Utility routines: NextRandomNumber

       Name: NextRandomNumber                                        [Show more]
       Type: Subroutine
   Category: Utility routines
    Summary: Point to the next item in the randomNumbers list
  Deep dive: Random numbers
    Context: See this subroutine in context in the source code
 References: This subroutine is called as follows:
             * ApplyFlightModel (Part 3 of 7) calls NextRandomNumber
             * ExplodeAlien calls NextRandomNumber


 The pointer for the randomNumbers list is stored in the first location. Each
 call to this routine increments the pointer through 0 to 10, after which it
 wraps back round to 0.


 Returns:

   A                    Points to the next number in the randomNumbers list


.NextRandomNumber

 LDA randomNumbers      \ Set A = randomNumbers + 1
 CLC                    \
 ADC #1                 \ so A points to the next item in the randomNumbers list

 CMP #11                \ If A < 11, skip the following instruction as the
 BCC rand1              \ pointer hasn't yet reached the end of the list

 LDA #0                 \ A >= 11, which is past the end of the list, so set
                        \ A = 0 to set the pointer back at the start of the list

.rand1

 STA randomNumbers      \ Store the updated pointer in randomNumbers

 RTS                    \ Return from the subroutine