Player: LandOnLaunchpad

       Name: LandOnLaunchpad                                         [Show more]
       Type: Subroutine
   Category: Player
    Summary: Check to see if the player has landed on the launchpad
  Deep dive: Collisions and bullets
    Context: See this subroutine in context in the source code
 References: This subroutine is called as follows:
             * MoveAndDrawPlayer (Part 3 of 5) calls LandOnLaunchpad


 We call this routine from MoveAndDrawPlayer when the player's altitude matches
 that of the launchpad, so this performs additional checks to see if the player
 just landed.


 Returns:

   R1                               The y-coordinate of the player's ship


.LandOnLaunchpad

 ADD     R3, R11, #xPlayer          \ Fetch the six words at xPlayer as follows:
 LDMIA   R3, {R0-R5}                \
                                    \  (R0, R1, R2) = the player's coordinates
                                    \           from (xPlayer, yPlayer, zPlayer)
                                    \
                                    \  (R3, R4, R5) = the player's velocity
                                    \     from (xVelocity, yVelocity, zVelocity)

 CMP     R0, #LAUNCHPAD_SIZE        \ If either of xPlayer or zPlayer is >=
 CMPLO   R2, #LAUNCHPAD_SIZE        \ LAUNCHPAD_SIZE then we are not over the
 BHS     LoseLife                   \ launchpad, as the launchpad is defined as
                                    \ this part of the map:
                                    \
                                    \   0 <= x 
                                    \
                                    \ and:
                                    \
                                    \   0 <= z 
                                    \
                                    \ So we just crashed into the ground rather
                                    \ than the launchpad, so jump to LoseLife to
                                    \ process the crash

 CMP     R3, #0                     \ Set R3 = |R3|
 RSBMI   R3, R3, #0                 \        = |xVelocity|

 CMP     R4, #0                     \ Set R4 = |R4|
 RSBMI   R4, R4, #0                 \        = |yVelocity|

 CMP     R5, #0                     \ Set R5 = |R5|
 RSBMI   R5, R5, #0                 \        = |zVelocity|

 ADD     R3, R3, R4                 \ If R3 + R4 + R5 >= LANDING_SPEED, then:
 ADD     R3, R3, R5                 \ 
 CMP     R3, #LANDING_SPEED         \   |xVelocity| + |yVelocity| + |zVelocity|
 MOVHS   PC, R14                    \
                                    \ is greater than the safe landing speed, so
                                    \ we can't be performing a safe landing, so
                                    \ return from the subroutine

                                    \ If we get here then we have landed, as the
                                    \ altitude checks were made before this
                                    \ routine was called, we know we are above
                                    \ the launchpad, and our speed is slow
                                    \ enough to land
                                    \
                                    \ So now we start refuelling

 MOV     R1, #LAUNCHPAD_Y           \ Set R1 to the y-coordinate of the player's
                                    \ ship as it sits on the launchpad (which is
                                    \ set to the launchpad altitude plus the
                                    \ height of the ship's undercarriage)

 LDR     R3, [R11, #fuelLevel]      \ Bump up the fuel level by 1/160 of a full
 ADD     R3, R3, #&20               \ tank
 CMP     R3, #&1400
 STRLO   R3, [R11, #fuelLevel]

 MOV     R3, #0                     \ Zero the player's velocity
 MOV     R4, #0
 MOV     R5, #0
 STR     R3, [R11, #xVelocity]
 STR     R4, [R11, #yVelocity]
 STR     R5, [R11, #zVelocity]

 STR     R1, [R11, #yPlayer]        \ Set the y-coordinate of the player to the
                                    \ y-coordinate of the launchpad, so the
                                    \ player's ship is resting correctly on the
                                    \ pad

 MOV     PC, R14                    \ Return from the subroutine