path: root/simloop/test/simloop_test.c

#include <simloop.h>

#include <test.h>

#include <stdint.h>

// -----------------------------------------------------------------------------
// Time.

static simloop_time_t time_delta_from_sec(double seconds) {
  static constexpr double NANOS_PER_SEC = 1e9;
  return (simloop_time_t)(seconds * NANOS_PER_SEC);
}

// -----------------------------------------------------------------------------
// Randomness.

typedef struct {
  uint64_t a;
} XorShift64State;

static uint64_t xorshift64(XorShift64State* state) {
  uint64_t x = state->a;
  x ^= x << 7;
  x ^= x >> 9;
  return state->a = x;
}

// -----------------------------------------------------------------------------
// Tests.

/// At time/frame 0:
///   1. An initial render is always triggered.
///   2. No update is triggered (not enough time passed).
TEST_CASE(simloop_initial_render) {
  Simloop    simloop = simloop_make(&(SimloopArgs){.update_fps = 10});
  SimloopOut simout;

  simloop_update(&simloop, 0, &simout);

  TEST_TRUE(simout.should_render);
  TEST_TRUE(!simout.should_update);
  TEST_EQUAL(simout.frame, 0);
}

/// The initial render is not re-triggered if there is a render frame rate cap
/// and time does not advance.
TEST_CASE(simloop_initial_render_not_retriggered) {
  Simloop simloop =
      simloop_make(&(SimloopArgs){.update_fps = 10, .max_render_fps = 10});
  SimloopOut simout;

  simloop_update(&simloop, 0, &simout);

  TEST_TRUE(simout.should_render);
  TEST_TRUE(!simout.should_update);
  TEST_EQUAL(simout.frame, 0);

  for (int i = 0; i < 10; i++) {
    // Note that time does not advance.
    simloop_update(&simloop, 0, &simout);
    TEST_TRUE(!simout.should_render);
    TEST_TRUE(!simout.should_update);
    TEST_EQUAL(simout.frame, 0);
  }
}

/// A simulation loop with no render frame cap:
///   1. Updates based on the desired update frame rate.
///   2. Renders at every loop (provided there are updates).
TEST_CASE(simloop_no_render_frame_cap) {
  constexpr int        UPDATE_FPS = 10; // 100ms delta
  const simloop_time_t UPDATE_DDT =
      time_delta_from_sec(1.0 / (double)UPDATE_FPS);
  const simloop_time_t STEP         = time_delta_from_sec(1);
  const simloop_time_t SIM_TIME_SEC = time_delta_from_sec(30);

  // We need simulation time to be an exact multiple of the desired deltas for
  // the modulo comparisons below.
  TEST_TRUE((STEP % UPDATE_DDT) == 0);

  simloop_time_t dt  = 0;
  Simloop    simloop = simloop_make(&(SimloopArgs){.update_fps = UPDATE_FPS});
  SimloopOut simout;

  for (simloop_time_t t = 0; t <= SIM_TIME_SEC; t += STEP) {
    simloop_update(&simloop, dt, &simout);
    const bool expect_update = (t > 0) && ((t % UPDATE_DDT) == 0);
    // A render is still expected at time 0.
    TEST_EQUAL(simout.should_render, (t == 0) || expect_update);
    TEST_EQUAL(simout.should_update, expect_update);
    dt = STEP;
  }
}

/// A simulation loop with a render frame cap:
///   1. Updates based on the desired update frame rate.
///   2. Renders based on the desired render frame rate.
TEST_CASE(simloop_with_render_frame_cap) {
  constexpr int        UPDATE_FPS = 10; // 100ms delta
  constexpr int        RENDER_FPS = 5;  // 200ms delta
  const simloop_time_t UPDATE_DDT =
      time_delta_from_sec(1.0 / (double)UPDATE_FPS);
  const simloop_time_t RENDER_DDT =
      time_delta_from_sec(1.0 / (double)RENDER_FPS);
  const simloop_time_t STEP         = time_delta_from_sec(0.1); // 100ms
  const simloop_time_t SIM_TIME_SEC = time_delta_from_sec(30);

  // We need simulation time to be an exact multiple of the desired deltas for
  // the modulo comparisons below.
  TEST_TRUE((UPDATE_DDT % STEP) == 0);
  TEST_TRUE((RENDER_DDT % STEP) == 0);

  simloop_time_t dt      = 0;
  Simloop        simloop = simloop_make(
      &(SimloopArgs){.update_fps = UPDATE_FPS, .max_render_fps = RENDER_FPS});
  SimloopOut simout;

  for (simloop_time_t t = 0; t <= SIM_TIME_SEC; t += STEP) {
    simloop_update(&simloop, dt, &simout);
    // A render is still expected at time 0.
    TEST_EQUAL(simout.should_render, (t % RENDER_DDT) == 0);
    TEST_EQUAL(simout.should_update, (t > 0) && ((t % UPDATE_DDT) == 0));
    dt = STEP;
  }
}

/// One benefit of fixed over variable time deltas is determinism. Test for
/// this by getting to t=10 by different clock time increments.
///
/// Note that the time increments must be able to keep up with the desired frame
/// delta, otherwise determinism is not maintained. We can guarantee determinism
/// at the expense of re-introducing divergence.
/// TODO: Perhaps the API should return an update count instead of a boolean,
///  advance simulation time per the number of updates, then leave it up to
///  the client to decide whether to update just once or as many times as
///  requested, depending on whether they want determinism or convergence.
TEST_CASE(simloop_determinism) {
  constexpr int        UPDATE_FPS     = 100; // 10ms delta
  const simloop_time_t RANDOM_STEPS[] = {
      time_delta_from_sec(0.007), // 7ms
      time_delta_from_sec(0.005), // 5ms
      time_delta_from_sec(0.003), // 3ms
  };
  constexpr uint64_t NUM_RANDOM_STEPS =
      sizeof(RANDOM_STEPS) / sizeof(RANDOM_STEPS[0]);
  const simloop_time_t SIM_TIME_SEC = time_delta_from_sec(10);
  constexpr float      ADD          = 0.123f;

  typedef struct Simulation {
    int   iter_count;
    float sum;
  } Simulation;

#define UPDATE_SIMULATION(SIM) \
  {                            \
    SIM.sum += ADD;            \
    SIM.iter_count++;          \
  }

  Simulation      sim[2] = {0};
  XorShift64State xss    = (XorShift64State){12069019817132197873ULL};

  // Perform two simulations with random clock-time steps.
  for (int s = 0; s < 2; ++s) {
    simloop_time_t dt  = 0;
    Simloop    simloop = simloop_make(&(SimloopArgs){.update_fps = UPDATE_FPS});
    SimloopOut simout;

    for (simloop_time_t t = 0; t <= SIM_TIME_SEC;) {
      simloop_update(&simloop, dt, &simout);

      if (simout.should_update) {
        UPDATE_SIMULATION(sim[s]);
      }

      // Advance time with a random step.
      const simloop_time_t step =
          RANDOM_STEPS[xorshift64(&xss) % NUM_RANDOM_STEPS];
      t += step;
      dt = step;
    }
  }

  // Make sure the simulations have advanced by the same number of updates so
  // that we can compare them. They may not have had the same update count
  // depending on the clock-time steps.
  while (sim[0].iter_count < sim[1].iter_count) {
    UPDATE_SIMULATION(sim[0]);
  }
  while (sim[1].iter_count < sim[0].iter_count) {
    UPDATE_SIMULATION(sim[1]);
  }
  TEST_EQUAL(sim[0].iter_count, sim[1].iter_count);

  // The sums should be exactly equal if determinism holds.
  // Check also that they are non-zero to make sure the simulation actually
  // advanced.
  TEST_TRUE(sim[0].sum > 0.f);
  TEST_EQUAL(sim[0].sum, sim[1].sum);
}

int main() { return 0; }