coord/comparator: add a trivial ordering comparator

CMakeLists.txt

@@ -258,9 +258,10 @@ list (
     cmdopt.hpp
     colour.cpp
     colour.hpp
+    coord.hpp
     coord/fwd.hpp
     coord/base.hpp
-    coord.hpp
+    coord/comparator.hpp
     coord/init.hpp
     coord/iostream.hpp
     coord/ops.hpp

coord/comparator.hpp

@@ -0,0 +1,38 @@
+/*
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/.
+ *
+ * Copyright 2019 Danny Robson <danny@nerdcruft.net>
+ */
+
+#pragma once
+
+#include "base.hpp"
+
+#include <cstddef>
+
+
+///////////////////////////////////////////////////////////////////////////////
+namespace cruft::coord {
+    /// A StrictWeakOrder over coordinate types.
+    ///
+    /// The comparison operates across successive elements of the two
+    /// coordinates.
+    template <typename CoordT>
+    struct ordering {
+        bool operator() (
+            CoordT const &a,
+            CoordT const &b
+        ) const noexcept {
+            for (std::size_t i = 0; i < CoordT::elements; ++i) {
+                if (a[i] < b[i])
+                    return true;
+                else if (a[i] > b[i])
+                    return false;
+            }
+
+            return false;
+        }
+    };
+}

test/coord.cpp

@@ -3,6 +3,7 @@
 #include "vector.hpp"
 
 #include "coord/iostream.hpp"
+#include "coord/comparator.hpp"
 
 #include <type_traits>
 
@@ -118,7 +119,26 @@ main (void)
             cruft::make_vector (3, 4, 0),
             "rshift, coord fill"
         );
-    };
+    }
+
+    {
+        struct {
+            cruft::point2i a;
+            cruft::point2i b;
+            bool expected;
+            char const *message;
+        } const TESTS[] {
+            { { 0, 0 }, { 0, 0 }, false, "equal zeroes" },
+            { { 1, 0 }, { 0, 0 }, false, "a, leading high" },
+            { { 0, 1 }, { 0, 0 }, false, "a, trailing high" },
+            { { 0, 0 }, { 1, 0 }, true,  "b, leading high" },
+            { { 0, 0 }, { 0, 1 }, true,  "b, trailing high" },
+        };
+
+        cruft::coord::ordering<cruft::point2i> const comp;
+        for (auto const &t: TESTS)
+            tap.expect (comp (t.a, t.b) == t.expected, "ordering; %!", t.message);
+    }
 
     return tap.status ();
 }