Add vec::permute to the standard library (#1013)

src/lib/vec.rs

@@ -734,6 +734,31 @@ fn riter2<T>(v: [mutable? T], f: block(uint, T)) {
     };
 }
 
+/*
+Function: permute
+
+Iterate over all permutations of vector `v`.  Permutations are produced in
+lexicographic order with respect to the order of elements in `v` (so if `v`
+is sorted then the permutations are lexicographically sorted).
+
+The total number of permutations produced is `len(v)!`.  If `v` contains
+repeated elements, then some permutations are repeated.
+*/
+fn permute<T>(v: [mutable? T], put: block([T])) {
+  let ln = len(v);
+  if ln == 0u {
+    put([]);
+  } else {
+    let i = 0u;
+    while i < ln {
+      let elt = v[i];
+      let rest = slice(v, 0u, i) + slice(v, i+1u, ln);
+      permute(rest) {|permutation| put([elt] + permutation)}
+      i += 1u;
+    }
+  }
+}
+
 /*
 Function: to_ptr
 

src/test/stdtest/vec.rs

@@ -366,6 +366,27 @@ fn riter2() {
     assert i == 3;
 }
 
+#[test]
+fn test_permute() {
+  let results: [[int]];
+
+  results = [];
+  permute([]) {|v| results += [v]; }
+  assert results == [[]];
+
+  results = [];
+  permute([7]) {|v| results += [v]; }
+  assert results == [[7]];
+
+  results = [];
+  permute([1,1]) {|v| results += [v]; }
+  assert results == [[1,1],[1,1]];
+
+  results = [];
+  permute([5,2,0]) {|v| results += [v]; }
+  assert results == [[5,2,0],[5,0,2],[2,5,0],[2,0,5],[0,5,2],[0,2,5]];
+}
+
 #[test]
 fn test_any_and_all() {
     assert (vec::any(is_three, [1u, 2u, 3u]));