AoC 2015, dc (part 2)

[ 1sp ] sP

0d                          # count = 0, sentinel
?
[
    0dd sp ss St            # init flags (pairs, sandwich), clear table/tmp with push

    # Process string (note: arrives backwards)
    [
        # pairs
        d3R d3R             # a b -> a b b a
        27*+ d              #     -> index index b a
        ;t 1=P              # set pairs if in table
        1 lt:t st           # delay add to table (with tmp) to avoid overlaps
        r                   # b a -> a b

        # sandwich
        3Rd3R =S r          # a b c -> b c (sandwich set if a = c)

        z 3<L               # leave: last-char sentinel count
    ] dsLx

    s.                      # clear last-char
    lpls*++                 # count += pair * sandwich + 0 (sentinel)

    0 ? z 2<M               # leave: sentinel count
] dsMx

+p