When I google "game loop"， I found deWiTTERS Game Loop, http://www.koonsolo.com/news/dewitters-gameloop/ which I still not fully understand.
(curr_frame_tick - prev_frame_tick) is the delta time of each loop. So this value is the time it took to do the previous loop.
It doesn't increase each timestep because prev_game_tick is recalculated each step.
Step1 (initial step):
- prev_frame_tick = 0
- curr_frame_tick = 0
- delta = 0
Step2:
- prev_frame_tick = 0
- curr_frame_tick = 200
- delta = 200
Step3:
- prev_frame_tick = 200
- curr_frame_tick = 356
- delta = 156
...
As an example, if we want to move something on screen at a constant rate in pixels per second, the number of pixels to move per update (ie frame) at 60 FPS is half as far as when updating at 30 FPS. The result is that the position at any time is the same regardless of how quickly the computer ran the simulation.