#include <simloop.h>

#include <assert.h>

static double min(double a, double b) { return a <= b ? a : b; }

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_ddt = ddt_from_fps(args->max_render_fps),
  };
}

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

  sim->clock += dt;

  // Simulation update.
  // If the simulation falls behind the clock, we advance by a single ddt
  // increment per loop iteration here and give it a chance to catch up over
  // subsequent iterations.
  // This has the implication that percent_frame can fall out of range (>1) if
  // we are not careful with how it is defined. See the logic below.
  // If the delta is too large, then we simply warp the simulation to the wall
  // clock. This avoids the appearance of the simulation playing in fast-forward
  // as it tries to catch up. Large time spikes can typically occur at the start
  // of the simulation when the application loads assets, compiles shaders, etc.
  static const uint64_t max_catchup_frames = 10;
  const simloop_time_t  delta              = sim->clock - sim->update.time;
  const uint64_t        delta_frames       = delta / sim->update.ddt;
  const bool            update_this_tick   = delta >= sim->update.ddt;
  const bool            warp               = delta_frames > max_catchup_frames;
  sim->update.time =
      warp ? sim->clock
           : (sim->update.time + (update_this_tick ? sim->update.ddt : 0));

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

  // Interpolator for smooth animation.
  // If the update falls behind the clock, then percent_frame can fall out of
  // range (>1) if we are not careful. We impose that it is strictly never >1
  // to account for this case.
  assert(sim->update.ddt > 0);
  assert(sim->update.time <= sim->clock);
  out->percent_frame = min(
      1., (double)(sim->clock - sim->update.time) / (double)sim->update.ddt);
  assert((0. <= out->percent_frame) && (out->percent_frame <= 1.));

  // Render frame rate throttle.
  // Note that if no max render fps is given, then render_ddt is 0. The logic
  // works for both render_ddt>0 and =0.
  // Need to be careful with subtraction since the quantities are unsigned.
  // Subtract an epsilon to account for delays in thread scheduling.
  static const simloop_time_t eps = 50'000; // 50us
  out->throttle =
      (sim->render_ddt > (dt - eps)) ? (sim->render_ddt - eps - dt) : 0;

  out->frame          = sim->frame;
  out->update_elapsed = sim->update.time;
  out->update_dt      = sim->update.ddt;
  out->should_update  = update_this_tick;
}