river-raid: Routine at 6B58

6B58: Calculate mirrored right edge (terrain extras mode 1)

For symmetric river sections. Right edge = 2*C - D where C is the default river width from state_terrain_element_23 low byte and D is the left edge coordinate. This mirrors the left edge around the river center.

Input
C	River center position (from state_terrain_element_23 low byte).
D	Left terrain edge coordinate.

Output
A	Right terrain edge coordinate = 2*C - D.

calc_terrain_right_mirrored

6B58

LD A,C

A = 2*C - D (mirror left around center), jump to draw_terrain_right_edge.

6B59

SUB D

6B5A

ADD A,C

6B5B

JP draw_terrain_right_edge