Robotron: 2084: Routine at 52560

52560: 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	52560	LD A,(50483)	Read the wave start delay from Wave_Start_Delay.
	52563	AND A	Jump to Process_Entity_Record_Counter_Up_Next7 if the wave has not started yet.
	52564	JP NZ,Process_Entity_Record_Counter_Up_Next7
	52567	CALL Generate_Random_Byte	Generate a random movement delay value.
	52570	AND %00001111	Keep the low four bits (delay range 0–15).
	52572	INC A	Increment to give a delay range of 1–16.
	52573	LD (50422),HL	Stash the entity record pointer and load the delay into B.
	52576	LD B,A	Stash the entity record pointer and load the delay into B.
	52577	LD A,(50460)	Read the animation frame counter from 50460.
	52580	SUB B	Skip movement this frame if the frame counter has not yet reached the delay value.
	52581	JP NC,Handle_Homing_Enemy_Advance
	52584	INC HL	Step past the handler address and erase the entity sprite.
	52585	CALL Erase_Entity_Sprite	Step past the handler address and erase the entity sprite.
	52588	LD HL,(50422)	Restore the entity record pointer and advance to the entity position bytes.
	52591	INC HL
	52592	INC HL
Move the entity one step toward the player along both axes.
	52593	LD A,(50388)	Compute the vertical distance from this entity to the player.
	52596	SUB (HL)
	52597	LD A,(50214)	Load the vertical movement speed and negate it if the entity is above the player.
	52600	JR NC,Handle_Homing_Enemy_Apply_Y
	52602	NEG
Handle_Homing_Enemy_Apply_Y	52604	ADD A,(HL)	Apply the vertical delta and step to the X position byte.
	52605	LD (HL),A
	52606	INC HL
	52607	LD A,(50389)	Compute the horizontal distance from this entity to the player.
	52610	SUB (HL)
	52611	LD A,(50214)	Load the horizontal movement speed and negate it if the entity is to the right of the player.
	52614	JR NC,Handle_Homing_Enemy_Apply_X
	52616	NEG
Handle_Homing_Enemy_Apply_X	52618	ADD A,(HL)	Apply the horizontal delta to the entity X position.
	52619	LD (HL),A	Apply the horizontal delta to the entity X position.
	52620	DEC HL	Step back to the entity Y position.
	52621	DEC HL	Step back to the entity Y position.
	52622	EXX	Set the draw colour to 70 in the shadow registers.
	52623	LD B,70
	52625	EXX
	52626	CALL Draw_Entity_Sprite	Redraw the entity sprite at its new position.
	52629	LD HL,(50422)	Restore the entity record pointer.
Handle_Homing_Enemy_Advance	52632	INC HL	Advance the entity record pointer by two bytes.
	52633	INC HL	Advance the entity record pointer by two bytes.
	52634	EXX	Jump to Handle_Homing_Enemy_Caught_Entity if this entity has caught another entity in the list.
	52635	CALL Find_Colliding_Entity
	52638	JR NC,Handle_Homing_Enemy_Caught_Entity
	52640	LD HL,50388	Point to the player position data and switch to the shadow registers.
	52643	EXX
	52644	CALL HitTestBounds8AtHL	Trigger the player death sequence if this entity has reached the player.
	52647	JP NC,Strip_Attributes_And_Fade
	52650	INC HL	Advance the entity pointer and return.
	52651	INC HL
	52652	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	52653	LD DE,6	Step back six bytes to the start of the caught entity's record.
	52656	SBC HL,DE
	52658	LD (50394),HL	Save the caught entity's record pointer to Entity_Record_Pointer.
	52661	CALL Erase_Narrow_Entity_Sprite	Erase the caught entity's sprite.
	52664	CALL Remove_Entity_From_List	Remove the caught entity from the entity list.
	52667	LD HL,50204	Decrement the active enemy count at Active_Enemy_Count.
	52670	DEC (HL)	Decrement the active enemy count at Active_Enemy_Count.
	52671	LD HL,50213	Decrement the wave entity kill counter.
	52674	DEC (HL)	Decrement the wave entity kill counter.
	52675	EXX	Install the score popup handler and update the entity state in the shadow record.
	52676	DEC HL
	52677	DEC HL
	52678	LD (HL),2
	52680	DEC HL
	52681	LD DE,52767
	52684	LD (HL),D
	52685	DEC HL
	52686	LD (HL),E
	52687	INC HL	Step forward to the entity position byte.
	52688	INC HL
	52689	INC HL
	52690	PUSH HL	Erase the score popup sprite.
	52691	CALL Erase_Entity_Sprite
	52694	POP HL
	52695	INC HL	Step to the entity Y position byte.
	52696	EXX	Set the draw colour to 2 in the shadow registers.
	52697	LD B,2
	52699	EXX
	52700	LD A,(HL)	Clamp the score popup Y position within the play area.
	52701	SUB 6
	52703	CP 16
	52705	JR C,Handle_Homing_Enemy_1
	52707	CP 185
	52709	JR C,Handle_Homing_Enemy_0
	52711	SUB 6
Handle_Homing_Enemy_0	52713	LD (HL),A
Handle_Homing_Enemy_1	52714	DEC HL
	52715	PUSH HL	Draw the score popup sprite at the entity's position.
	52716	CALL Draw_Clipped_Wide_Sprite
	52719	POP HL
	52720	LD DE,6	Step forward to the player's BCD score bytes.
	52723	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.
	52724	EXX	Add the enemy's point value to the least significant BCD score byte.
	52725	LD HL,50182
	52728	LD A,(HL)
	52729	ADD A,0
	52731	DAA
	52732	LD (HL),A
	52733	LD A,1	Propagate BCD carry from the lowest score byte into the next score byte.
	52735	DEC HL
	52736	JR NC,Handle_Homing_Enemy_2
	52738	INC A
Handle_Homing_Enemy_2	52739	ADD A,(HL)
	52740	DAA
	52741	LD (HL),A
	52742	JR NC,Handle_Homing_Enemy_3
	52744	DEC HL	Propagate BCD carry through the second score byte.
	52745	LD A,1
	52747	ADD A,(HL)
	52748	DAA
	52749	LD (HL),A
	52750	JR NC,Handle_Homing_Enemy_3
	52752	DEC HL	Propagate BCD carry into the most significant score byte.
	52753	LD A,1
	52755	DAA
	52756	LD (HL),A
Handle_Homing_Enemy_3	52757	CALL Clear_Colour_Bar_Column	Refresh the score display column.
	52760	LD E,0	Initialise the score popup entity state via Initialise_Entity_State.
	52762	CALL Initialise_Entity_State
	52765	EXX	Restore the main register set and return.
	52766	RET	Restore the main register set and return.
Per-frame handler for the floating score popup; scrolls the popup upward each frame until 15 frames have elapsed.
Handle_Homing_Enemy_Float_Score	52767	LD A,(HL)	Load the entity state byte into shadow B'.
	52768	EXX
	52769	LD B,A
	52770	EXX
	52771	PUSH HL	Draw the score popup sprite at the current entity position.
	52772	INC HL
	52773	CALL Draw_Clipped_Wide_Sprite
	52776	POP HL
	52777	INC (HL)	Advance the animation frame and jump to Handle_Homing_Enemy_Float_Score_Exit when the score popup has completed.
	52778	LD A,(HL)
	52779	CP 15
	52781	JR Z,Handle_Homing_Enemy_Float_Score_Exit
	52783	EXX	Update shadow B' with the frame count and advance to the entity Y position.
	52784	LD B,A
	52785	EXX
	52786	INC HL
	52787	INC HL
	52788	LD A,(HL)	Select the vertical scroll speed based on the Y position.
	52789	CP 22
	52791	JR NC,Handle_Homing_Enemy_4
	52793	CP 173
	52795	LD A,0
	52797	JR NC,Handle_Homing_Enemy_Float_Score_Done
	52799	JR Handle_Homing_Enemy_Float_Score_Move
Handle_Homing_Enemy_4	52801	CP 176	Adjust the scroll speed for the upper boundary.
	52803	LD A,250
	52805	JR C,Handle_Homing_Enemy_Float_Score_Move
	52807	LD A,244
Handle_Homing_Enemy_Float_Score_Move	52809	ADD A,(HL)	Apply the Y delta, redraw the score popup, and continue at Process_Entity_Record_Next6.
	52810	LD (HL),A
	52811	DEC HL
	52812	PUSH HL
	52813	CALL Draw_Clipped_Wide_Sprite
	52816	POP HL
	52817	JP Process_Entity_Record_Next6
Handle_Homing_Enemy_Float_Score_Done	52820	DEC HL	Step back to the handler address field.
	52821	DEC HL	Step back to the handler address field.
Handle_Homing_Enemy_Float_Score_Exit	52822	LD DE,57388	Install Process_Entity_Record_Next7 as the score popup completion handler.
	52825	DEC HL
	52826	LD (HL),D
	52827	DEC HL
	52828	LD (HL),E
	52829	INC HL
	52830	INC HL
	52831	JP Process_Entity_Record_Next7	Jump to Process_Entity_Record_Next7 to finalise the score popup.
	52834	POP DE	Return.
	52835	RET	Return.