algo/sort: add explicit index sorting routine

algo/sort.hpp

@@ -89,6 +89,27 @@ namespace cruft::sort {
         }
     }
 
+
+    ///////////////////////////////////////////////////////////////////////////
+    /// Fill a supplied index buffer with a sorted range of indices over an
+    /// identically size collection of data elements addressed by the
+    /// RandomIterator `data`.
+    ///
+    /// Does not modify any data element pointed to by `data`.
+    template <typename IndexT, typename RandomT>
+    void
+    indices (IndexT idx_first, IndexT idx_last, RandomT data)
+    {
+        // 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);
+        });
+    }
+
+
     ///////////////////////////////////////////////////////////////////////////
     // sort an array specified by the iterators key_first:key_last using a
     // comparator, and optionally a series of additional value iterators