I’d forgotten that this day was also pretty hard. My “unpack” function (snap!) also delivered the goods for both parts. It works through the message calling either “literal” or “operator”. literal is self-contained, returning a value and an unused message fragment, while operator calls unpack in a mode depending on the length-type id.
So it’s recursive, and I found it very tricky to see what was happening! But it worked, eventually. Far too ugly to share here. It looks at first glance as if you translated the whole message into a J-type string. Non-recursive, anyway, unless I’ve overlooked something. Cheers, Mike Sent from my iPad > On 6 Jan 2022, at 01:50, Raul Miller <rauldmil...@gmail.com> wrote: > > https://adventofcode.com/2021/day/16 > > Here, we were decoding messages encoded using the AoC "BITS" protocol. > > A significant part of the puzzle was about comprehending the somewhat > shiftless protocol specification included in the puzzle. > > The approach I found useful was to set up a "test bench" with the > puzzle examples, and have a 'trace' feature enabled so that I could > see all intermediate results when parsing those examples. (I turned > this off for the large puzzle value, because that was considerably > bulkier.) > > For part A, it looked like this: > > trace=: 1 > assert 16= a15 '8A004A801A8002F478' > assert 12= a15 '620080001611562C8802118E34' > assert 23= a15 'C0015000016115A2E0802F182340' > assert 31= a15 'A0016C880162017C3686B18A3D4780' > > With that in place, I could relatively quickly compare notes about > examples in the puzzle with what my own code was doing. That said, for > part A we only had to have part of the protocol implemented. > > The protocol itself was something of a virtual machine implementation. > And, at this point, having solved part B, my part A solution includes > that virtual machine implementation which was not strictly necessary > for part A. > > Anyways... here is what my code which handles part A looks like. > There's some initial waffling about where I attempt to conceptualize > some of the different abstractions the protocol spec uses when talking > about the data. And then I wound up encoding part A specific > mechanisms into my 'unpack' routine. And, the code itself consists of > a 'parse' routine implementing a "core" of the protocol and an > 'operator' routine implementing a 'core' of the associated virtual > machine. > > require'convert' > hex=: ,toupper hfd i.16 > > BITS=:{{ > ,2 2 2 2 #: hex i.y -.LF > }} > > bits=: {{ > r=. packet{~pos+i.y > pos=: pos+y > r > }} > > unpack=: {{ > pos=: 0 > packet=: BITS y > vsum=: 0 > parse '' > }} > > operator=: {{ > lentype=. bits 1 > payload=. i.0 > if. lentype do. > sublen=. #. bits 11 > for. i.sublen do. > payload=. payload,parse'' > end. > else. > bitlen=. #. bits 15 > limit=. pos+bitlen > while. pos < limit-6 do. > payload=. payload,parse'' > end. > assert. pos <: limit > end. > if. trace do. echo x,":payload end. > payload > }} > > parse=: {{ > vsum=:vsum+V=. #. bits 3 > ID=. #. bits 3 > payload=. i.0 > select. ID > case. 0 do. payload=. +/ '+/'operator'' > case. 1 do. payload=. */ '*/'operator'' > case. 2 do. payload=. <./ '<./'operator'' > case. 3 do. payload=. >./ '>./'operator'' > case. 4 do. > value=. i.0 > while. {. group=. bits 5 do. > value=. value,}.group > end. > payload=. #.x:value,}.group > case. 5 do. payload=. >/0 0+'>/' operator'' > case. 6 do. payload=. </0 0+'</' operator'' > case. 7 do. payload=. =/0 0+'=/' operator'' > end. > payload > }} > > a15=:{{ > unpack y > vsum > }} > > My part B implementation looks like this: > > b15=: unpack > > assert 3= b15 'C200B40A82' > assert 54= b15 '04005AC33890' > assert 7= b15 '880086C3E88112' > assert 9= b15 'CE00C43D881120' > assert 1= b15 'D8005AC2A8F0' > assert 0= b15 'F600BC2D8F' > assert 0= b15 '9C005AC2F8F0' > assert 1= b15 '9C0141080250320F1802104A08' > > Running this against the puzzle input with trace=: 1 is kind of > amusing, but trace=:0 avoids quite a lot of 'trace spam'. > > FYI, > > -- > Raul > ---------------------------------------------------------------------- > For information about J forums see http://www.jsoftware.com/forums.htm ---------------------------------------------------------------------- For information about J forums see http://www.jsoftware.com/forums.htm
