AoC 2021 day1 (dc)

#!/usr/bin/dc -finput

[q] SX                  # exit loop macro

[ lx 1+ sx ] S1         # inc counter for part 1
[ ly 1+ sy ] S2         # inc counter for part 2

# Main loop:
[
    z 0 =X              # stop when stack empty

    d li;p >1           # top -> (p(i) > top) top -> top

    d li 1+ 3% d si     # top -> (i+1)%3 top top, next i = (i+1)%3
    ;p >2               #     -> (p(next i) > top) top -> top

    li:p                # p(next i) = pop

    lLx                 # loop
] SL

# Mainline:
0d sx sy                # init counters for results

# Initialize previous array
# Note that input is pushed, so we're going backwards (first is actually last)
0:p d 0;p >1            # compare top two, store first
1:p d 1;p >1            # compare next two, store second
2:p                     # store third

2 si                    # init index into previous array
lLx                     # start loop

[Part 1: ]n lxp
[Part 2: ]n lyp