trapezoid joint

hi,

im trying to get a joint simulation of a simple trapezoid joint (not sure if the phrase is right)
@ the pic you see what im trying to achieve. the problem that i have is that i dont know how to deal with the movements of the points.

P1 rotates around FP1 to the left, until distance between P1 and FP2 is maximum (length P1P2+P1FP2) then P1 is pushed back the other way. so i have to build in a query to switch rotations i assume.

maybe someone could point me in the right direction
big thanx