Fixed timestep vs Delta time

main.go

package main

import (
	"fmt"
	"log"
	"strings"
	"time"

	"github.com/hajimehoshi/ebiten/v2"
	"github.com/hajimehoshi/ebiten/v2/ebitenutil"
)

func main() {
	ebiten.SetWindowSize(640, 480)
	ebiten.SetWindowTitle("Slow Game")
	if err := ebiten.RunGame(&Game{useDelta: false, prevTime: time.Now()}); err != nil {
		log.Fatal(err)
	}
}

type Game struct {
	// useDelta selects the time computation strategy.
	// false - rely on the fixed TPS rate.
	// true - compute the frame delta manually.
	useDelta bool

	prevTime time.Time

	timer time.Duration
}

func (g *Game) timeDelta() time.Duration {
	if g.useDelta {
		// Calculating a time delta manually.
		now := time.Now()
		timeDelta := now.Sub(g.prevTime)
		g.prevTime = now
		return timeDelta
	}
	// Relying on a fact that TPS rate is constant.
	// 60 TPS is a default.
	return time.Second / 60
}

func (g *Game) Update() error {
	g.timer += g.timeDelta()
	heavyWork()
	return nil
}

func (g *Game) Draw(screen *ebiten.Image) {
	ebitenutil.DebugPrint(screen, fmt.Sprintf("TPS: %f\nFPS: %f\nElapsed: %s", ebiten.CurrentTPS(), ebiten.CurrentFPS(), g.timer))
}

func (g *Game) Layout(outsideWidth, outsideHeight int) (int, int) {
	return 640, 480
}

func heavyWork() {
	// Do something that wastes CPU cycles.
	for i := 0; i < 2000; i++ {
		slice := make([]string, 100)
		for j := range slice {
			slice[j] = strings.Repeat("s", j)
		}
		res := ""
		for _, part := range slice {
			res += part
		}
		sink = res
	}
}

var sink interface{}