Krakout: Set a ball's velocity from its heading code

A2D8: Set a ball's velocity from its heading code

Derives the active ball's per-axis step thresholds and signs from its heading code, through the velocity table.
set_ball_velocity	A2D8	LD A,(active_ball)	Select the active ball state record; this arm takes ball A.
	A2DB	OR A
	A2DC	JR NZ,set_ball_velocity_0
	A2DE	LD IY,ball_a_x_accum
	A2E2	JR set_ball_velocity_1
set_ball_velocity_0	A2E4	LD IY,ball_b_x_accum	This arm takes ball B.
set_ball_velocity_1	A2E8	LD E,A	Index the heading array by the ball's slot.
	A2E9	LD D,$00
	A2EB	LD HL,ball_a_heading
	A2EE	ADD HL,DE
	A2EF	LD A,(HL)	Read this ball's heading code.
	A2F0	CP $48	A heading code below the range is out of bounds.
	A2F2	JR C,set_ball_velocity_2	A heading code below the range is out of bounds.
	A2F4	CP $90	One above it is out of bounds too.
	A2F6	JR C,set_ball_velocity_3	One above it is out of bounds too.
set_ball_velocity_2	A2F8	LD A,$57	Either way reset it to the default heading, 72° up-right.
	A2FA	LD (HL),A
	A2FB	JR set_ball_velocity_5
set_ball_velocity_3	A2FD	CP $48	Test the heading code against the four pure-axis headings – due right, straight up, due left and straight down.
	A2FF	JR Z,set_ball_velocity_4
	A301	CP $5A
	A303	JR Z,set_ball_velocity_4
	A305	CP $6C
	A307	JR Z,set_ball_velocity_4
	A309	CP $7E
	A30B	JR NZ,set_ball_velocity_5
set_ball_velocity_4	A30D	ADD A,$06	Any of them is nudged six codes counterclockwise, which avoids a flat endless bounce. Due right becomes 27°, straight up 125°, due left 207° and straight down 305°.
	A30F	LD (HL),A
set_ball_velocity_5	A310	SUB $48	Index the velocity table by the heading code's offset into the range, three bytes each.
	A312	LD C,A
	A313	ADD A,A
	A314	ADD A,C
	A315	LD HL,velocity_table
	A318	LD E,A
	A319	LD D,$00
	A31B	ADD HL,DE
	A31C	LD A,(HL)
	A31D	LD (IY+$02),A	Store the X step threshold.
	A320	INC HL	Store the X step threshold.
	A321	LD A,(HL)	Store the Y step threshold.
	A322	LD (IY+$03),A
	A325	INC HL
	A326	LD A,(HL)
	A327	AND $10	X direction sign from the sign byte
	A329	RRCA
	A32A	RRCA
	A32B	RRCA
	A32C	RRCA
	A32D	LD (IY+$04),A
	A330	LD A,(HL)
	A331	AND $01	Y direction sign from the same byte
	A333	LD (IY+$05),A	Y direction sign from the same byte
	A336	XOR A	The new heading takes the ball off the bat and restarts both sub-pixel accumulators.
	A337	LD (IY+$0B),A
	A33A	LD (IY+$00),A
	A33D	LD (IY+$01),A
	A340	RET

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