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#!/usr/bin/sbcl --script
(defparameter *seen*
(make-hash-table :test #'equal))
(defconstant +farm+
(with-open-file (stream "input")
(let ((lines (loop for line = (read-line stream nil)
while line
collect (coerce line 'array))))
(make-array (list (length lines)
(length (first lines)))
:initial-contents lines))))
(defun main ()
(loop with dimensions = (array-dimensions +farm+)
for i from 0 below (first dimensions)
sum (loop for j from 0 below (second dimensions)
sum (multiple-value-call #'* (flood-fill i j)))))
(defun flood-fill (i j)
(flood-fill-1 (aref +farm+ i j) i j))
(defun flood-fill-1 (char i j)
(block nil
(let ((pos (cons i j)))
(when (gethash pos *seen*)
(return (values 0 0)))
(setf (gethash pos *seen*) t))
(when (char/= char (try-aref i j))
(return (values 0 0)))
(loop with neighbors = (locate-neighbors char i j)
with a = 1
;; START PART 1
with c = (- 4 (length neighbors))
;; END PART 1 START PART 2
with c = (corner-count char neighbors i j)
;; END PART 2
for (i . j) in neighbors
do (multiple-value-bind (a1 c1) (flood-fill-1 char i j)
(incf a a1)
(incf c c1))
finally (return (values a c)))))
(defun locate-neighbors (char i j)
(loop for (i . j) in (list (cons (1- i) j) (cons i (1- j))
(cons (1+ i) j) (cons i (1+ j)))
if (char= char (try-aref i j))
collect (cons i j)))
;; START PART 2
(defun corner-count (char neighbors i j)
;; Figuring out how many corners the current chunk of the farm has is a
;; bit tricky. If the chunk has less than 2 neighbors then the number
;; of corners is fixed. When there are 2 neighbors then if the three
;; chunks form a line there are no corners, and if they form an ‘L’
;; then there may be 1 or 2 corners. When there are 4 neighbors then
;; there are as many corners as there aren’t diagonal neighbors.
;; Finally the trickiest is 3 neighbors. In this case we need to
;; figure out which 2 diagonal-neighbors would turn our ‘T’ shape into
;; a rectangle, and use them to determine how many corners we have.
(let ((n (length neighbors)))
(cond
((= n 0) 4)
((= n 1) 2)
((= n 2)
(let* ((l (first neighbors))
(r (second neighbors))
(li (car l)) (ri (car r))
(lj (cdr l)) (rj (cdr r)))
(cond
((or (= li ri i) (= lj rj j)) 0)
((char= char (try-aref
(if (= li i) ri li)
(if (= lj j) rj lj)))
1)
(:else 2))))
((= n 3)
(let ((wart (loop for n in neighbors
if (/= (car n) i)
collect n into odd-one-out-i
else
collect n into odd-one-out-j
finally (return
(first (if (= 1 (length odd-one-out-i))
odd-one-out-i
odd-one-out-j))))))
(loop for n in neighbors
count (not (or (equal n wart)
(char= char (try-aref
(if (= (car n) i)
(car wart)
(car n))
(if (= (cdr n) j)
(cdr wart)
(cdr n)))))))))
((= n 4)
(loop for (i . j) in (list (cons (1- i) (1- j)) (cons (1- i) (1+ j))
(cons (1+ i) (1- j)) (cons (1+ i) (1+ j)))
count (char/= (try-aref i j) char))))))
;; END PART 2
(defun try-aref (i j)
(let ((dimensions (array-dimensions +farm+)))
(if (and (< -1 i (first dimensions))
(< -1 j (second dimensions)))
(aref +farm+ i j)
#\Nul)))
(format t "~d~%" (main))
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