AoC 2024, day 7, part 2 (dc)

# Command: tr -d ':' <input | dc -fdc-p2.dc

[ 0*  ] sZ
[ lt+ ] sT

# exit recursion: idx=0 acc -> acc or 0
[
    + dlt!=Z            # zero top of stack it acc != target
    q
] sX

# recursion: stack frame: acc idx; return acc or 0
[
    d lt<X              # prune if acc > targ
    r 1-d 0=X           # idx--, check base case, stack: idx acc
    d3Rd                # acc acc idx idx
    3R d;l 3R+          # list(idx)+acc idx acc idx
    lRx                 # res acc idx
    _3R                 # tuck res

    d3Rd                # idx idx acc acc res+
    d;l 4R*
    lRx                 # res* idx acc res+
    _3R                 # tuck res

    d;l
    dZ10r^ 4R*+         # concat acc||val
    lRx                 # res|| res* res+
    ++                  # "or" results with +
] sR

0
?
[
    0
    [
        1+
        d3R r:l         # store list (backwards n to 1)
        z3<I
    ] dsIx              # stack: n=size-of-list target sum

    rst                 # target = input(0); stack: n sum
    d;l                 # acc = list(n) (first item); stack: acc i=n sum
    lRx                 # res sum
    0d 3R>T             # res = (res > 0) ? targ : 0
    +                   # sum += res
    ? z1<M
] dsMx

[Part 2: ]n p