September Morn Update

Task/Permutations/Modula-3/permutations-1.mod3

@@ -0,0 +1,58 @@
+MODULE Permutations EXPORTS Main;
+
+IMPORT IO, IntSeq;
+
+CONST n = 3;
+
+TYPE Domain = SET OF [ 1.. n ];
+
+VAR
+
+  chosen: IntSeq.T;
+  values := Domain { };
+
+PROCEDURE GeneratePermutations(VAR chosen: IntSeq.T; remaining: Domain) =
+(*
+  Recursively generates all the permutations of elements
+  in the union of "chosen" and "values".
+  Values in "chosen" have already been chosen;
+  values in "remaining" can still be chosen.
+  If "remaining" is empty, it prints the sequence and returns.
+  Otherwise, it picks each element in "remaining", removes it,
+  adds it to "chosen", recursively calls itself,
+  then removes the last element of "chosen" and adds it back to "remaining".
+*)
+BEGIN
+  FOR i := 1 TO n DO
+    (* check if each element is in "remaining" *)
+    IF i IN remaining THEN
+      (* if so, remove from "remaining" and add to "chosen" *)
+      remaining := remaining - Domain { i };
+      chosen.addhi(i);
+      IF remaining # Domain { } THEN
+        (* still something to process? do it *)
+        GeneratePermutations(chosen, remaining);
+      ELSE
+        (* otherwise, print what we've chosen *)
+        FOR j := 0 TO chosen.size() - 2 DO
+          IO.PutInt(chosen.get(j)); IO.Put(", ");
+        END;
+        IO.PutInt(chosen.gethi());
+        IO.PutChar('\n');
+      END;
+      (* add "i" back to "remaining" and remove from "chosen" *)
+      remaining := remaining + Domain { i };
+      EVAL chosen.remhi();
+    END;
+  END;
+END GeneratePermutations;
+
+BEGIN
+
+  (* initial setup *)
+  chosen := NEW(IntSeq.T).init(n);
+  FOR i := 1 TO n DO values := values + Domain { i }; END;
+
+  GeneratePermutations(chosen, values);
+
+END Permutations.