collision detection in joshdown

; swap using just A + stack
.macro swapa mema, memb
  lda mema
  pha
  lda memb
  sta mema
  pla
  sta memb
.endmacro

.proc ChkTouch16x16 ; Carry is set if collision is detected between TouchLeftA,TouchTopA and TouchLeftB,TouchTopB
  jsr :+
  swapa TouchLeftA, TouchLeftB
  swapa TouchTopA,  TouchTopB
  jsr :+
  clc ; no collision
  rts
: lda TouchLeftB
  add #16
  sta TouchRight
  lda TouchTopB
  add #16
  sta TouchBottom

  lda TouchLeftA
  cmp TouchLeftB
  bcc :+

  lda TouchLeftA
  cmp TouchRight
  bcs :+

  lda TouchTopA
  cmp TouchTopB
  bcc :+

  lda TouchTopA
  cmp TouchBottom
  bcs :+
  pla
  pla
  sec ; collision detected
: rts
.endproc