if (dir_change)
	step_acc= (1/2 step, for whichever direction we're now headed in)

...

if (step_acc > addval) {
	step_acc -= addval;	// keep track of our 1/2 "pseudo-step)
	accum -= addval;
	if (step should have happened)
		set a flag
} else {
	step_acc=0  (was going to go to 0 anyway)
	deal with accum as normal
}

...

when doing the step, the check should be
if (step_flag && !step_acc)  { step }