pumpkinzzz - 1 year ago 87
jQuery Question

# setTimeout - counter increasing speed exponentially

what i am trying to do is a function that prints out a sequence of numbers (eg. 1-100) in a given time (eg. 2 seconds). super easy.

The hardest part is that the sequence animation should begin slowly and speed up exponentially. This is what i have so far:

``````var animationLength = 2000; //ms

var counter = 0,
counterEnd = 100,
countInterval = animationLength / counterEnd; // 20ms

function animate() {
\$('#result').text(counter++);
if (counter <= counterEnd) {

//Calculate here dynamically newInterval
var newInterval = countInterval;

countInterval = newInterval;
setTimeout(animate, newInterval);
}
}

animate();
``````

So, now
`countInterval`
is constantly 20ms, but it should be variable, decreasing exponentially. Eg:

``````counter = 1; => countInterval = 40ms //not sure about that
...
counter = 100; => countInterval = 1ms
``````

And the summatory of these intervals has to be 2000ms

https://jsfiddle.net/fvxf7mby/5/

UPDATE:

thanks to @Mats Lind i finally found out. That's the final code (JSFIDDLE)

``````var animationLength = 2000; //ms

var counter = 0,
counterEnd = 100,
countInterval = animationLength / counterEnd, // 20 ms,
a = 1.05; //speed factor

var summatory = 0;

function animate() {
\$('#result').text(counter++);
if (counter <= counterEnd) {

//that's the formula:
var newInterval = (animationLength-summatory) / ( (a - Math.pow(a, -(counterEnd-1))) / (a-1))

summatory += newInterval;
countInterval = newInterval;
setTimeout(animate, newInterval);
} else {
\$('#summatory').text(summatory); //should be 2000
}
}

animate();
``````

Looking at the "hardest part", the math question about speeding up exponentially:

With time-interval inversely proportional to speed, we like the time-interval to decrease. Exponentially mean a decrease with the same factor between each interval.

Call the factor 1/a and set the first time-interval to b, then next time-interval will be be b/a, the third b/a^2 and the n:th b/a^(n-1).

Total time for N timesteps is b*(a-a^-(N-1))/(a-1) (summation formula for an exponential series), excel formula; `=(a-a^-(N-1))/(a-1)`

Now we know total time 2000ms and number of steps 100, and have two unknown, b and a, but only one equation. So we can set a and have b given via the equation above b=total_time/((a-a^-(N-1))/(a-1)). The more we want time to "accelerate" the higher we should set a. To conclude:

time interval length for interval n>0: b/a^(n-1)

total_time: choose, in this example total_time = 2000ms

N: choose, in this example N=100

a: choose, higher the faster you want speed to increase

b=total_time/((a-a^-(N-1))/(a-1)).

Try for instance a = 1.05 which gives b = 95.96788204ms and all timesteps:

95.96788204 91.39798289 87.04569799 82.90066476 78.95301405 75.19334672 71.61271116 68.20258206 64.95484005 61.86175243 58.9159547 56.11043304 53.43850766 50.89381682 48.47030173 46.16219213 43.9639925 41.87046905 39.87663719 37.97774971 36.16928543 34.44693851 32.8066081 31.24438867 29.75656064 28.33958156 26.99007768 25.70483588 24.48079608 23.31504388 22.2048037 21.14743209 20.14041152 19.1813443 18.26794696 17.39804472 16.5695664 15.78053943 15.02908517 14.31341445 13.63182328 12.98268884 12.36446556 11.77568149 11.21493475 10.68089024 10.17227642 9.687882303 9.226554574 8.787194832 8.368756983 7.970244746 7.590709282 7.229246935 6.884997081 6.557140077 6.244895312 5.947519344 5.664304138 5.394575369 5.137690828 4.893038884 4.660037032 4.438130507 4.226790959 4.025515199 3.833823999 3.651260951 3.477391382 3.311801316 3.154096492 3.003901421 2.860858496 2.724627139 2.594882989 2.471317133 2.353635365 2.24155749 2.134816657 2.033158721 1.936341639 1.844134894 1.756318947 1.672684712 1.593033059 1.517174342 1.444927944 1.376121852 1.31059224 1.248183085 1.188745796 1.132138853 1.078227479 1.026883313 0.977984108 0.931413436 0.887060415 0.844819443 0.804589946 0.766276139

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