CODEWITHSUNDEEP
javascriptleafletmodulodiscrete-mathematicsnumber-theory
APL
APL

Reputation: 355

What is the correct way to get a number modulo range such that the returned value lies between the min and max range values?

I was checking out the leaflet js code. There is a wrapNum function.

// @function wrapNum(num: Number, range: Number[], includeMax?: Boolean): Number
// Returns the number `num` modulo `range` in such a way so it lies within
// `range[0]` and `range[1]`. The returned value will be always smaller than
// `range[1]` unless `includeMax` is set to `true`.
function wrapNum(x, range, includeMax) {
    var max = range[1],
        min = range[0],
        d = max - min;
    return x === max && includeMax ? x : ((x - min) % d + d) % d + min;
}

So, they are simply trying to find a number modulo range so that the number lies between the given min - max range.

Instead of using ((x - min) % d + d) % d + min expression if I write ((x - min) % d) + min, will that miss any test case that is covered by original expression?

Upvotes: 1

Views: 5419

Answers (1)

RobG
RobG

Reputation: 147413

…will that miss any test case that is covered by original expression?

Yes. You will get incorrect values for most values of x that are less than the range minimum range[0].

That is because the % multiplicative operator returns a remainder, not a proper modulo, e.g. 

-1 % 12

returns -1, not 1 (see JavaScript % (modulo) gives a negative result for negative numbers) hence the use of the convoluted:

((x - min) % d + d) % d

which gets the correct modulo value. Some play code:

function wrapNum0(x, range, includeMax) {
  var max = range[1],
    min = range[0],
    d = max - min;
  return x === max && includeMax ? x : ((x - min) % d + d) % d + min;
}

function wrapNum1(x, range, includeMax) {
  var max = range[1],
    min = range[0],
    d = max - min;
  return x === max && includeMax ? x : ((x - min) % d) + min;
}


function doCalc(form) {
  var num = +form.num.value;
  var range = form.range.value.split(',').map(Number);
  var includeMax = form.includeMax.checked;
  form.result0.value = wrapNum0(num, range, includeMax);
  form.result1.value = wrapNum1(num, range, includeMax);
}
<form onsubmit="event.preventDefault();doCalc(this); return false;">
  <input value="3" name="num">Value<br>
  <input value="5,10" name="range">Range (<em>min,max</em>)<br>
  <input type="checkbox" name="includeMax">Inclue max?<br>
  <input readonly name="result0">Original (wrapNum0)<br>
  <input readonly name="result1">Modified (wrapNum1)<br>
  <input type="reset"><button>Do calc</button>
</form>

Upvotes: 2

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