September Morn Update

Task/Permutations/Modula-3/permutations-2.mod3

@@ -0,0 +1,28 @@
+GENERIC INTERFACE GenericPermutations(DomainSeq, DomainSet, DomainSeqSeq);
+
+(*
+  "Domain" is where the things to permute come from (unused in interface).
+  "DomainSeq" is a "Sequence" of "Domain".
+  "DomainSet" is a "Set" of "Domain".
+  "DomainSeqSeq" is a "Sequence" of "DomainSeq".
+*)
+
+PROCEDURE GeneratePermutations(
+  READONLY chosen: DomainSeq.T;
+  READONLY remaining: DomainSet.T;
+  READONLY result: DomainSeqSeq.T
+);
+(*
+  Recursively generates all the permutations of elements
+  in the union of "chosen" and "remaining".
+  Values in "chosen" have already been chosen;
+  values in "remaining" can still be chosen.
+  If "remaining" is empty, it adds the permutation to "result".
+  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".
+  Although the parameters are modified, we can describe them as "READONLY"
+  because we do not re-assign them.
+*)
+
+END GenericPermutations.