Robotron: 2084: Routine at CD50

CD50: Handle Homing Enemy

Per-frame entity handler for a homing enemy. Each frame the entity moves one step toward the player. If it catches another entity it removes that entity from play, awards the player points, and shows a floating score popup. If it reaches the player it triggers the player death sequence.
Handle_Homing_Enemy	CD50	LD A,($C533)	Read the wave start delay from Wave_Start_Delay.
	CD53	AND A	Jump to Process_Entity_Record_Counter_Up_Next7 if the wave has not started yet.
	CD54	JP NZ,Process_Entity_Record_Counter_Up_Next7
	CD57	CALL Generate_Random_Byte	Generate a random movement delay value.
	CD5A	AND %00001111	Keep the low four bits (delay range 00–0F).
	CD5C	INC A	Increment to give a delay range of 01–10.
	CD5D	LD ($C4F6),HL	Stash the entity record pointer and load the delay into B.
	CD60	LD B,A	Stash the entity record pointer and load the delay into B.
	CD61	LD A,($C51C)	Read the animation frame counter from C51C.
	CD64	SUB B	Skip movement this frame if the frame counter has not yet reached the delay value.
	CD65	JP NC,Handle_Homing_Enemy_Advance
	CD68	INC HL	Step past the handler address and erase the entity sprite.
	CD69	CALL Erase_Entity_Sprite	Step past the handler address and erase the entity sprite.
	CD6C	LD HL,($C4F6)	Restore the entity record pointer and advance to the entity position bytes.
	CD6F	INC HL
	CD70	INC HL
Move the entity one step toward the player along both axes.
	CD71	LD A,($C4D4)	Compute the vertical distance from this entity to the player.
	CD74	SUB (HL)
	CD75	LD A,($C426)	Load the vertical movement speed and negate it if the entity is above the player.
	CD78	JR NC,Handle_Homing_Enemy_Apply_Y
	CD7A	NEG
Handle_Homing_Enemy_Apply_Y	CD7C	ADD A,(HL)	Apply the vertical delta and step to the X position byte.
	CD7D	LD (HL),A
	CD7E	INC HL
	CD7F	LD A,($C4D5)	Compute the horizontal distance from this entity to the player.
	CD82	SUB (HL)
	CD83	LD A,($C426)	Load the horizontal movement speed and negate it if the entity is to the right of the player.
	CD86	JR NC,Handle_Homing_Enemy_Apply_X
	CD88	NEG
Handle_Homing_Enemy_Apply_X	CD8A	ADD A,(HL)	Apply the horizontal delta to the entity X position.
	CD8B	LD (HL),A	Apply the horizontal delta to the entity X position.
	CD8C	DEC HL	Step back to the entity Y position.
	CD8D	DEC HL	Step back to the entity Y position.
	CD8E	EXX	Set the draw colour to 46 in the shadow registers.
	CD8F	LD B,$46
	CD91	EXX
	CD92	CALL Draw_Entity_Sprite	Redraw the entity sprite at its new position.
	CD95	LD HL,($C4F6)	Restore the entity record pointer.
Handle_Homing_Enemy_Advance	CD98	INC HL	Advance the entity record pointer by two bytes.
	CD99	INC HL	Advance the entity record pointer by two bytes.
	CD9A	EXX	Jump to Handle_Homing_Enemy_Caught_Entity if this entity has caught another entity in the list.
	CD9B	CALL Find_Colliding_Entity
	CD9E	JR NC,Handle_Homing_Enemy_Caught_Entity
	CDA0	LD HL,$C4D4	Point to the player position data and switch to the shadow registers.
	CDA3	EXX
	CDA4	CALL HitTestBounds8AtHL	Trigger the player death sequence if this entity has reached the player.
	CDA7	JP NC,Strip_Attributes_And_Fade
	CDAA	INC HL	Advance the entity pointer and return.
	CDAB	INC HL
	CDAC	RET
This entity has reached another entity; remove the caught entity from play, award the player points, and display a floating score popup.
Handle_Homing_Enemy_Caught_Entity	CDAD	LD DE,$0006	Step back six bytes to the start of the caught entity's record.
	CDB0	SBC HL,DE
	CDB2	LD ($C4DA),HL	Save the caught entity's record pointer to Entity_Record_Pointer.
	CDB5	CALL Erase_Narrow_Entity_Sprite	Erase the caught entity's sprite.
	CDB8	CALL Remove_Entity_From_List	Remove the caught entity from the entity list.
	CDBB	LD HL,$C41C	Decrement the active enemy count at Active_Enemy_Count.
	CDBE	DEC (HL)	Decrement the active enemy count at Active_Enemy_Count.
	CDBF	LD HL,$C425	Decrement the wave entity kill counter.
	CDC2	DEC (HL)	Decrement the wave entity kill counter.
	CDC3	EXX	Install the score popup handler and update the entity state in the shadow record.
	CDC4	DEC HL
	CDC5	DEC HL
	CDC6	LD (HL),$02
	CDC8	DEC HL
	CDC9	LD DE,$CE1F
	CDCC	LD (HL),D
	CDCD	DEC HL
	CDCE	LD (HL),E
	CDCF	INC HL	Step forward to the entity position byte.
	CDD0	INC HL
	CDD1	INC HL
	CDD2	PUSH HL	Erase the score popup sprite.
	CDD3	CALL Erase_Entity_Sprite
	CDD6	POP HL
	CDD7	INC HL	Step to the entity Y position byte.
	CDD8	EXX	Set the draw colour to 02 in the shadow registers.
	CDD9	LD B,$02
	CDDB	EXX
	CDDC	LD A,(HL)	Clamp the score popup Y position within the play area.
	CDDD	SUB $06
	CDDF	CP $10
	CDE1	JR C,Handle_Homing_Enemy_1
	CDE3	CP $B9
	CDE5	JR C,Handle_Homing_Enemy_0
	CDE7	SUB $06
Handle_Homing_Enemy_0	CDE9	LD (HL),A
Handle_Homing_Enemy_1	CDEA	DEC HL
	CDEB	PUSH HL	Draw the score popup sprite at the entity's position.
	CDEC	CALL Draw_Clipped_Wide_Sprite
	CDEF	POP HL
	CDF0	LD DE,$0006	Step forward to the player's BCD score bytes.
	CDF3	ADD HL,DE	Step forward to the player's BCD score bytes.
Award the player points for catching the entity; add the point value to the BCD score with carry propagation through all four score bytes.
	CDF4	EXX	Add the enemy's point value to the least significant BCD score byte.
	CDF5	LD HL,$C406
	CDF8	LD A,(HL)
	CDF9	ADD A,$00
	CDFB	DAA
	CDFC	LD (HL),A
	CDFD	LD A,$01	Propagate BCD carry from the lowest score byte into the next score byte.
	CDFF	DEC HL
	CE00	JR NC,Handle_Homing_Enemy_2
	CE02	INC A
Handle_Homing_Enemy_2	CE03	ADD A,(HL)
	CE04	DAA
	CE05	LD (HL),A
	CE06	JR NC,Handle_Homing_Enemy_3
	CE08	DEC HL	Propagate BCD carry through the second score byte.
	CE09	LD A,$01
	CE0B	ADD A,(HL)
	CE0C	DAA
	CE0D	LD (HL),A
	CE0E	JR NC,Handle_Homing_Enemy_3
	CE10	DEC HL	Propagate BCD carry into the most significant score byte.
	CE11	LD A,$01
	CE13	DAA
	CE14	LD (HL),A
Handle_Homing_Enemy_3	CE15	CALL Clear_Colour_Bar_Column	Refresh the score display column.
	CE18	LD E,$00	Initialise the score popup entity state via Initialise_Entity_State.
	CE1A	CALL Initialise_Entity_State
	CE1D	EXX	Restore the main register set and return.
	CE1E	RET	Restore the main register set and return.
Per-frame handler for the floating score popup; scrolls the popup upward each frame until 0F frames have elapsed.
Handle_Homing_Enemy_Float_Score	CE1F	LD A,(HL)	Load the entity state byte into shadow B'.
	CE20	EXX
	CE21	LD B,A
	CE22	EXX
	CE23	PUSH HL	Draw the score popup sprite at the current entity position.
	CE24	INC HL
	CE25	CALL Draw_Clipped_Wide_Sprite
	CE28	POP HL
	CE29	INC (HL)	Advance the animation frame and jump to Handle_Homing_Enemy_Float_Score_Exit when the score popup has completed.
	CE2A	LD A,(HL)
	CE2B	CP $0F
	CE2D	JR Z,Handle_Homing_Enemy_Float_Score_Exit
	CE2F	EXX	Update shadow B' with the frame count and advance to the entity Y position.
	CE30	LD B,A
	CE31	EXX
	CE32	INC HL
	CE33	INC HL
	CE34	LD A,(HL)	Select the vertical scroll speed based on the Y position.
	CE35	CP $16
	CE37	JR NC,Handle_Homing_Enemy_4
	CE39	CP $AD
	CE3B	LD A,$00
	CE3D	JR NC,Handle_Homing_Enemy_Float_Score_Done
	CE3F	JR Handle_Homing_Enemy_Float_Score_Move
Handle_Homing_Enemy_4	CE41	CP $B0	Adjust the scroll speed for the upper boundary.
	CE43	LD A,$FA
	CE45	JR C,Handle_Homing_Enemy_Float_Score_Move
	CE47	LD A,$F4
Handle_Homing_Enemy_Float_Score_Move	CE49	ADD A,(HL)	Apply the Y delta, redraw the score popup, and continue at Process_Entity_Record_Next6.
	CE4A	LD (HL),A
	CE4B	DEC HL
	CE4C	PUSH HL
	CE4D	CALL Draw_Clipped_Wide_Sprite
	CE50	POP HL
	CE51	JP Process_Entity_Record_Next6
Handle_Homing_Enemy_Float_Score_Done	CE54	DEC HL	Step back to the handler address field.
	CE55	DEC HL	Step back to the handler address field.
Handle_Homing_Enemy_Float_Score_Exit	CE56	LD DE,$E02C	Install Process_Entity_Record_Next7 as the score popup completion handler.
	CE59	DEC HL
	CE5A	LD (HL),D
	CE5B	DEC HL
	CE5C	LD (HL),E
	CE5D	INC HL
	CE5E	INC HL
	CE5F	JP Process_Entity_Record_Next7	Jump to Process_Entity_Record_Next7 to finalise the score popup.
	CE62	POP DE	Return.
	CE63	RET	Return.