day12-1.prdc

l]µIg:0Ó‡}1kχ:{‹m~u:au{¯U}zyÁeÁk‡D¸}v}{LšmÞ}š .. 45 bytes

{
l .. lines
] .. list
µ .. map:
  Ig .. extract ints
  :  .. copy
  0Ó .. vectorizing multiply by 0, to get an equally long list of 0s
  ‡  .. wrap into a list of size 2
}
.. we now have a list of four [pt, velocity] pairs
1kχ .. do this 1000 times:
  : .. copy, as we want to vectorize this against itself
  {  .. take a [pt, velocity] and the list of all [pt,velocity] pairs
     .. gravity of a moon against itself is fine, yay.
     .. X is the single [pt, velocity] so we don't particularly need to save it
     ‹m .. get all the pts out
     ~u .. expand the bottom list: stack has pt, velocity, list of pts
     :au .. copy pt again: stack has pt, velocity, pt, list of points
     .. to compute a single dimension gravity diff, take the minus the other
     .. way, then U to get the signum
     .. zip by that to get a full gravity diff vector
     .. bindmap that to get the list of gravity diffs
     {¯U}zy
     .. stack has pt, velocity, list of gravity diff
     Áe .. add every gravity diff onto velocity
     .. stack has pt, new velocity
     Ák .. stack has pt, new velocity, new pt
     ‡D .. make size-2 list, reverse
     ¸  .. pop the old pt
  }v
}
.. we have four [pt, velocity] pairs
.. sum by product by sum by absolute value
{LšmÞ}š
.. why isn't product a trailer, i'm sad
};