Add library from last year

lib/lib-advent.org

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+#+title: A collection of helper functions for Advent of Code
+#+property: header-args :results none
+
+* Input manipulation
+#+begin_src emacs-lisp
+  (require 'dash)
+  (defun advent/replace-multiple-regex-buffer (regex-list)
+    "Takes REGEX-LIST as an alist ( regex . replacement )
+  and applies each substitiution to the current buffer"
+    (save-excursion
+      (-each regex-list (lambda (x)
+                          (goto-char (point-min))
+                          (replace-regexp (car x) (cdr x))))))
+#+end_src
+
+* Maze processing
+  #+begin_src emacs-lisp
+    (defun advent/split-string-into-char-list (str)
+      "Split the string into a list of chars"
+      (--map (string-to-char it) (split-string str "\\|.+" t)))
+  #+end_src
+
+  #+begin_src emacs-lisp
+      (defun advent/char-at (p l)
+        "return the char at position P in the 2D list l"
+    (nth (car p) (nth (cadr p) l)))
+  #+end_src
+
+  #+begin_src emacs-lisp
+    (defun advent/coordinates-of (e li)
+      "This takes a 2D list (a list of lists) of chars LI and an element E
+    and returns a list of positions in LI that contain the element E.
+    "
+      (-map #'cadr
+            (--filter (eq (car it) e)
+                      (-mapcat #'identity
+                               (-map-indexed (lambda (y l)
+                                               (-map-indexed (lambda (x el) (list el (list x y))) l))
+                                             li)))))
+  #+end_src
+* 2D geometry auxiliary functions
+  Here a vector is just a list with two components; the "dot"
+  function is a bit of an overkill tbh
+
+  #+begin_src emacs-lisp :results none
+  (defun advent/dot (a b)
+    "Return the dot product of the two vectors"
+    (-reduce #'+ (-map (lambda (x) (* (car x) (cdr x))) (-zip-pair a b))))
+
+  (defun advent/neighbour (p dir)
+    "Returns the neighbour to P in the direction DIR"
+    (list (+ (car p) (car dir))
+          (+ (cadr p) (cadr dir))))
+
+  (defun advent/taxicab-distance (p q)
+    "Returns the taxicab distance from P to Q"
+    (+ (abs (- (car p) (car q)))
+       (abs (- (cadr p) (cadr q)))))
+#+end_src