640px
□□□□□
□□□□□ 480px
□□□□□
□□□□□
□□□□□

□ = ↑/5 = 128*96px

// kuukan bunkatsu
db[ realBullet ] = reset
loop(allBullet){
　x = bullet.x / 128px
　y = bullet.y / 96px
　index = x + y * 5
　db[ realBullet ][ index ][ n ] = bullet.id
　n += 1
}

// hitCheck
x = actor.x / 128px
y = actor.y / 96px
index = x + y * 5
bullet = db[ realbullet ][ index ]
hitCheck( actor, bullet )

>>877
was?