From 43e44bc35b18c7e6938d8b5103ae8a76dd06f5a0 Mon Sep 17 00:00:00 2001
From: Felix Morgner <felix.morgner@gmail.com>
Date: Tue, 22 Sep 2026 16:27:21 +0200
Subject: lib/routers: introduce router concept

---
 CMakeLists.txt                     |  1 +
 ttwhy/routers/echo.tests.cpp       | 24 ++++++++++++++++++++++++
 ttwhy/routers/mod.cppm             |  1 +
 ttwhy/routers/router_concepts.cppm | 20 ++++++++++++++++++++
 4 files changed, 46 insertions(+)
 create mode 100644 ttwhy/routers/router_concepts.cppm

diff --git a/CMakeLists.txt b/CMakeLists.txt
index 84d89a3..4228310 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -60,6 +60,7 @@ target_sources("ttwhy-core" PUBLIC
 
   "ttwhy/routers/echo.cppm"
   "ttwhy/routers/mod.cppm"
+  "ttwhy/routers/router_concepts.cppm"
 
   "ttwhy/scanners/mod.cppm"
   "ttwhy/scanners/terminal_concepts.cppm"
diff --git a/ttwhy/routers/echo.tests.cpp b/ttwhy/routers/echo.tests.cpp
index f12d08b..c6ea10a 100644
--- a/ttwhy/routers/echo.tests.cpp
+++ b/ttwhy/routers/echo.tests.cpp
@@ -45,8 +45,32 @@ namespace
           token, buffers);
     }
   };
+
+  struct incomplete_router_no_policy
+  {
+    auto process(ttwhy::scanners::input_event) -> asio::awaitable<void>;
+  };
+
+  struct incomplete_router_no_process
+  {
+    using terminal_policy = ttwhy::scanners::xterm_policy;
+  };
+
+  struct incomplete_router_wrong_return
+  {
+    using terminal_policy = ttwhy::scanners::xterm_policy;
+    auto process(ttwhy::scanners::input_event) -> void;
+  };
 }  // namespace
 
+TEST_CASE("The router concept validates router candidate interfaces", "[router][concept]")
+{
+  STATIC_REQUIRE(ttwhy::router<ttwhy::routers::echo<mock_stream>>);
+  STATIC_REQUIRE_FALSE(ttwhy::router<incomplete_router_no_policy>);
+  STATIC_REQUIRE_FALSE(ttwhy::router<incomplete_router_no_process>);
+  STATIC_REQUIRE_FALSE(ttwhy::router<incomplete_router_wrong_return>);
+}
+
 SCENARIO("The echo router handles terminal navigation within the valid printed area", "[router][echo]")
 {
   GIVEN("An initialized echo router with a mock stream")
diff --git a/ttwhy/routers/mod.cppm b/ttwhy/routers/mod.cppm
index fbab282..f5e151b 100644
--- a/ttwhy/routers/mod.cppm
+++ b/ttwhy/routers/mod.cppm
@@ -1,3 +1,4 @@
 export module ttwhy.routers;
 
 export import :echo;
+export import :router_concepts;
diff --git a/ttwhy/routers/router_concepts.cppm b/ttwhy/routers/router_concepts.cppm
new file mode 100644
index 0000000..ce143cb
--- /dev/null
+++ b/ttwhy/routers/router_concepts.cppm
@@ -0,0 +1,20 @@
+module;
+
+#include <asio.hpp>
+
+#include <concepts>
+
+export module ttwhy.routers:router_concepts;
+
+import ttwhy.scanners;
+
+namespace ttwhy
+{
+
+  export template<typename Candidate>
+  concept router = requires(Candidate candidate, scanners::input_event event) {
+    typename Candidate::terminal_policy;
+    { candidate.process(event) } -> std::same_as<asio::awaitable<void>>;
+  };
+
+}  // namespace ttwhy
-- 
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