The Theme: GetAlienWeakSpot

       Name: GetAlienWeakSpot                                        [Show more]
       Type: Subroutine
   Category: The Theme
    Summary: Calculate the coordinates of an alien's weak spot
  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 GetAlienWeakSpot


 The alien's weak spot is calculated as follows:

   * x-coordinate = (xObjectHi xObjectLo) + (5 Q)

   * y-coordinate = (yObjectHi yObjectLo)

   * z-coordinate = (zObjectHi zObjectLo) + (5 Q)

 where Q is the low byte of the amount to add, depending on the feeding state
 of the alien.

 This is called STIP in the original source code.


 Arguments:

   Y                    The object ID of the alien (30 to 33)

   Q                    The low byte of the amount to add


 Returns:

   (I+2 W+2)            The x-coordinate of the alien's weak spot

   (I+1 W+1)            The y-coordinate of the alien's weak spot

   (I W)                The z-coordinate of the alien's weak spot


.GetAlienWeakSpot

 LDX #2                 \ Set a counter in X to iterate through 2, 1, 0, which
                        \ has the following effect:
                        \
                        \ When X = 2:
                        \
                        \   * (I+2 W+2) = (xObjectHi xObjectLo) + (5 Q)
                        \
                        \ When X = 1:
                        \
                        \   * (I+1 W+1) = (yObjectHi yObjectLo)
                        \
                        \ When X = 0:
                        \
                        \   * (I W) = (zObjectHi zObjectLo) + (5 Q)
                        \
                        \ note that

.weak1

 LDA xObjectLo,Y        \ Set (I+X W+X) = (xObjectHi xObjectLo) + (5 Q)
 CLC                    \
 ADC Q                  \ starting with the low bytes
 STA W,X

 LDA xObjectHi,Y        \ And then the high bytes
 ADC #5
 STA I,X

.weak2

 TYA                    \ Point Y to the next axis (xObject, yObject, zObject)
 CLC
 ADC #nextAxis
 TAY

 DEX                    \ Decrement the loop counter

 BPL weak3              \ If we haven't yet done all three calculations, jump
                        \ to weak3

 RTS                    \ Return from the subroutine

.weak3

 BEQ weak1              \ If X = 0, jump up to weak1 to add (5 Q)

 LDA xObjectLo,Y        \ If we get here then X = 1, so do the calculation
 STA W,X                \ without adding (5 Q)
 LDA xObjectHi,Y
 STA I,X

 JMP weak2              \ Jump back to weak2 to move on to X = 0