DaniGuardiola · February 2, 2016 10:56
diff --git a/circle-in-rectangle.html b/circle-in-rectangle.html
 <!DOCTYPE html>
 <html>

 <head>
    <meta charset="UTF-8">
    <title>Circle in rectangle</title>
 </head>

 <body>
    <!--
  By Dani Guardiola
    
 	The diameter() function calculates the diameter that a circle
 	must have to cover a rectangle	based on its center position and
 	the rectangle's dimensions.

 	The run() function reflects it into the DOM for demonstration purposes.

 	Further explanation:

 	Imagine that we want to cover a rectangle completely with a circle with
 	the same center as the rectangle. To find the correct diameter in this
 	case, we just have to calculate the diagonal of the rectangle and use it
 	as the circle's diameter.
 	This is very easy to calculate just by using the Pythagoras' Theorem:
 	outer_radius = 0.5 * sqrt(width * width + height * height)

 	Source:
 	http://stackoverflow.com/questions/2891222/choosing-circle-radius-to-fully-fill-a-rectangle

 	But what happens when the center of the circle is not at the same
 	position as the center of the rectangle?

 	Well in that case, we must readjust the width and the height used to
 	calculate the radius by "simulating" another rectangle with the same
 	center position as the circle.
 	The way to do this is simple, we just have to find out what distance to
 	the edge from the circle center position is	greater in X and Y, and
 	duplicate it to find out the new rectangle dimensions.

 	For example, to find out the width of the new rectangle, we must first
 	find the distance from X to the left edge (which is X itself) and to
 	the right edge (initial width minus X).

 	Then we duplicate the greater value of those two and we have the width
 	we are gonna use to calculate the radius of the circle.
 	-->
    <div id="rectangle" style="margin: 150px; height: 100px; width: 100px; background-color: blue; position: relative;">
        <div id="circle" style="border-radius: 50%; height: 50px; width: 50px; background-color: rgba(0, 0, 0, 0.5); top: 25px; left: 25px; position: absolute;"></div>
    </div>
    <script type="text/javascript">
    function run(x, y) {
        var rectangle = document.querySelector("#rectangle");
        var circle = document.querySelector("#circle");
        var rect = rectangle.getBoundingClientRect();
        var d = diameter(rect.width, rect.height, x, y);
        circle.style.height = circle.style.width = d + "px";
        circle.style.top = y - (d / 2) + "px";
        circle.style.left = x - (d / 2) + "px";
    }

    function diameter(w, h, x, y) {
        var width = Math.max(w - x, x) * 2;
        var height = Math.max(h - y, y) * 2;
        var diameter = Math.sqrt(width * width + height * height);
        return diameter;
    }
    </script>
 </body>

 </html>
	<!DOCTYPE html>
	<html>

	<head>
	<meta charset="UTF-8">
	<title>Circle in rectangle</title>
	</head>

	<body>
	<!--
	By Dani Guardiola

	The diameter() function calculates the diameter that a circle
	must have to cover a rectangle based on its center position and
	the rectangle's dimensions.

	The run() function reflects it into the DOM for demonstration purposes.

	Further explanation:

	Imagine that we want to cover a rectangle completely with a circle with
	the same center as the rectangle. To find the correct diameter in this
	case, we just have to calculate the diagonal of the rectangle and use it
	as the circle's diameter.
	This is very easy to calculate just by using the Pythagoras' Theorem:
	outer_radius = 0.5 * sqrt(width * width + height * height)

	Source:
	http://stackoverflow.com/questions/2891222/choosing-circle-radius-to-fully-fill-a-rectangle

	But what happens when the center of the circle is not at the same
	position as the center of the rectangle?

	Well in that case, we must readjust the width and the height used to
	calculate the radius by "simulating" another rectangle with the same
	center position as the circle.
	The way to do this is simple, we just have to find out what distance to
	the edge from the circle center position is greater in X and Y, and
	duplicate it to find out the new rectangle dimensions.

	For example, to find out the width of the new rectangle, we must first
	find the distance from X to the left edge (which is X itself) and to
	the right edge (initial width minus X).

	Then we duplicate the greater value of those two and we have the width
	we are gonna use to calculate the radius of the circle.
	-->
	<div id="rectangle" style="margin: 150px; height: 100px; width: 100px; background-color: blue; position: relative;">
	<div id="circle" style="border-radius: 50%; height: 50px; width: 50px; background-color: rgba(0, 0, 0, 0.5); top: 25px; left: 25px; position: absolute;"></div>
	</div>
	<script type="text/javascript">
	function run(x, y) {
	var rectangle = document.querySelector("#rectangle");
	var circle = document.querySelector("#circle");
	var rect = rectangle.getBoundingClientRect();
	var d = diameter(rect.width, rect.height, x, y);
	circle.style.height = circle.style.width = d + "px";
	circle.style.top = y - (d / 2) + "px";
	circle.style.left = x - (d / 2) + "px";
	}

	function diameter(w, h, x, y) {
	var width = Math.max(w - x, x) * 2;
	var height = Math.max(h - y, y) * 2;
	var diameter = Math.sqrt(width * width + height * height);
	return diameter;
	}
	</script>
	</body>

	</html>