Track geometry: ShuffleSectionList

       Name: ShuffleSectionList                                      [Show more]
       Type: Subroutine
   Category: Track geometry
    Summary: Shuffle the track section list along by one position
  Deep dive: Data structures for the track calculations
    Context: See this subroutine in context in the source code
 References: This subroutine is called as follows:
             * GetSectionAngles (Part 1 of 3) calls ShuffleSectionList


 The track section list lives in the first six bytes of the following variable
 blocks, and contains data on a number of track sections that we use when
 calculating the positions of the track verges:

   * xVergeLeftLo
   * xVergeLeftHi
   * yVergeLeft
   * xVergeRightLo
   * xVergeRightHi
   * yVergeRight

 This routine shuffles the list along by one position, so we can insert a new
 track section into position 0 while  pushing the data in position 5 off the
 end of the list.

 The remaining bytes in the blocks are used to store track segment data.


.ShuffleSectionList

 LDX #44                \ Set X = 44 so we start by shuffling the first batch:
                        \
                        \   * xVergeLeftLo+0-4 to xVergeLeftLo+1-5
                        \
                        \   * xVergeLeftHi+0-4 to xVergeLeftHi+1-5
                        \
                        \   * yVergeLeft+0-4 to yVergeLeft+1-5
                        \
                        \ This works because:
                        \
                        \   * xVergeRightLo + 40 = xVergeLeftLo
                        \
                        \   * xVergeRightHi + 40 = xVergeLeftHi
                        \
                        \   * yVergeRight + 40 = yVergeLeft

.shuf1

 LDA xVergeRightLo,X    \ Shuffle the X-th byte of xVergeRightLo up by one
 STA xVergeRightLo+1,X

 LDA xVergeRightHi,X    \ Shuffle the X-th byte of xVergeRightHi up by one
 STA xVergeRightHi+1,X

 LDA yVergeRight,X      \ Shuffle the X-th byte of yVergeRight up by one
 STA yVergeRight+1,X

 CPX #40                \ If X <> 40 then we are either still shuffling the
 BNE shuf2              \ first batch and haven't yet done the last shuffle, or
                        \ we are already shuffling the second batch, so in
                        \ either case jump to shuf2 to skip the following

 LDX #5                 \ We have just done the last shuffle in the first batch,
                        \ so set X = 5 so we now shuffle the second batch:
                        \
                        \   * xVergeRightLo+0-4 to xVergeRightLo+1-5
                        \
                        \   * xVergeRightHi+0-4 to xVergeRightHi+1-5
                        \
                        \   * yVergeRight+0-4 to yVergeRight+1-5

.shuf2

 DEX                    \ Decrement the loop counter

 BPL shuf1              \ Loop back until we have shuffled all the bytes in both
                        \ batches

 LDA #6                 \ Set sectionListStart = 6 - sectionListSize
 SEC                    \
 SBC sectionListSize    \ so sectionListStart is the start index of the track
 STA sectionListStart   \ section list, as the list ends at index 6

 JSR IncSectionPointers \ Update the list pointer variables so the new entry is
                        \ not marked as valid

 RTS                    \ Return from the subroutine