Routines
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C5A9: Do two objects overlap in U?
Used by the routines at DO_ANY_OBJS_INTERSECT, CAN_PICK_UP, ADJ_DU_FOR_OBJ_INTERSECT, ADJ_DV_FOR_OBJ_INTERSECT and ADJ_DZ_FOR_OBJ_INTERSECT.
Boxes are centred in U and V, with +$04 and +$05 the half-sizes. They overlap in U if the distance between the centres, after the mover's dU, is less than the two half-sizes added: exactly touching is not overlapping. The distance is taken as a signed byte, so centres 128 or more apart would be misjudged; no room is that big. Also used by DO_ANY_OBJS_INTERSECT and CAN_PICK_UP.
Input
IX the moving object
IY the other object
C the mover's dU
Output
F carry set if they overlap
DO_OBJS_INTERSECT_ON_U C5A9 LD A,(IX+$04) D = the two half-sizes
C5AC ADD A,(IY+$04)
C5AF LD D,A
C5B0 LD A,(IX+$01) A = |U + dU - the other's U|
C5B3 ADD A,C
C5B4 SUB (IY+$01)
C5B7 JP P,DO_OBJS_INTERSECT_ON_U_0
C5BA NEG
DO_OBJS_INTERSECT_ON_U_0 C5BC SUB D Carry if less than D
C5BD RET
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