[AoC2023] day06 lisp part 1

2 files changed, 42 insertions, 0 deletions

diff --git a/AoC2023/day06/input b/AoC2023/day06/input
new file mode 100644
index 0000000..973ba45
--- /dev/null
+++ b/AoC2023/day06/input
@@ -0,0 +1,2 @@
+Time:        47     70     75     66
+Distance:   282   1079   1147   1062
diff --git a/AoC2023/day06/solver.lisp b/AoC2023/day06/solver.lisp
new file mode 100644
index 0000000..460b554
--- /dev/null
+++ b/AoC2023/day06/solver.lisp
@@ -0,0 +1,40 @@
+(ql:quickload '(fiveam str arrows))
+
+(defparameter eg-input "Time:      7  15   30
+Distance:  9  40  200")
+
+(defun bounds (time r &optional lower)
+  (multiple-value-bind (point reminder)
+      (if lower
+          (ceiling (/ (- time (sqrt (- (* time time) (* 4 r)))) 2))
+          (floor   (/ (+ time (sqrt (- (* time time) (* 4 r)))) 2)))
+    (if (zerop reminder)
+        (funcall (if lower #'1+ #'1-) point)
+        point)))
+
+(defun charge-time-solutions (pair)
+  (destructuring-bind (race-time . record-distance) pair
+    ;; quadratic equation: -charge_time**2 + race_time*charge_time - record_distance
+    (let ((lower (bounds race-time record-distance 'lower))
+          (upper (bounds race-time record-distance)))
+      (1+ (- upper lower)))))
+
+(defun solver1 (lines)
+  (arrows:->>
+   lines
+   (mapcar
+    (lambda (line)
+      (mapcar #'parse-integer (cdr (str:split-omit-nulls #\Space line)))))
+   (apply #'mapcar #'cons)
+   (mapcar #'charge-time-solutions)
+   (apply #'*)))
+
+(fiveam:test solutions
+  (fiveam:is
+   (= 288
+      (solver1
+       (uiop:split-string eg-input :separator '(#\Newline)))))
+  (fiveam:is
+   (= 281600
+      (solver1
+       (uiop:read-file-lines "input")))))