Bench underestimates covered search area by a factor of 4

[muendlein]

Currently Math.random() * boxSize is used to set the delta between min and max (=box length) which will on average only give 0.5*boxSize reducing the box area by a factor of 4.

flatbush/bench.js

Lines 14 to 20 in 3a6f0f0

	function addRandomBox(arr, boxSize) {
	const x = Math.random() * (100 - boxSize);
	const y = Math.random() * (100 - boxSize);
	const x2 = x + Math.random() * boxSize;
	const y2 = y + Math.random() * boxSize;
	arr.push(x, y, x2, y2);
	}