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⌂ ∞ ▽ Revs on the BBC Micro

Track geometry: GetFirstSegment

       Name: GetFirstSegment                                         [Show more]
       Type: Subroutine
   Category: Track geometry
    Summary: Get the track section coordinates and flags from the track data
             and populate the first track segment
  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:
             * GetTrackSegment (Part 1 of 3) calls GetFirstSegment




 This routine is called when we move into a new track section, and is used to
 populate the front track segment with the data from the start of the new
 track section.



 Arguments:

   sectionBehind        The number * 8 of the track section behind us, for when
                        we are facing backwards

   frontSegmentIndex    The index * 3 of the new front track segment in the
                        track segment buffer



 Returns:

   sectionListSize      Bits 0-2 from trackSectionData, or 2 if we are facing
                        backwards

   xSegmentCoordI       Start coordinates for the inside of the track section,
                        stored in the new front track segment

   xSegmentCoordO       Start coordinates for the outside of the track section,
                        stored in the new front track segment

   thisVectorNumber     The vector number from the start of the track section

   thisSectionFlags     The track section flags for the section containing the
                        new front track segment

   segmentFlags         Zeroed for the new front track segment



.GetFirstSegment

 LDX frontSegmentIndex  \ Set X to the index * 3 of the front track segment in
                        \ the track segment buffer

 LDY #6                 \ Set Y = 6

 STY newSectionFetched  \ Set newSectionFetched to a non-zero value to indicate
                        \ that we have just fetched a new section (as the
                        \ GetFirstSegment routine is only called when the
                        \ player's car enters a new section

 LDA resetSectionList   \ If resetSectionList = 0, then we do not need to reset
 BEQ getf1              \ the track section list, so jump to getf1 to skip the
                        \ following instruction

                        \ If we get here then resetSectionList is non-zero,
                        \ which means we need to reset the track section list

 STY sectionListValid   \ Set sectionListValid = 6 to indicate that the track
                        \ section list contains no valid entries, so the list
                        \ gets regenerated over the coming iterations

.getf1

 LDA directionFacing    \ If our car is facing backwards, jump to getf2
 BMI getf2

 LDY objTrackSection+23 \ Set Y to the number * 8 of the track section for the
                        \ front segment of the track segment buffer

 JSR GetSectionCoords   \ Copy the start coordinates for the track section
                        \ into xSegmentCoordI and xSegmentCoordO, and set
                        \ thisVectorNumber to trackSectionFrom

 LDA trackSectionData,Y \ Set A = trackSectionData for the track section, so
                        \ bits 0-2 can be set as the value for sectionListSize
                        \ below

 JMP getf3              \ Jump to getf3 to skip the following

.getf2

 LDY sectionBehind      \ Set Y to the number * 8 of the track section in
                        \ sectionBehind

 JSR GetSectionCoords   \ Copy the start coordinates for the track section
                        \ into xSegmentCoordI and xSegmentCoordO, and set
                        \ thisVectorNumber to trackSectionFrom

 JSR UpdateVectorNumber \ Update thisVectorNumber to the next vector along the
                        \ track in the direction we are facing

 LDA #2                 \ Set A = 2, to use as the value for sectionListSize
                        \ below

.getf3

 AND #%00000111         \ Set sectionListSize = bits 0-2 from A
 STA sectionListSize

 LDY objTrackSection+23 \ Set Y to the number * 8 of the track section for the
                        \ new front segment

 LDA trackSectionFlag,Y \ Set thisSectionFlags to the track section flags for
 STA thisSectionFlags   \ the section containing the new front track segment

 LDA #0                 \ Zero the flags for the front track segment
 STA segmentFlags,X

 RTS                    \ Return from the subroutine