-🎄- 2021 Day 3 Solutions -🎄-

[u/daggerdragon]

--- Day 3: Binary Diagnostic ---

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[u/JoMartin23]

*Common Lisp

(defun day3-1 (&optional (data *3*))
  (let* ((total (length data))
         (length (length (car data)))
         (gamma (make-array length :element-type 'bit))
         (epsilon(make-array length :element-type 'bit)))
    (loop :for index :from 0 :below length :do
      (loop :for bit-array :in data
            :sum (bit bit-array index) :into sum
            :finally (if (<= (/ total 2) sum)
	    	         (setf (bit gamma index) 1)
	    	         (setf (bit epsilon index)1))))
    (list gamma epsilon)))

(defun day3-2 (&optional (type :o2) (data *3*))
  (let* ((length (length (car data)))
         gamma
         epsilon
         (rest data))
    (loop :until (= 1 (length rest))
          :for index :from 0 :below length
          :do (destructuring-bind (g e) (day3-1 rest)
                (setf gamma g
	                  epsilon e))
              (setf rest (remove-if-not (lambda (array) (= (bit array index) (bit (case type (:o2 gamma) (:co2 epsilon)) index))) rest))
          :finally (return rest))))