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-rw-r--r--AoC2023/day06/input2
-rw-r--r--AoC2023/day06/solver.lisp40
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")))))