aoc-2021/6/fish.lisp

(defparameter *fish-input* "./input.txt")
(ql:quickload :cl-strings)
(use-package :cl-strings)

(defun read-fish ()
  "Read input file, returning a list of its fish ages."
  (let ((input (open *fish-input* :if-does-not-exist nil))
        (fish-ages '()))
    (when input
      (setq fish-ages (map 'list 'parse-integer (cl-strings:split (read-line input nil) ",")))
      (close input))

    fish-ages))

(defun process-fish (input-fish)
  (defun fish-iter (fish new-fish-ages new-fish-count)
    (if (eq fish '())
        (append new-fish-ages
                (loop for x upto (1- new-fish-count) collect 8))
        (let ((current-fish (car fish)))
          (cond ((= current-fish 0)
                 (fish-iter (cdr fish) (cons 6 new-fish-ages) (1+ new-fish-count)))
                ('true
                 (fish-iter (cdr fish) (cons (1- current-fish) new-fish-ages) new-fish-count))))))

  (reverse (fish-iter input-fish '() 0)))

(defun process-fish-generations (generations fish)
  (defun my-fish-iter (gens fish-ages)
    (if (= gens 0)
        fish-ages
        (my-fish-iter (1- gens) (process-fish fish-ages))))
  (my-fish-iter generations fish))

(length (process-fish-generations 80 (read-fish)))
;; -> 352872

;; Part 2: 256 generations
;;  my solution is too naive to process this many generations
;;  efficiently :/