controller.asm

; Goal: to turn controller button samples, incoming on the x register each
; frame, into three variables:
;   - cur, which represents the current state of the controller buttons
;   - prs, which represents the buttons pressed this frame
;   - rls, which represents the buttons released this frame
; 
; Example:
;   x                 cur               prs               rls
;                     ................  ................  ................  <- initial values
;   ...........1....  ...........1....  ...........1....  ................
;   ...1.......1....  ...1.......1....  ...1............  ................
;   ...1............  ...1............  ................  ...........1....
;   ...1.11.........  ...1.11.........  .....11.........  ................
;   ...1.11.........  ...1.11.........  ................  ................
;   ...1.1.....1....  ...1.1.....1....  ...........1....  ......1.........
;   .....1.....1....  .....1.....1....  ................  ...1............
;   .........1.1....  .........1.1....  .........1......  .....1..........
;   .........1.1....  .........1.1....  ................  ................
;   .........1.1....  .........1.1....  ................  ................
;   ....1....1.1....  ....1....1.1....  ....1...........  ................
;   ....1....1.1....  ....1....1.1....  ................  ................
;   ....1......1....  ....1......1....  ................  .........1......
;   ...11......1....  ...11......1....  ...1............  ................
;   ...11......1....  ...11......1....  ................  ................
;   ...........1....  ...........1....  ................  ...11...........
; 
; prs == !old & cur == (old ^ cur) & cur
; rls == old & !cur == (old ^ cur) & old


            ;b c    a   x   y   cur prs rls
            ;------------------------------
            ;           cur     old
lda cur     ;2 4    old cur     old             a = cur
stx cur     ;2 4    old cur     cur             cur = x
stx prs     ;2 4    old cur     cur cur         prs = x
sta rls     ;2 4    old cur     cur cur old     rls = a
trb prs     ;2 7    old cur     cur prs old     prs &= !a
txa         ;1 2    cur cur     cur prs old     a = x
trb rls     ;2 7    cur cur     cur prs rls     rls &= !a

; 13 bytes, 32 cycles

            ;b c    a   x   y   cur prs rls
            ;------------------------------
            ;           cur     old
txa         ;1 2    cur cur     old             a = x
eor cur     ;2 4    o^c cur     old             a ^= cur
tay         ;1 2    o^c cur o^c old             y = a
and cur     ;2 4    rls cur o^c old             a &= cur
sta rls     ;2 4    rls cur o^c old     rls     rls = a
stx cur     ;2 4    rls cur o^c cur     rls     cur = x
tya         ;1 2    o^c cur o^c cur     rls     a = y
and cur     ;2 4    prs cur o^c cur     rls     a &= cur
sta prs     ;2 4    prs cur o^c cur prs rls     prs = a

; 15 bytes, 30 cycles

;---------------------------------------------------------------------------
; Or without rls:

            ;b c    a   x   y   cur prs
            ;--------------------------
            ;           cur     old
lda cur     ;2 4    old cur     old             a = cur
stx cur     ;2 4    old cur     cur             cur = x
stx prs     ;2 4    old cur     cur cur         prs = x
trb prs     ;2 7    old cur     cur prs         prs &= !a

; 8 bytes, 19 cycles

            ;b c    a   x   y   cur prs
            ;--------------------------
            ;           cur     old
txa         ;1 2    cur cur     old             a = x
eor cur     ;2 4    o^c cur     old             a ^= cur
stx cur     ;2 4    o^c cur     cur             cur = x
and cur     ;2 4    prs cur     cur             a &= cur
sta prs     ;2 4    prs cur     cur prs         prs = a

; 9 bytes, 18 cycles