The Theme: CheckAlienWeakSpot

       Name: CheckAlienWeakSpot                                      [Show more]
       Type: Subroutine
   Category: The Theme
    Summary: Check whether an object is close enough to an alien's weak spot to
             be hitting it
  Deep dive: Detecting alien hits
    Context: See this subroutine in context in the source code
 References: This subroutine is called as follows:
             * CheckIfAlienIsHit (Part 2 of 2) calls CheckAlienWeakSpot


 If the object is hitting the alien's weak spot, the routine returns to the
 caller of the caller - in other words, it returns from the original call to
 the CheckIfAlienIsHit routine, in part 6 of the main loop.

 This is called HITS in the original source code.


 Arguments:

   Y                    The bullet trail to be calculated:

                          * 216 = object 0, which we set to points along the
                                  bullet trail at 1/32 intervals

                          * 228 = the trailing end of the left bullet trail

                          * 230 = the trailing end of the right bullet trail


.CheckAlienWeakSpot

 LDX #2                 \ Set a counter in X to iterate through the three axes

.spot1

 TYA                    \ Point Y to the next axis (xObject, yObject, zObject)
 CLC                    \
 ADC #nextAxis          \ The routine is called is with Y = 216, 228 or 230, so
 TAY                    \ this bumps X onto 256, 268 or 260, which gets
                        \ truncated in the 8-bit register to 0, 12 or 14, so
                        \ this moves X through the xObject, yObject and zObject
                        \ values for object 0 (when called with X = 216), object
                        \ 12 (when called with X = 228) or object 14 (when
                        \ called with X = 230)
                        \
                        \ Objects 12 and 14 are the trailing ends of the two
                        \ bullet trails, so the following checks whether the
                        \ trailing end is within the weak spot, rather than the
                        \ bullets themselves

 LDA xObjectLo,Y        \ Set (A T) = (xObjectHi xObjectLo) - (I+X W+X)
 SEC                    \
 SBC W,X                \ starting with the low bytes
 STA T

 LDA xObjectHi,Y        \ And then the high bytes
 SBC I,X

                        \ So we now have:
                        \
                        \   (A T) = xObject - (I W)
                        \
                        \ where xObject is the coordinate of the bullet, and
                        \ (I W) is the coordinate of the alien's weak spot
                        \
                        \ In other words, (A T) is the distance between the
                        \ bullet and the alien's weak spot

 BNE spot2              \ If the high byte in A is non-zero, then the bullet is
                        \ too far from the weak spot to do any damage, so jump
                        \ to spot2 to return from the subroutine

 LDA T                  \ If the low byte in T is >= the corresponding value in
 CMP PP,X               \ PP, QQ or RR (for the z, y and x-axes respectively),
 BCS spot2              \ then the bullet is too far away from the weak spot to
                        \ do any damage, so jump to spot2 to return from the
                        \ subroutine

 DEX                    \ Otherwise the bullet is close enough to the weak spot
                        \ to cause damage in this axis, so decrement X to point
                        \ to the next axis

 BPL spot1              \ Loop back until we have checked all three axes

                        \ If we get here then the bullet is close enough to the
                        \ weak spot in all three axes, so we have a hit

 LDA objectId           \ Store the object ID of the hit alien in hitObjectId
 STA hitObjectId

 TSX                    \ Remove two bytes from the top of the stack, so the
 INX                    \ RTS below returns us to the JSR CheckIfAlienIsHit in
 INX                    \ part 6 of the main loop
 TXS

 LDA #27                \ Set the hitTimer to 27 to start the explosion counter
 STA hitTimer

.spot2

 RTS                    \ Return from the subroutine