sm83 asm backwardly iterate over all pairs of particles for an n-body sim

AccelerateBodies:
    ; Accelerate bodies
    ld a, [wNumParticles]
    cp 2
    ret c ; Not enough bodies (need at least 2)
    ; For b backwards from len - 1 inclusive to 1 inclusive
    ld b, a
    dec b
.outerLoop
    ; For c backwards from i - 1 inclusive to 0 inclusive
    ld c, b
    dec c
.innerLoop
    ; b: first particle's index, c: second particle's index

    ; Get and apply forces

    ; Has c finished counting down?
    dec c
    ld a, c
    cp -1 ; Check for underflow
    jr nz, .innerLoop
    ; Has b finished counting down?
    dec b
    jr nz, .outerLoop
    ret