Advent of Code with Common Lisp

Posted: lisp

One observation I've had working through Advent of Code with Common Lisp is that the LOOP macro is an absolute powerhouse.

When first learning Common Lisp, it's common to hear that the language is actually comprised of three separate languages: Common Lisp, FORMAT, and LOOP. Common Lisp itself is made up of the parenthetical soup that is easily recognizable. FORMAT and LOOP, on the other hand, each have their own bespoke syntax that looks next to nothing like Lisp.

Although the unique syntax of FORMAT and LOOP brings a learning curve on top of Common Lisp itself, both tools bring an incredible amount of power to the language. LOOP in particular has been fantastically useful in this year's Advent of Code.

Most Advent of Code exercises involve (a) reading lines of data from a file and (b) accumulating or counting some result from the data. This sort of operation is handled very gracefully by LOOP.

Here's a simple example from Day 2, summing scores from a file:

(defun score (line) ...)

(with-open-file (in "day02.txt")
  (loop for line = (read-line in nil)
        while line
        summing (score line) into total
        finally (return total)))

Within the LOOP macro, I'm reading one line from the input file at a time, storing it into the variable, LINE. After checking that the LINE is non-empty or EOF, I pass it into the SCORE function. The result of that function call is accumulated into a variable, TOTAL, which is incremented on each iteration of the loop. Finally, once the entire file is processed (and LINE is NIL) I return TOTAL.

Not very "Lispy", but very expressive and readable.

Day 12 has a more complex example. The following loops over all possible starting positions for a graph (the input) and determines the path to the given endpoint via a breadth-first search. While collecting these paths, the macro saves the shortest path into a variable, MIN, then returns it as the result. This variable is compared against all iterations, only changing value if the iteration is less than the stored result.

(defun bfs (start end) ...)

(loop for start in *starting-positions*
      for path = (bfs start #\E)
      when (not (null path))
      minimizing (length path) into min
      finally (return (1- min))

Note that you can assign as many iterators as you want by adding additional for VAR = EXPRESSION. You can similarly accumulate iterators from an automatically incremented value, like an index: for VAR from 0.

Another cool feature is the ability to loop over successive CDRs of a list. This feature was super handy for Day 13, when I needed to zip together pairs from a list of inputs:

(loop for el on '(a b c d e f)
      if (and (>= (length el) 2)
              (= (mod (length el) 2) 0))
        collect (subseq el 0 2))

;; Result: ((A B) (C D) (E F))

When looping via on, the value bound into EL is the CDR of that list, similar to calling NTHCDR with the index of that iteration:

(nthcdr 0 '(a b c d))
;; '(a b c d)

(nthcdr 1 '(a b c d))
;; '(b c d)

(nthcdr 2 '(a b c d))
;; '(c d)

By collecting SUBSEQ, I'm only collecting a slice of the list from the starting index to the end, forming pairs of two.

(subseq '(a b c d) 0 2)
;; '(a b)

Combine these two things, and protect against out-of-index errors, and pairs are achieved. A two-length slice is collected on each iteration from consecutive CDRs of the list.

The LOOP rabbit hole goes far deeper than what's shown in this post. A chapter of Peter Seibel's Practical Common Lisp has an entire chapter dedicated to the macro, the aptly named LOOP for Black Belts.

Thanks for reading! Send your comments to