Explicitly test for canonicity

This will come in handy later, with more complex test cases.

src/include/interval-map/interval-map.hh

@@ -2,6 +2,9 @@
 
 #include <map>
 
+// Testing class forward declaration
+class IntervalMapTest;
+
 namespace amby {
 
 template <typename K, typename V> class interval_map {
@@ -21,6 +24,9 @@ public:
         return std::prev(it)->second;
     }
 
+    // Used in testing
+    friend class ::IntervalMapTest;
+
 private:
     V init_;
     std::map<K, V> underlying_{};

tests/unit/unit_test.cc

@@ -69,6 +69,27 @@ protected:
     void SetUp() override {
         map = map_type{0};
     }
+
+    void TearDown() override {
+        check_canonicity();
+    }
+
+    void check_canonicity() const {
+        // Consecutive map entries must not contain the same value
+        for (auto it = map.underlying_.begin(); it != map.underlying_.end();
+             ++it) {
+            const auto next = std::next(it, 1);
+            if (next == map.underlying_.end())
+                break;
+            EXPECT_NE(it->second, next->second);
+        }
+
+        // The first entry must not contain the initial value
+        if (const auto it = map.underlying_.begin();
+            it != map.underlying_.end()) {
+            EXPECT_NE(it->second, map.init_);
+        }
+    }
 };
 
 TEST_F(IntervalMapTest, minimal_interface) {