diff --git a/caching/mapping_array.cc b/caching/mapping_array.cc
index c26460e..93e2d09 100644
--- a/caching/mapping_array.cc
+++ b/caching/mapping_array.cc
@@ -26,3 +26,49 @@ mapping_traits::pack(value_type const &value, disk_type &disk)
 }
 
 //----------------------------------------------------------------
+
+missing_mappings::missing_mappings(run<uint32_t> const &keys,
+				   std::string const &desc)
+	: keys_(keys),
+	  desc_(desc)
+{
+
+}
+
+void
+missing_mappings::visit(damage_visitor &v) const
+{
+	v.visit(*this);
+}
+
+namespace {
+	struct no_op_visitor {
+		virtual void visit(uint32_t index,
+				   mapping_traits::value_type const &v) {
+		}
+	};
+
+	class ll_damage_visitor {
+	public:
+		ll_damage_visitor(damage_visitor &v)
+		: v_(v) {
+		}
+
+		virtual void visit(array_detail::damage const &d) {
+			v_.visit(missing_mappings(d.lost_keys_, d.desc_));
+		}
+
+	private:
+		damage_visitor &v_;
+	};
+}
+
+void
+caching::check_mapping_array(mapping_array const &array, damage_visitor &visitor)
+{
+	no_op_visitor vv;
+	ll_damage_visitor ll(visitor);
+	array.visit_values(vv, ll);
+}
+
+//----------------------------------------------------------------
diff --git a/caching/mapping_array.h b/caching/mapping_array.h
index 030ec4f..9d22b26 100644
--- a/caching/mapping_array.h
+++ b/caching/mapping_array.h
@@ -26,10 +26,37 @@ namespace caching {
 			static void pack(value_type const &value, disk_type &disk);
 		};
 
-		// FIXME: damage visitor stuff
+		class damage_visitor;
+
+		struct damage {
+			virtual ~damage() {}
+			virtual void visit(damage_visitor &v) const = 0;
+		};
+
+		struct missing_mappings : public damage {
+			missing_mappings(run<uint32_t> const &keys, std::string const &desc);
+			virtual void visit(damage_visitor &v) const;
+
+			run<uint32_t> keys_;
+			std::string desc_;
+		};
+
+		class damage_visitor {
+		public:
+			virtual ~damage_visitor() {}
+
+			virtual void visit(mapping_array_detail::damage const &d) {
+				d.visit(*this);
+			}
+
+			virtual void visit(missing_mappings const &d) = 0;
+		};
 	}
 
 	typedef persistent_data::array<mapping_array_detail::mapping_traits> mapping_array;
+
+	void check_mapping_array(mapping_array const &array,
+				 mapping_array_detail::damage_visitor &visitor);
 }
 
 //----------------------------------------------------------------