Damage based on movement distance

How would I go about making a trigger that did damage to a unit depending on the distance it moved. Not necessarily by using the "Amov" command, but through actual movement of the unit. Example:

A unit is debuffed so that for every 20 distance it moves it will take 1 damage. The unit moves 650 distance, so it would take 7.5 damage over the amount of time it took to get to the point it's trying to move to.

How would I do that, is there a way to detect idle movement (like when switching targets), and can it be implemented in GUI with minmal custom scripts?

Forget GUI. This is impossible to do in GUI (if you want to do it at least a lil' accurate). You would have to use a periodic, stor the units x,y , get the distance between the last x,y and the current x,y you wcould get the distance and damage depending on that instantly, or store the distance additivly and damage after a fixed period of time.(The smaller the timer cycle is, the smaller the 'failure' will be).

Oh wow... nevermind then.