http://adventofcode.com/2017/day/1

day1.el

;; Part 1
(defun sum (list)
  "Solution for Part #1 http://adventofcode.com/2017/day/1"
  (let ((s 0) next-i next val (n (length list)))
    (dotimes (i n)
      (progn
        (setq val (nth i list))
        (setq next-i (+ i 1))
        (when (= next-i n) (setq next-i 0))
        (setq next (nth next-i list))
        (when (eq val next) (setq s (+ s val)))
        )
      )
    s
    )
  )

(sum '(1 1 2 2)) ;; 3
(sum '(1 1 1 1)) ;; 4
(sum '(1 2 3 4)) ;; 0
(sum '(9 1 2 1 2 1 2 9)) ;; 9

(defun read-numbers-from-file (file-path)
  (with-temp-buffer
    (insert-file-contents file-path)
    (mapcar 'string-to-number (split-string (string-trim (buffer-string)) "" t))
    )
  )

(sum (read-numbers-from-file "./input.txt"))

;; Part 2
(defun sum1 (list)
  "Solution for Part #2 http://adventofcode.com/2017/day/1"
  (let ((s 0) next-i next val d (n (length list)))
    (setq d (/ n 2))
    (dotimes (i n)
      (progn
        (setq val (nth i list))
        (setq next-i (+ i d))
        (when (> next-i (1- n)) (setq next-i (- next-i n)))
        (setq next (nth next-i list))
        (when (eq val next) (setq s (+ s val)))
        )
      )
    (print s)
    )
  )

(sum1 '(1 2 1 2)) ;; 6
(sum1 '(1 2 2 1)) ;; 0
(sum1 '(1 2 3 4 2 5)) ;; 4
(sum1 '(1 2 3 1 2 3)) ;; 12
(sum1 '(1 2 1 3 1 4 1 5)) ;; 4

(sum1 (read-numbers-from-file "./input1.txt"))