AoC 2024, day 14, part 1 (dc)

# Command: tr -- '-p=v,' '_ ' <input | dc -fdc-p1.dc

# Calculate quadrant and increment
[
    ly dd*v/                    # y /= abs(y)
    lx dd*v/                    # x /= abs(x); stack: sign(x) sign(y)
    2*3r- +                     # index = 3 - sign(x) * 2 + sign(y) => {0, 2, 4, 6}
    d;q 1+ r:q
] sQ

?                               # stack: vy vx y x
[
    # ASSUME: velocities on (-dim,dim) and position always non-negative
    # This means we can just add dim to velocities to have mod residues be non-negative.
    A3+ A0* 3R+ A3% 51-         # y+=100*vy (mod 103), residue -51..51
    r A1+ A0* 3R+ A1% 50-       # x+=100*vx (mod 101), residue -50..50

    # stack: x y (moved)
    dsx r dsy* 0!=Q             # if (x*y) != 0, add to quad (pass by reg for clean stack)

    ? z0<M
] dsMx

[Part 1: ]n 6;q4;q*2;q*0;q*p