Extra tracks: HookSectionFrom

       Name: HookSectionFrom                                         [Show more]
       Type: Subroutine
   Category: Extra tracks
    Summary: Initialise and calculate the current segment vector
  Deep dive: Secrets of the extra tracks
             Dynamic track generation in the extra tracks
             Code hooks in the extra tracks
    Context: See this subroutine in context in the source code
 References: This subroutine is called as follows:
             * newContentHi calls HookSectionFrom
             * newContentLo calls HookSectionFrom


 This routine is called from GetSectionCoords when fetching the coordinates for
 a track section. It initialises the segment vector calculation process by
 doing the following:

   * Fetch the section's yaw angle from the trackYawAngle tables

   * Fetch the section's slope from the trackSlope table

   * Initialise the sub-section and sub-section segment variables

   * Modify the GetSectionAngles routine so the horizon level check is skipped
     if the section's trackSubConfig has bit 1 set

   * If we are facing forwards along the track, calculate and store the current
     segment vector


 Arguments:

   Y                    The number of the track section * 8 whose coordinates we
                        want to fetch


.HookSectionFrom

 STY yStore             \ Store the section number in yStore, so we can retrieve
                        \ it at the end of the hook routine

 LDA trackSectionFrom,Y \ Set thisVectorNumber= the Y-th trackSectionFrom, just
 STA thisVectorNumber   \ like the code that we overwrote with the call to the
                        \ hook routine

 TYA                    \ Set Y= Y / 8
 LSR A                  \
 LSR A                  \ So Y now contains the number of the track section (as
 LSR A                  \ trackSectionFrom contains the track section * 8)
 TAY

 LDA trackYawAngleLo,Y  \ Set (yawAngleHi yawAngleLo) to this section's entry
 STA yawAngleLo         \ from (trackYawAngleHi trackYawAngleLo)
 LDA trackYawAngleHi,Y
 STA yawAngleHi

 LDA trackSlope,Y       \ Set segmentSlope to this section's entry from
 STA segmentSlope       \ trackSlope

 LDA trackSubConfig,Y   \ Set A to this section's configuration byte

 LSR A                  \ Set A= A >> 2, with bit 6 cleared, bit 7 set to the
 ROR A                  \ bit 0 of the trackSubConfig entry, and the C flag set
                        \ to bit 1 of the trackSubConfig entry

 STA subSection         \ Store A in subSection, so it contains the index
                        \ from bits 2-7 of trackSubConfig, and bit 7 is set if
                        \ bit 0 of trackSubConfig is set

 LDA #14                \ Set A= 7, with bit 7 set to the C flag (so if this
 ROR A                  \ section's trackSubConfig has bit 1 set, then A is 135,
                        \ otherwise it is 7)

 STA &23B3              \ Modify the GetSectionAngles routine, at instruction
                        \ #4 after gsec11, to test prevHorizonIndex against the
                        \ value we just calculated in A rather than 7
                        \
                        \ So if this section's trackSubConfig has bit 1 set, the
                        \ test becomes prevHorizonIndex <= 135, which is always
                        \ true, so this modification makes us never set the
                        \ horizon line to 7 for sections that have bit 1 of
                        \ trackSubConfig set

 LDA #0                 \ Set subSectionSegment= 0, so we start counting from
 STA subSectionSegment  \ the first segment in the sub-section

 BIT directionFacing    \ If we are facing backwards along the track, jump to
 BMI from1              \ from1 to skip the following call to SetSegmentVector

 JSR SetSegmentVector   \ We are facing forwards along the track, so calculate
                        \ and store the current segment vector

.from1

 LDY yStore             \ Retrieve the section number from yStore

 LDA thisVectorNumber   \ Set A to the Y-th trackSectionFrom that we set above,
                        \ so the routine sets A to the segment vector number,
                        \ just like the code that we overwrote with the call to
                        \ the hook routine

 RTS                    \ Return from the subroutine