Files are now in a src directory and I'm using a src/meson.build and tests/meson.build to specify what to build.

src/simplefsm.hpp

@@ -0,0 +1,32 @@
+#pragma once
+
+#include <fmt/core.h>
+
+#ifndef FSM_DEBUG
+#define FSM_STATE(C, S, E, ...) case C::S: S(E, ##__VA_ARGS__); break
+#else
+static int last_event=-1;
+#define FSM_STATE(C, S, E, ...) case C::S: if(last_event != int(E)) { last_event = int(E); fmt::println(">> " #C " " #S " event={}, state={}", int(E), int($state));}; S(E, ##__VA_ARGS__); break
+#endif
+
+template<typename S, typename E>
+class DeadSimpleFSM {
+protected:
+  // BUG: don't put this in your class because state() won't work
+ S $state = S::START;
+
+public:
+  template<typename... Types>
+  void event(E event, Types... args);
+
+  void state(S next_state) {
+#ifdef FSM_DEBUG
+    fmt::println("<< STATE: {} -> {}", int($state), int(next_state));
+#endif
+    $state = next_state;
+  }
+
+  bool in_state(S state) {
+    return $state == state;
+  }
+};