Create a test for the ECS before we add more functionality.

tests/dinkyecs.cpp

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+#include "dinkyecs.hpp"
+
+#include <iostream>
+#include <fmt/core.h>
+
+using namespace fmt;
+using DinkyECS::Entity;
+
+struct Player {
+  std::string name;
+  Entity eid;
+};
+
+struct Position {
+  double x, y;
+};
+
+struct Velocity {
+  double x, y;
+};
+
+struct Gravity {
+  double level;
+};
+
+/*
+ * Using a function catches instances where I'm not copying
+ * the data into the world.
+ */
+void configure(DinkyECS::World &world, Entity &test) {
+  println("---Configuring the base system.");
+  Entity test2 = world.entity();
+
+  world.set<Position>(test, {10,20});
+  world.set<Velocity>(test, {1,2});
+
+  world.set<Position>(test2, {1,1});
+  world.set<Velocity>(test2, {10,20});
+
+  println("---- Setting up the player as a fact in the system.");
+
+  auto player_eid = world.entity();
+  Player player_info{"Zed", player_eid};
+  // just set some player info as a fact with the entity id
+  world.set_the<Player>(player_info);
+
+  world.set<Velocity>(player_eid, {0,0});
+  world.set<Position>(player_eid, {0,0});
+
+  auto enemy = world.entity();
+  world.set<Velocity>(enemy, {0,0});
+  world.set<Position>(enemy, {0,0});
+
+  println("--- Creating facts (singletons)");
+  world.set_the<Gravity>({0.9});
+}
+
+int main() {
+  DinkyECS::World world;
+  Entity test = world.entity();
+
+  configure(world, test);
+
+  Position &pos = world.get<Position>(test);
+  println("GOT POS x={}, y={}", pos.x, pos.y);
+
+  Velocity &vel = world.get<Velocity>(test);
+  println("GOT VELOCITY x={}, y={}", vel.x, vel.y);
+
+  println("--- Position only system:");
+  world.query<Position>([](const auto &ent, auto &pos) {
+      println("entity={}, pos.x={}, pos.y={}", ent, pos.x, pos.y);
+  });
+
+  println("--- Velocity only system:");
+  world.query<Velocity>([](const auto &, auto &vel) {
+      println("vel.x={}, vel.y={}", vel.x, vel.y);
+  });
+
+  println("--- Manually get the velocity in position system:");
+  world.query<Position>([&](const auto &ent, auto &pos) {
+      Velocity &vel = world.get<Velocity>(ent);
+      println("entity={}, vel.x, vel.y, pos.x={}, pos.y={}", ent, vel.x, vel.y, pos.x, pos.y);
+  });
+
+  println("--- Query only entities with Position and Velocity:");
+  world.query<Position, Velocity>([&](const auto &ent, auto &pos, auto &vel) {
+      Gravity &grav = world.get_the<Gravity>();
+      println("grav={}, entity={}, vel.x, vel.y, pos.x={}, pos.y={}", grav.level, ent, vel.x, vel.y, pos.x, pos.y);
+  });
+
+  // now remove Velocity
+  world.remove<Velocity>(test);
+
+  println("--- After remove test, should only result in test2:");
+  world.query<Position, Velocity>([&](const auto &ent, auto &pos, auto &vel) {
+      println("entity={}, vel.x, vel.y, pos.x={}, pos.y={}", ent, vel.x, vel.y, pos.x, pos.y);
+  });
+
+  // to avoid repeatedly getting the player just make a closure with it
+  // QUESTION: could I just capture it and not have the double function wrapping?
+  auto playerVsEnemies = [&]() {
+    auto& player = world.get_the<Player>(); // grabbed it
+    world.query<Position>([&](const auto &ent, auto &pos) {
+      if(player.eid != ent) {
+        println("{} is enemy attacking player {}", ent, player.name);
+      } else {
+        println("{} is player", player.name);
+      }
+    });
+  };
+
+  playerVsEnemies();
+
+  return 0;
+}