#include <simloop.h>

#include <assert.h>

static uint64_t ddt_from_fps(int fps) {
  static constexpr double NANOSECONDS = 1e9;
  return (fps == 0) ? 0 : (uint64_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),
                                  .last_step = args->timer->start_time},
      .render = (SimloopTimeline){  .ddt = ddt_from_fps(args->max_render_fps),
                                  .last_step = args->timer->start_time},
      .timer  = args->timer,
  };
}

static time_delta time_elapsed(const Simloop* sim, time_point t) {
  assert(sim);
  return time_diff(sim->timer->start_time, t);
}

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

  const time_delta dt = time_diff(timeline->last_step, sim->timer->last_tick);
  const time_delta steps = dt / timeline->ddt;
  timeline->last_step    = time_add(timeline->last_step, dt);
  return (int)steps;
}

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) > 0;
  } else {
    timeline->last_step = sim->timer->last_tick;
    render              = true;
  }
  return render;
}

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

  const int new_frames = step_update(sim, &sim->update);
  out->updates_pending = new_frames;
  out->should_render =
      step_render(sim, &sim->render) ||
      (sim->frame == 0); // Trigger an initial render on the first frame.
  sim->frame += new_frames;
  out->frame          = sim->frame;
  out->render_elapsed = time_elapsed(sim, sim->render.last_step);
  out->update_elapsed = time_elapsed(sim, sim->update.last_step);
  out->update_dt      = sim->update.ddt;
}