Robotron: 2084: Routine at 50906

Used by the routines at Primary_Enemy_Entity_Handler, Find_Colliding_Entity, Handle_Homing_Enemy, Spawn_Primary_Wave_Enemies, Generate_Random_Byte, Process_Entity_Record_Fast, Handle_Horizontal_Enemy_Entity, Find_Colliding_Wave_Enemy, Pick_Random_Spawn_Position_Y, Process_Tracked_Enemy_Entity, Process_Cluster_Entity, Process_Player_Tracking_Enemy, Process_Wall_Bouncing_Enemy and Process_Final_Phase_Enemy_Entity.

Reads four little-endian words at HL (8 bytes). The first two load into main BC and DE; EXX swaps to the shadow set so the next two words load into shadow BC and DE while HL walks the same block. After swapping back, four ADD/SUB pairs (with further EXX) each end in RET C if the subtraction borrows (no hit). If none borrow, falls through to RET with carry clear (hit). HL ends just past the eighth byte. Used throughout the gameplay object loop for collisions (among others 52442, 52517, 53577, 53805, 57405, 60188).

Used by the routines at Primary_Enemy_Entity_Handler, Find_Colliding_Entity, Handle_Homing_Enemy, Spawn_Primary_Wave_Enemies, Generate_Random_Byte, Process_Entity_Record_Fast, Handle_Horizontal_Enemy_Entity, Find_Colliding_Wave_Enemy, Pick_Random_Spawn_Position_Y, Process_Tracked_Enemy_Entity, Process_Cluster_Entity, Process_Player_Tracking_Enemy, Process_Wall_Bouncing_Enemy and Process_Final_Phase_Enemy_Entity. Reads four little-endian words at HL (8 bytes). The first two load into main BC and DE; EXX swaps to the shadow set so the next two words load into shadow BC and DE while HL walks the same block. After swapping back, four ADD/SUB pairs (with further EXX) each end in RET C if the subtraction borrows (no hit). If none borrow, falls through to RET with carry clear (hit). HL ends just past the eighth byte. Used throughout the gameplay object loop for collisions (among others 52442, 52517, 53577, 53805, 57405, 60188).
HitTestBounds8AtHL	50906	LD B,(HL)	B=*HL (first word, low byte).
	50907	INC HL	Advance HL.
	50908	LD C,(HL)	C=*HL (first word, high byte).
	50909	INC HL	Advance HL.
	50910	LD D,(HL)	D=*HL (second word, low byte).
	50911	INC HL	Advance HL.
	50912	LD E,(HL)	E=*HL (second word, high byte).
	50913	EXX	EXX (shadow HL continues the load; shadow BC/DE hold first pair).
	50914	LD B,(HL)	B=*HL (third word, low byte).
	50915	INC HL	Advance HL.
	50916	LD C,(HL)	C=*HL (third word, high byte).
	50917	INC HL	Advance HL.
	50918	LD D,(HL)	D=*HL (fourth word, low byte).
	50919	INC HL	Advance HL.
	50920	LD E,(HL)	E=*HL (fourth word, high byte).
	50921	EXX	EXX (main holds first pair; shadow second pair; HL past all 8 bytes).
Each test adds two bytes from one pair, switches to the shadow pair, subtracts a byte from the other rectangle, and RET C on borrow (no overlap on that check).
	50922	LD A,B	A=B.
	50923	ADD A,D	A+=D (high bytes from main BC/DE).
	50924	EXX	EXX.
	50925	SUB B	Subtract shadow B from A.
	50926	EXX	EXX.
	50927	RET C	RET C if no hit (carry from SUB).
	50928	EXX	EXX.
	50929	LD A,B	A=B.
	50930	ADD A,D	A+=D (second high-byte sum).
	50931	EXX	EXX.
	50932	SUB B	Subtract shadow B from A.
	50933	RET C	RET C if no hit.
	50934	LD A,C	A=C.
	50935	ADD A,E	A+=E (low-byte sum).
	50936	EXX	EXX.
	50937	SUB C	Subtract shadow C from A.
	50938	EXX	EXX.
	50939	RET C	RET C if no hit.
	50940	EXX	EXX.
	50941	LD A,C	A=C.
	50942	ADD A,E	A+=E (final low-byte sum).
	50943	EXX	EXX.
	50944	SUB C	Subtract shadow C from A (last test).
	50945	RET	RET (carry clear if overlapping).