Robotron: 2084: Routine at E6F7

Used by the routine at Process_Cluster_Entity.

Per-frame entity handler for the slow player-tracking enemy. Tests player contact via HitTestBounds8AtHL; triggers death via Strip_Attributes_And_Fade if touching. Computes the signed Y and X distance from the player at C4D4 and C4D5 halves it five times to derive a velocity nudge, and negates if the enemy is above or to the left of the player. Reads the entity facing byte; on a random 1-in-8192 chance sets it to 1A (freeze state). If the facing is 1A flips both velocity components and tests for a 1-in-256 chance to clear the freeze. Applies the computed Y and X nudge to the entity position, clamping to the play area, and draws the entity via Draw_Entity_Sprite.

Used by the routine at Process_Cluster_Entity. Per-frame entity handler for the slow player-tracking enemy. Tests player contact via HitTestBounds8AtHL; triggers death via Strip_Attributes_And_Fade if touching. Computes the signed Y and X distance from the player at C4D4 and C4D5 halves it five times to derive a velocity nudge, and negates if the enemy is above or to the left of the player. Reads the entity facing byte; on a random 1-in-8192 chance sets it to 1A (freeze state). If the facing is 1A flips both velocity components and tests for a 1-in-256 chance to clear the freeze. Applies the computed Y and X nudge to the entity position, clamping to the play area, and draws the entity via Draw_Entity_Sprite.
Process_Player_Tracking_Enemy	E6F7	LD HL,$C4D4	Set the player Y address in alternate HL and test player contact via HitTestBounds8AtHL; trigger death via Strip_Attributes_And_Fade if touching.
	E6FA	EXX
	E6FB	CALL HitTestBounds8AtHL
	E6FE	JP NC,Strip_Attributes_And_Fade	Step back three bytes to the entity Y position and load the player Y coordinate at C4D4.
	E701	DEC HL
	E702	DEC HL
	E703	DEC HL
	E704	LD C,(HL)
	E705	EX DE,HL
	E706	LD A,($C4D4)
	E709	LD L,A	Compute the signed Y nudge: subtract the entity Y from the player Y, halve five times, and negate if the entity is above the player.
	E70A	LD H,$00
	E70C	LD B,$00
	E70E	AND A
	E70F	SBC HL,BC
	E711	BIT 0,H
	E713	LD A,L
	E714	JP Z,Process_Player_Tracking_Enemy_Nudge_Y
	E717	NEG
Process_Player_Tracking_Enemy_Nudge_Y	E719	SRL A	Scale the Y nudge and restore its sign.
	E71B	SRL A
	E71D	SRL A
	E71F	SRL A
	E721	SRL A
	E723	BIT 0,H
	E725	JP Z,Process_Player_Tracking_Enemy_Nudge_X
	E728	NEG
Process_Player_Tracking_Enemy_Nudge_X	E72A	EX AF,AF'	Compute the signed X nudge from the player X coordinate at C4D5 in the same way.
	E72B	LD A,($C4D5)
	E72E	LD L,A
	E72F	LD H,$00
	E731	INC DE
	E732	LD A,(DE)
	E733	LD C,A
	E734	LD B,$00
	E736	AND A
	E737	SBC HL,BC
	E739	BIT 0,H
	E73B	LD A,L
	E73C	JP Z,Process_Player_Tracking_Enemy_0
	E73F	NEG
Process_Player_Tracking_Enemy_0	E741	SRL A
	E743	SRL A
	E745	SRL A
	E747	SRL A
	E749	SRL A
	E74B	BIT 0,H
	E74D	JP Z,Process_Player_Tracking_Enemy_Face
	E750	NEG
Process_Player_Tracking_Enemy_Face	E752	LD B,A	Store the scaled Y nudge in B and X nudge in C; step back to the entity facing byte.
	E753	EX AF,AF'
	E754	LD C,A
	E755	EX DE,HL
	E756	DEC HL
	E757	DEC HL
	E758	LD A,(HL)
	E759	CP $1A
	E75B	JP Z,Process_Player_Tracking_Enemy_Freeze	Read the facing byte; if 1A jump to Process_Player_Tracking_Enemy_Freeze; otherwise test a 1-in-8192 random chance (Generate_Random_Byte twice, masked to 1F and 3F) to set the facing to 1A (freeze).
	E75E	CALL Generate_Random_Byte
	E761	AND $1F
	E763	JP NZ,Process_Player_Tracking_Enemy_Move
	E766	CALL Generate_Random_Byte
	E769	AND $3F
	E76B	JP NZ,Process_Player_Tracking_Enemy_Move
	E76E	LD (HL),$1A
	E770	JP Process_Player_Tracking_Enemy_Move
Process_Player_Tracking_Enemy_Freeze	E773	LD A,B	Facing is 1A (frozen): negate both velocity components; test a 1-in-256 chance to promote the facing to 1F (thaw).
	E774	NEG
	E776	LD B,A
	E777	LD A,C
	E778	NEG
	E77A	LD C,A
	E77B	CALL Generate_Random_Byte
	E77E	CP $F0
	E780	JP NZ,Process_Player_Tracking_Enemy_Move
	E783	LD (HL),$1F
Process_Player_Tracking_Enemy_Move	E785	INC HL	Apply the Y nudge to the entity Y position; clamp to 10-AD and write back.
	E786	LD A,C
	E787	ADD A,(HL)
	E788	CP $10
	E78A	JP C,Process_Player_Tracking_Enemy_Move_X
	E78D	CP $AD
	E78F	JP NC,Process_Player_Tracking_Enemy_Move_X
	E792	LD (HL),A
Process_Player_Tracking_Enemy_Move_X	E793	INC HL	Apply the X nudge to the entity X position; clamp to 08-EF and write back.
	E794	LD A,B
	E795	ADD A,(HL)
	E796	CP $08
	E798	JP C,Process_Player_Tracking_Enemy_Draw
	E79B	CP $EF
	E79D	JP NC,Process_Player_Tracking_Enemy_Draw
	E7A0	LD (HL),A
Process_Player_Tracking_Enemy_Draw	E7A1	PUSH HL	Stash HL, step back two bytes, set colour INK: CYAN, PAPER: BLACK (BRIGHT) in alternate B, draw the entity via Draw_Entity_Sprite, restore HL, and advance via Process_Cluster_Entity_Next.
	E7A2	DEC HL
	E7A3	DEC HL
	E7A4	EXX
	E7A5	LD B,$45
	E7A7	EXX
	E7A8	CALL Draw_Entity_Sprite
	E7AB	POP HL
	E7AC	JP Process_Cluster_Entity_Next