Filtered visitor (#18)

New `visit()` overload taking a filter.

Closes #17.

test/visitor.cpp

@@ -0,0 +1,86 @@
+#include <cppast/cpp_entity.hpp>
+using namespace cppast;
+
+#include "test_parser.hpp"
+#include <iostream>
+
+TEST_CASE("visitor_filtered")
+{
+    auto code = R"(
+        namespace the_ns {
+            class foo {
+                enum inner_enum {};
+                class bar {};
+            };
+            class one {}; class two {}; class three {};
+            enum quaz {};
+        }
+        enum outer {};
+    )";
+
+    cpp_entity_index idx;
+    auto             file           = parse(idx, "cpp_class.cpp", code);
+    unsigned         filtered_count = 0;
+    auto visitor_callback           = [&](const cpp_entity&, cppast::visitor_info info) {
+        if (info.event != cppast::visitor_info::container_entity_exit)
+            ++filtered_count;
+        return true;
+    };
+
+    constexpr auto all_node_count = 10, enum_count = 3, class_count = 5;
+
+    SECTION("all nodes are visited")
+    {
+        filtered_count = 0;
+        cppast::visit(*file, [](const cpp_entity&) { return true; }, visitor_callback);
+        REQUIRE(filtered_count == all_node_count);
+    }
+
+    SECTION("filtered callback on both enter and exit")
+    {
+        filtered_count = 0;
+        cppast::visit(*file, [](const cpp_entity&) { return true; },
+                      [&](const cpp_entity&, cppast::visitor_info info) {
+                          (void)info;
+                          filtered_count++;
+                          return true;
+                      });
+        REQUIRE(filtered_count == all_node_count * 2);
+    }
+
+    SECTION("whitelist")
+    {
+        SECTION("only one kind whitelisted")
+        {
+            filtered_count = 0;
+            cppast::visit(*file, whitelist<cpp_entity_kind::enum_t>(), visitor_callback);
+            REQUIRE(filtered_count == enum_count);
+        }
+
+        SECTION("many kinds whitelisted")
+        {
+            filtered_count = 0;
+            cppast::visit(*file, whitelist<cpp_entity_kind::enum_t, cpp_entity_kind::class_t>(),
+                          visitor_callback);
+            REQUIRE(filtered_count == enum_count + class_count);
+        }
+    }
+
+    SECTION("blacklist")
+    {
+        SECTION("only one kind blacklisted")
+        {
+            filtered_count = 0;
+            cppast::visit(*file, blacklist<cpp_entity_kind::file_t>(), visitor_callback);
+            REQUIRE(filtered_count == all_node_count - 1);
+        }
+
+        SECTION("many kinds blacklisted")
+        {
+            filtered_count = 0;
+            cppast::visit(*file, blacklist<cpp_entity_kind::enum_t, cpp_entity_kind::class_t>(),
+                          visitor_callback);
+            REQUIRE(filtered_count == all_node_count - enum_count - class_count);
+        }
+    }
+}