[Swarm-Modelling] calculating distance on a torus

[lpremo]

Hello,

Could anyone point me in the direction of some code for calculating
Euclidean distance between two coordinates [(x1, y1) and (x2, y2)] on a
torus?  I imagine one would calculate distance a bit differently on a
torus than on a grid, so I thought I'd quickly check to see if anyone
had already tackled this problem before setting out to do so myself. 
Thanks for your time,

Luke Premo
anthropology
u of arizona