AoC 2021 day 3, part 1 (dc)

[q] SX

[
    [
        li;c 1+ li:c    # (c(i) = c(i)++) i
    ] SC

    [
        li 12=X         # (exit if i = 12) top
        2~              # top -> top%2 top/2
        1=C             # (top%2 = 1) top/2
        li 1+ si        # (i--) top/2

        lIx             # loop, next top = top/2
    ] SI

    z 0 =X              # stop when stack empty

    0si lIx             # i = 0, run loop
    s.                  # pop remains of top

    lLx                 # loop
] SL

#
# Mainline start
#

1010i                   # back to decimal
z 2/ sn                 # n = half size of input

# Initialize counts
[
    1-                  # i--
    d 0 r:c             # 0 i i -> (c(i) = 0) i
    d 0=X               # (exit if i = 0)
    lIx                 # loop
] SI
12 lIx s.               # push i, run loop, pop i

lLx                     # start loop

# Calculate gamma and epsilon (XOR via subtraction)
0 sg                    # init gamma, epsilon

[ 1+ ] SS

[
    0 lb;c ln >S        # (n > c(b)) 0, S changes 0 to 1
    2 lb ^ * lg+ sg     # (0|1) -> (g = g + 2^b * (0|1))
    lb 1+ d sb 12=X

    lBx                 # loop
] SB
0 sb lBx

[Part 1: ]n lg 4095 lg-* p