Routines
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C91F: Turn the player one quarter
Used by the routine at turn_by_parity.
How a facing is stored explains the shape of this. Bit 3 of the type chooses the sprite seen from behind (clear: W or N) or from the front (set: E or S), and bit 6 of +$07 mirrors it (clear: W or E; set: N or S). A quarter turn always toggles the mirror bit; half the time it also toggles the view. For a left turn that is when the sprite is not mirrored.
Input
F Z for a left turn, NZ for a right turn
left_right_calc_sprite C91F JR NZ,turn_right right turn
C921 BIT 6,(IX+$07) left: mirrored sprites only change mirror (N to W, S to E)
C925 JR NZ,turn_toggle_mirror
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