Chase H.Q.: Routine at A89C

A89C: Returns the range of lanes that cars or hazards should spawn within

Returning (1,1) means that cars will only spawn in the leftmost lane; returning (3,4) makes them spawn in the third or fourth lane; and so on.

The lanes byte at the given road buffer offset decides. (1,4) is a four lane road, a dirt track or a fork; (1,3) a three lane road or a tunnel; (2,4) a three lane road biased right; (1,2) a two lane road biased left; and (3,4) a two lane road biased right.

Input
C	Additional lanes buffer offset (e.g. 20)

Output
B	Lowest lane
C	Highest lane

get_spawn_lanes

A89C

LD A,($A240)

Calculate [current buffer offset] + [lanes offset of 64] + 2 bytes + [additional buffer offset] (wrapping around)

A89F

ADD A,$42

A8A1

ADD A,C

A8A2

LD L,A

Point DE at road buffer lane data as calculated

A8A3

LD H,$EE

A8A5

LD A,(HL)

Read lanes_byte

A8A6

AND A

Set flags

A8A7

LD BC,$0104

Set lowest lane = 1, highest lane = 4

A8AA

RET Z

Return if lanes_byte was zero (4 lanes)

A8AB

LD E,A

Preserve lanes_byte

A8AC

AND $C1

Return if ((lanes_byte & $C1) == $C1) (4 lane dirt track) [would also choose 2 lane dirt track but that's nonstandard]

A8AE

CP $C1

A8B0

RET Z

A8B1

CP $41

Test for tunnel (three lanes)

A8B3

LD BC,$0103

Set lowest = 1, highest = 3

A8B6

RET Z

Return if tunnel

A8B7

LD A,E

Compute (lanes_byte & $82)

A8B8

AND $82

A8BA

ADD A,A

Set carry from top bit while discarding it

A8BB

JR NC,gsl_two_lanes

Jump to gsl_two_lanes if top bit not set

gsl_three_lanes

A8BD

LD BC,$0204

Set lowest = 2, highest = 4

A8C0

RET NZ

Return if non-zero (3 lanes right aligned)

A8C1

LD BC,$0103

Set lowest = 1, highest = 3

A8C4

RET

Return (3 lanes left aligned)

gsl_two_lanes

A8C5

LD BC,$0102

Set lowest = 1, highest = 2

A8C8

RET Z

Return if zero (2 lanes left aligned)

A8C9

LD BC,$0304

Set lowest = 3, highest = 4

A8CC

RET

Return