Track geometry: UpdateVectorNumber

       Name: UpdateVectorNumber                                      [Show more]
       Type: Subroutine
   Category: Track geometry
    Summary: Move to the next segment vector along the track in the direction
             we are facing
  Deep dive: Building a 3D track from sections and segments
    Context: See this subroutine in context in the source code
 References: This subroutine is called as follows:
             * GetFirstSegment calls UpdateVectorNumber


 Returns:

   thisVectorNumber     Set to the number of the next segment vector along the
                        track in the direction in which we are facing


.UpdateVectorNumber

 LDA directionFacing    \ If our car is facing backwards, jump to uvec1
 BMI uvec1

 LDY thisVectorNumber   \ Set Y to the current segment vector number

 INY                    \ Increment Y to point to the next segment vector number
                        \ along the track

 CPY trackVectorCount   \ If Y <> trackVectorCount, then we have not reached the
 BNE uvec3              \ last vector, so jump to uvec3 to store the new value
                        \ of thisVectorNumber

 LDY #0                 \ If we get here then we have reached the last vector,
                        \ so set Y = 0 to wrap around to the first vector

 BEQ uvec3              \ Jump to uvec3 to set thisVectorNumber = 0 (this BEQ is
                        \ effectively a JMP as Y is always zero)

.uvec1

                        \ If we get here then our car is facing backwards

 LDY thisVectorNumber   \ Set Y to the current segment vector number

 BNE uvec2              \ If Y <> 0, then we are not on the first segment
                        \ vector, so jump to uvec2 to decrement to the previous
                        \ vector

 LDY trackVectorCount   \ Set Y = trackVectorCount, so we wrap around to the
                        \ last vector

.uvec2

 DEY                    \ Decrement Y to point to the previous segment vector
                        \ number, i.e. backwards along the track

.uvec3

 STY thisVectorNumber   \ Update thisVectorNumber with the new value that we set
                        \ above

.uvec4

 RTS                    \ Return from the subroutine