Routines
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34861: Steer Creature
Used by the routines at HandlerMummy, HandlerDracula, HandlerFrankenstein, HandlerDevil and HandlerHunchback.
Sets the velocities of a creature so that it heads towards a target position, which is one pixel per step on each axis (or nothing on an axis where it has already arrived). Bit 0 of C is set if the creature is at the Y position of the target and bit 1 is set if it is at the X position, so C is 3 when it has arrived.
Input
IX The creature entity
DE The target position (X in E and Y in D)
Output
C Bit 0 set if the Y positions match, bit 1 set if the X ones do
SteerCreature 34861 LD C,0 Clear the result in C, which flags which axes have been reached.
34863 LD A,D Fetch the target Y position.
34864 CP (IX+4) Compare the target Y position with the Y position of the creature.
34867 JR Z,SteerCreature_SameY Jump to SteerCreature_SameY if they are the same.
34869 JR NC,SteerCreature_Below
34871 LD (IX+9),255 Set the Y velocity of the creature to -1 (the target is above).
SteerCreature_X 34875 LD A,E Fetch the target X position.
34876 CP (IX+3) Compare the target X position with the X position of the creature.
34879 JR Z,SteerCreature_SameX Jump to SteerCreature_SameX if they are the same.
34881 JR NC,SteerCreature_Right
34883 LD (IX+8),255 Set the X velocity of the creature to -1 (the target is to the left).
34887 RET Return.
SteerCreature_SameY 34888 LD (IX+9),0 Set the Y velocity of the creature to 0.
34892 SET 0,C Flag that the creature is at the target Y position.
34894 JR SteerCreature_X Jump to SteerCreature_X to work out the X velocity.
SteerCreature_Below 34896 LD (IX+9),1 Set the Y velocity of the creature to 1.
34900 JR SteerCreature_X Jump to SteerCreature_X to work out the X velocity.
SteerCreature_SameX 34902 LD (IX+8),0 Set the X velocity of the creature to 0.
34906 SET 1,C Flag that the creature is at the target X position.
34908 RET Return.
SteerCreature_Right 34909 LD (IX+8),1 Set the X velocity of the creature to 1.
34913 RET Return.
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