#include <simloop.h>

#include <assert.h>

static simloop_time_t ddt_from_fps(int fps) {
  static constexpr double NANOSECONDS = 1e9;
  return (fps == 0) ? 0 : (simloop_time_t)(NANOSECONDS / (double)fps);
}

Simloop simloop_make(const SimloopArgs* args) {
  assert(args);
  assert(args->update_fps > 0);

  return (Simloop){
      .frame = 0,
      .update =
          (SimloopTimeline){
                            .ddt  = ddt_from_fps(args->update_fps),
                            .time = 0,
                            },
      .render =
          (SimloopTimeline){
                            .ddt  = ddt_from_fps(args->max_render_fps),
                            .time = 0,
                            },
      .first_iter                = true,
      .updates_since_last_render = false,
  };
}

static bool step_update(const Simloop* sim, SimloopTimeline* timeline) {
  assert(sim);
  assert(timeline);
  assert(timeline->ddt > 0);

  const simloop_time_t dt          = sim->clock - timeline->time;
  const bool           should_step = dt >= timeline->ddt;
  timeline->time += should_step ? timeline->ddt : 0;
  return should_step;
}

static bool step_render(const Simloop* sim, SimloopTimeline* timeline) {
  assert(sim);
  assert(timeline);

  bool render = false;
  if (timeline->ddt > 0) {
    render = step_update(sim, timeline);
  } else {
    timeline->time = sim->clock;
    render         = true;
  }
  return render;
}

void simloop_update(Simloop* sim, simloop_time_t dt, SimloopOut* out) {
  assert(sim);
  assert(out);

  sim->clock += dt;

  // Simulation update.
  const bool updated             = step_update(sim, &sim->update);
  sim->updates_since_last_render = sim->updates_since_last_render || updated;

  // Simulation render.
  const bool rendered =
      (sim->updates_since_last_render && step_render(sim, &sim->render)) ||
      (sim->first_iter); // Trigger an initial render on the first frame.
  sim->updates_since_last_render =
      sim->updates_since_last_render && !out->should_render;

  // Loop state update.
  sim->frame += (updated ? 1 : 0);
  sim->first_iter = false;

  out->frame          = sim->frame;
  out->render_elapsed = sim->render.time;
  out->update_elapsed = sim->update.time;
  out->update_dt      = sim->update.ddt;
  out->should_update  = updated;
  out->should_render  = rendered;
}