(asdf:load-system :adventofcode2020)
(in-package #:adventofcode2020)
(named-readtables:in-readtable fn-reader)

(defun make-preambled-list (list k i)
  (subseq list (- i k) (1+ i)))

(defun check-consistency (list)
  (let ((preamble (butlast list))
        (checksum (car (last list))))
    (block find-sum
           (dolist (i preamble)
             (dolist (j preamble)
               (unless (= i j)
                 (when (= (+ i j) checksum)
                   (return-from find-sum '(i j)))))))))

(defun test-preambled-lists (list preamble)
  (loop for i upfrom preamble below (length list)
        unless (check-consistency (make-preambled-list list preamble i))
        return (nth i list)))

(defun find-sum-list (list checksum)
  (labels ((test-bounds (i j)
             (let ((sum (apply #'+ (subseq list i j))))
               (cond ((< sum checksum) :lt)
                     ((= sum checksum) :eq)
                     ((> sum checksum) :gt))))
           (find-bounds (i j)
             (case (test-bounds i j)
               (:lt (find-bounds i (1+ j)))
               (:eq (subseq list i j))
               (:gt (find-bounds (1+ i) j)))))
    (find-bounds 0 1)))

(day 09 input
  (let ((numbers (int-list-from input)))
    (part1 (test-preambled-lists numbers 25))
    (part2 (let ((sum-list (find-sum-list numbers (test-preambled-lists numbers 25))))
             (+ (apply #'min sum-list)
                (apply #'max sum-list))))))

(def-suite day09)
(in-suite day09)

(defvar *simple-numbers* 
  '(35 20 15 25 47 40 62 55
    65 95 102 117 150 182 
    127 219 299 277 309 576))

(test make-preambled-lists
  (is (equal
        '(35 20 15 25 47 40)
        (make-preambled-list *simple-numbers* 5 5)))
  (is (equal
        '(40 62 55 65 95 102)
        (make-preambled-list *simple-numbers* 5 10))))

(test check-list-consistencies
  (is-true  (check-consistency '(35 20 15 25 47 40)))
  (is-false (check-consistency '(95 102 117 150 182 127))))

(test test-preambled-lists
  (is (equal 127 (test-preambled-lists *simple-numbers* 5))))

(test find-sum-lists
  (is (equal
        '(15 25 47 40)
        (find-sum-list *simple-numbers* 
                       (test-preambled-lists *simple-numbers* 5)))))

(run! 'day09)