First cut of pulling out the relevant parts of my original game to make a little framework.

tests/systems.cpp

@@ -0,0 +1,36 @@
+#include <catch2/catch_test_macros.hpp>
+#include <fmt/core.h>
+#include "game/systems.hpp"
+#include <cmath>
+#include <numbers>
+
+
+TEST_CASE("figure out best rotation direction", "[systems-rotate]") {
+  Matrix map = matrix::make(3, 3);
+
+  Point player_at{1, 1};
+  map[player_at.y][player_at.x] = 2;
+
+
+  for(matrix::box target{map, player_at.x, player_at.y, 1}; target.next();) {
+    for(matrix::box aiming_at{map, player_at.x, player_at.y, 1}; aiming_at.next();) {
+      map[aiming_at.y][aiming_at.x] = 10;
+
+      float target_dx = float(player_at.x) - float(target.x);
+      float target_dy = float(player_at.y) - float(target.y);
+      float aiming_dx = float(player_at.x) - float(aiming_at.x);
+      float aiming_dy = float(player_at.y) - float(aiming_at.y);
+
+      float target_angle = atan2(-target_dy, target_dx) * (180.0 / std::numbers::pi);
+      float aiming_angle = atan2(-aiming_dy, aiming_dx) * (180.0 / std::numbers::pi);
+
+      float diff = target_angle - aiming_angle;
+      double normalized = fmod(diff + 360.0, 360.0);
+
+      REQUIRE(normalized >= 0);
+      REQUIRE(normalized <= 360);
+
+      map[aiming_at.y][aiming_at.x] = 0;
+    }
+  }
+}