Flight model: ApplyFlightModel (Part 3 of 7)

       Name: ApplyFlightModel (Part 3 of 7)                          [Show more]
       Type: Subroutine
   Category: Flight model
    Summary: If we are near to an exploding alien, apply turbulence
  Deep dive: The flight model
             Explosions and turbulence
    Context: See this subroutine in context in the source code
 References: No direct references to this subroutine in this source file


 This part does the following:

   * If we are near an exploding alien, calculate the amount of turbulence and
     apply it to the (xControlsSc, yControlsSc, zControlsSc) vector, as
     follows:

       * Set the level of turbulence in the range 0 to 1920, to be inversely
         proportional to the distance from the explosion (so it is 0 if we are
         far away, 1920 if we are very close).

       * We randomly add or subtract this amount from each axis (with each axis
         being individually signed randomly), to give:

         xControlsSc = xControlsSc +/- turbulence / 2

         yControlsSc = yControlsSc +/- turbulence / 2

         zControlsSc = zControlsSc +/- turbulence / 2


 LDA hitTimer           \ If the hit timer is zero, then there are no exploding
 BEQ fmod3              \ aliens, so jump to fmod3 to skip this part

                        \ We now apply turbulence to the plane by applying a
                        \ random amount to xControlsSc, yControlsSc and
                        \ zControlsSc, with the random amount being positive or
                        \ negative, and with a larger magnitude the closer we
                        \ are to the alien

 LDX #&6A               \ Set X so in the following loop, the call to AddScaled
                        \ updates xControlsSc, yControlsSc and zControlsSc, one
                        \ on each iteration of the loop
                        \
                        \ The comments below are for xControlsSc

.fmod2

 JSR NextRandomNumber   \ Set A to point to the next item in the randomNumbers
                        \ list

 TAY                    \ Set P to the next random number from the list
 LDA randomNumbers+1,Y
 STA P

 LDA distanceFromHit    \ If distanceFromHit >= 16, jump to fmod3 to skip the
 CMP #16                \ following, as we are too far from the exploding alien
 BCS fmod3              \ to feel the effects

 EOR #&0F               \ As A < 16, this flips the value of A so instead of
                        \ being in the range 0 to 15, it's reversed to the
                        \ range 15 to 0 - so A is now 15 if we are really close
                        \ to the exploding alien, or 0 if we are further away

 ASL A                  \ Double the value of A, so it's in the range 0 to 30,
                        \ with the number being higher the closer we are to the
                        \ exploding alien

 LSR P                  \ We set P to a random number above, so this sets the C
                        \ flag randomly, so the call to AddScaled randomly adds
                        \ or subtracts the amount of turbulence in A

 LDY #1                 \ Set the scale factor in Y so we add or subtract
                        \ (A 0) >> 2, which is in the range 0 to 1920, as
                        \ (30 0) >> 2 is 1920

 JSR AddScaled          \ Set xControlsSc = xControlsSc +/- (A 0) >> 2
                        \
                        \ So this changes xControlsSc by an amount in the range
                        \ 0 to 1920, with a higher amount the closer we are to
                        \ the exploding alien, with the sign being random (i.e.
                        \ it is random whether we add or subtract the amount,
                        \ but the amount itself is not random)

 INX                    \ Increment the value of X so the next iteration updates
                        \ the next of xControlsSc, yControlsSc and zControlsSc

 CPX #&6D               \ Loop back until we have applied turbulence to all
 BNE fmod2              \ three axes