sort: use a comparator object for index sort

algo/sort.hpp

@@ -96,16 +96,27 @@ namespace cruft::sort {
     /// RandomIterator `data`.
     ///
     /// Does not modify any data element pointed to by `data`.
-    template <typename IndexT, typename RandomT>
+    template <
+        typename IndexT,
+        typename RandomT,
+        typename ComparatorT = std::less<>
+    >
     void
-    indices (IndexT idx_first, IndexT idx_last, RandomT data)
-    {
+    indices (
+        IndexT idx_first,
+        IndexT idx_last,
+        RandomT data,
+        ComparatorT &&cmp
+    ) {
         // initialise a monotonic sequence of indices
         std::iota (idx_first, idx_last, 0);
 
         // sort using the indices
         std::sort (idx_first, idx_last, [&] (auto a, auto b) {
-            return *(data + a) < *(data + b);
+            return cmp (
+                *(data + a),
+                *(data + b)
+            );
         });
     }