se, proc(seq: openArray[int])=
+    echo seq
+    )