AoC 2023 day 9 part 2 (dc)

# Command: tr '-' '_' <input | dc -fpart2.dc

0                           # sum = 0
?
[
    zsn                     # n = length
    [
        zlnr-:a             # array(n-z) = top
        z1<L
    ] dsLx

    ln2-                    # stack: sum i=len (array is 2 less than n marked)
    [
        dd;a                # sum i j=i a(j)
        d5R+                # i j a(j) sum+=a(j)

        _4Rr                # sum i a(j) j=i
        [
            1-              # j--

            d;a             # sum a(j+1) j a(j)
            d 4Rr-          # diff = a(j+1) - a(j)

            3Rd             # sum a(j) diff j j
            _3R:a           # a(j)=diff; stack: sum old-a(j) j

            d 0<J           # actually need only to 2<J, but this makes clearing junk easy
        ] dsJx

        # stack: sum i junk 0
        *+                  # clear junk

        1- d1<I             # only need 2<I again, but saves a stroke cleaning up
    ] dsIx

    *                       # clear i (*1)

    ? z1<M
] dsMx

[Part 2: ]np