Scoring: ScoreHitPoints

       Name: ScoreHitPoints                                          [Show more]
       Type: Subroutine
   Category: Scoring
    Summary: Award points for destroying an alien, and remove the alien from
             its slot and the radar, if required
    Context: See this subroutine in context in the source code
 References: This subroutine is called as follows:
             * ExplodeAlien calls ScoreHitPoints


.ScoreHitPoints

 LDA #255               \ Set A to a negative number so we can clear out the
                        \ alien slot below

 LDY hitObjectId        \ Set Y to the ID of the object we just hit with our
                        \ bullets

 CPY #30                \ If Y is not 30, then jump to hitp1 to clear out the
 BNE hitp1              \ slot

 LDX alienSlot          \ Set X to the number of the alien in slot 30

 JMP hitp2              \ Jump to hitp2 to skip the following

.hitp1

                        \ If we get here then we just hit an alien in slot 31
                        \ to 33

 LDX alienSlot-30,Y     \ Set X to the number of the alien in slot Y

 STA alienSlot-30,Y     \ Store A (which is negative) in slot Y to empty the
                        \ slot

.hitp2

 STA alienObjectId,X    \ Reset the object ID associated with the alien that
                        \ we just hit (i.e. the one in the range 16 to 29)

 CPY #33                \ If Y is not 33, then jump to hitp3 as only alien 33
 BNE hitp3              \ appears on the radar

 JSR ResetRadar         \ Remove the alien from the radar

 LDA #3                 \ Set A = 3 and jump down to hitp5 to award 30 points
 BNE hitp5              \ for destroying the flying alien (this BNE is
                        \ effectively a JMP as A is never zero)

.hitp3

 LDX feedingStage       \ Set X to feedingStage, the feeding stage of the alien
                        \ we just hit (if it isn't dormant or flying)

 CPY #31                \ If Y >= 31, i.e. this is not alien 30, then jump to
 BCS hitp4              \ hitp4 to award a score based on the alien's feeding
                        \ stage

 LDA dormantAlienScore  \ This is alien 30, which is always dormant, so fetch
 JMP hitp5              \ the score for killing a dormant alien (400 points)
                        \ and jump down to hitp5 to increase the score

.hitp4

 LDA alienScore,X       \ Fetch the score for killing an alien in the feeding
                        \ state in A (0 = fattest alien, 3 = slimmest alien)

.hitp5

 LDX #0                 \ Add A * 10 points to the score by calling UpdateScore
 JSR UpdateScore        \ with (X A) = (0 A) = A

 RTS                    \ Return from the subroutine