day15

1 files changed, 48 insertions, 0 deletions

diff --git a/day15.lisp b/day15.lisp
new file mode 100644
index 0000000..a27ff7d
--- /dev/null
+++ b/day15.lisp
@@ -0,0 +1,48 @@
+(defun split-by-char (char string)
+  (loop for i = 0 then (1+ j)
+        as j = (position char string :start i)
+        collect (subseq string i j)
+        while j))
+
+(defun read-seq (filespec)
+  (with-open-file (stream filespec)
+    (split-by-char #\, (read-line stream))))
+
+(defun hash-algorithm (str)
+  (let ((res 0))
+    (loop for c across str do (setf res (mod (* 17 (+ res (char-code c))) 256))) res))
+
+(defun solve1 (filespec)
+  (reduce #'+ (mapcar #'hash-algorithm (read-seq filespec))))
+
+(defun solve2 (filespec)
+  (let ((seq (read-seq filespec))
+        (boxes (make-array 256 :initial-element ())))
+    (loop for str in seq
+          as symbp = (loop for c across str
+                           for i from 0
+                           if (or (equal c #\-) (equal c #\=)) return i)
+          as symb = (aref str symbp)
+          as label = (subseq str 0 symbp)
+          as hash = (hash-algorithm label)
+          if (equal symb #\=)
+            do (let* ((value (parse-integer (subseq str (1+ symbp))))
+                      (box (aref boxes hash))
+                      (lmember (assoc label box :test 'equal)))
+                 (if lmember (setf (cdr lmember) value)
+                     (setf (aref boxes hash) (acons label value box))))
+          else
+            do (setf (aref boxes hash)
+                     (remove-if (lambda (a) (equal label (car a)))
+                                (aref boxes hash) :count 1)))
+    (loop for box across boxes
+          for i from 1
+          sum (loop for lens in (reverse box)
+                    for j from 1
+                    sum (* i j (cdr lens))))))
+
+(print (solve1 "data/15/example.txt")) ;; 1320
+(print (solve1 "data/15/input.txt"))   ;; 516804
+(print (solve2 "data/15/example.txt")) ;; 145
+(print (solve2 "data/15/input.txt"))   ;; 231844
+