1 files changed, 59 insertions, 0 deletions
diff --git a/day06.lisp b/day06.lisp
new file mode 100644
index 0000000..7d87c59
--- /dev/null
+++ b/day06.lisp
@@ -0,0 +1,59 @@
+(defstruct problem
+  time
+  dist)
+
+(defun parse-integers (string &key (start 0))
+  (loop for i = start then (cadr o)
+        as o = (multiple-value-list (parse-integer string :start i :junk-allowed t))
+        as n = (car o)
+        while n
+        collect n))
+
+(defun parse-integers-after (string item)
+  (parse-integers string :start (1+ (position item string))))
+
+(defun parse-problem (f)
+  (with-open-file (s f)
+    (let* ((time (parse-integers-after (read-line s) #\:))
+           (dist (parse-integers-after (read-line s) #\:)))
+      (make-problem :time time
+                    :dist dist))))
+
+;; Assumes answer cannot be 0
+(defun ways-to-win-race (time dist)
+  (labels ((compute-dist (i)
+             (* (- time i) i))
+           (upper-bound (l r)
+             (let ((m (ash (+ l r) -1)))
+               (if (= l r)
+                   l
+                   (if (> (compute-dist m) dist)
+                       (upper-bound l m)
+                       (upper-bound (1+ m) r))))))
+    (let ((l (upper-bound 0 (ash time -1))))
+      (- (1+ time) (ash l 1)))))
+
+(defun solve1(f)
+  (let ((problem (parse-problem f)))
+    (reduce #'* (loop for time in (problem-time problem)
+                      for dist in (problem-dist problem)
+                      collect (ways-to-win-race time dist))
+            :initial-value 1)))
+
+(defun parse-problem2 (f)
+  (with-open-file (s f)
+    (let* ((time (parse-integers-after (remove #\Space (read-line s)) #\:))
+           (dist (parse-integers-after (remove #\Space (read-line s)) #\:)))
+      (make-problem :time time
+                    :dist dist))))
+
+(defun solve2(f)
+  (let* ((problem (parse-problem2 f))
+         (time (car (problem-time problem)))
+         (dist (car (problem-dist problem))))
+    (ways-to-win-race time dist)))
+
+(print (solve1 "data/06/example.txt")) ;; 288
+(print (solve1 "data/06/input.txt"))   ;; 1731600
+(print (solve2 "data/06/example.txt")) ;; 71503
+(print (solve2 "data/06/input.txt"))   ;; 40087680