Universe: SpawnSpaceStation

       Name: SpawnSpaceStation                                       [Show more]
       Type: Subroutine
   Category: Universe
    Summary: Add a space station to the local bubble of universe if we are
             close enough to the station's orbit
    Context: See this subroutine in context in the source code
 References: This subroutine is called as follows:
             * Main flight loop (Part 14 of 16) calls SpawnSpaceStation


.SpawnSpaceStation

                        ; We now check the distance from our ship (at the
                        ; origin) towards the point where we will spawn the
                        ; space station if we are close enough
                        ;
                        ; This point is calculated by starting at the planet's
                        ; centre and adding 2 * nosev, which takes us to a point
                        ; above the planet's surface, at an altitude that
                        ; matches the planet's radius
                        ;
                        ; This point pitches and rolls around the planet as the
                        ; nosev vector rotates with the planet, and if our ship
                        ; is within a distance of (100 0) from this point in all
                        ; three axes, then we spawn the space station at this
                        ; point, with the station's slot facing towards the
                        ; planet, along the nosev vector
                        ;
                        ; This works because in the following, we calculate the
                        ; station's coordinates one axis at a time, and store
                        ; the results in the INWK block, so by the time we have
                        ; calculated and checked all three, the ship data block
                        ; is set up with the correct spawning coordinates

 SETUP_PPU_FOR_ICON_BAR ; If the PPU has started drawing the icon bar, configure
                        ; the PPU to use nametable 0 and pattern table 0

 LDX #0                 ; Call MAS1 with X = 0, Y = 9 to do the following:
 LDY #9                 ;
 JSR MAS1               ;   (x_sign x_hi x_lo) += (nosev_x_hi nosev_x_lo) * 2
                        ;
                        ;   A = |x_sign|

 BNE MA23S2             ; If A > 0, jump to MA23S2 to skip the following, as we
                        ; are too far from the planet in the x-direction to
                        ; bump into a space station

 LDX #3                 ; Call MAS1 with X = 3, Y = 11 to do the following:
 LDY #11                ;
 JSR MAS1               ;   (y_sign y_hi y_lo) += (nosev_y_hi nosev_y_lo) * 2
                        ;
                        ;   A = |y_sign|

 BNE MA23S2             ; If A > 0, jump to MA23S2 to skip the following, as we
                        ; are too far from the planet in the y-direction to
                        ; bump into a space station

 LDX #6                 ; Call MAS1 with X = 6, Y = 13 to do the following:
 LDY #13                ;
 JSR MAS1               ;   (z_sign z_hi z_lo) += (nosev_z_hi nosev_z_lo) * 2
                        ;
                        ;   A = |z_sign|

 BNE MA23S2             ; If A > 0, jump to MA23S2 to skip the following, as we
                        ; are too far from the planet in the z-direction to
                        ; bump into a space station

 LDA #100               ; Call FAROF2 to compare x, y and z with 100 * 2, which
 JSR FAROF2             ; will clear the C flag if the distance to the point is
                        ; < 200 or set the C flag if it is >= 200

 BCS MA23S2             ; Jump to MA23S2 if the distance to point (x, y, z) is
                        ; >= 100 (i.e. we must be near enough to the planet to
                        ; bump into a space station)

 JSR NWSPS              ; Add a new space station to our local bubble of
                        ; universe

 SEC                    ; Set the C flag to indicate that we have added the
                        ; space station

 RTS                    ; Return from the subroutine

.MA23S2

 SETUP_PPU_FOR_ICON_BAR ; If the PPU has started drawing the icon bar, configure
                        ; the PPU to use nametable 0 and pattern table 0

 CLC                    ; Clear the C flag to indicate that we have not added
                        ; the space station

 RTS                    ; Return from the subroutine