aoc 2025 - still need to 7

Forgot to push these all onto the cloud lol

2025/puzzle-3.lisp

@@ -0,0 +1,65 @@
+(load "util.lisp")
+
+(defpackage "aoc:3"
+  (:use :cl "aoc-util"))
+
+(in-package "aoc:3")
+
+(fn compose-joltage (x y) (=> (integer integer) integer)
+  (->> x (* 10) (+ y)))
+
+(fn maximum (xs) (=> list cons)
+  (--> _
+    (loop for x in xs maximizing x)
+    (cons _ (position _ xs))))
+
+(fn best-joltage-1 (bank) (=> list cons)
+  (destructuring-bind (max-val . max-pos) (maximum bank)
+    (if (->> bank length 1- (= max-pos))
+        ;; best value at end => next best is the first digit
+        (cons (->> bank length 1-
+                   (subseq bank 0)
+                   maximum car)
+              max-val)
+        ;; best value not at end => next best is the second digit
+        (->> max-pos 1+
+             (subseq bank)
+             maximum car
+             (cons max-val)))))
+
+(fn round-1 (banks) (=> list fixnum)
+  (loop for bank in banks
+        for (first-digit . second-digit) = (best-joltage-1 bank)
+        sum (compose-joltage first-digit second-digit)))
+
+(fn best-joltage-2 (bank) (=> list fixnum)
+  #| Sliding window greedy search?
+
+  We look at a sequence of digits in bank, choose the best one, then move onto
+  the next window.  We need the windows, at all times, to have enough digits for
+  us to pick a good one from.  In this case we need to choose 12, so at any one
+  time we need to be examining at most 12 digits (decrementing as we get more
+  digits).  The next window needs to be _after_ the position of the best value
+  we picked in our current window.  |#
+  (loop
+    with window-start = 0
+    for n from 12 downto 1
+    for window = (subseq bank window-start (-<> bank length 1+ (- n)))
+    for (max-val . max-pos) = (maximum window)
+    do (setf window-start (+ max-pos 1 window-start))
+    collect max-val into digits
+    finally (return (reduce #'compose-joltage digits))))
+
+(fn round-2 (banks) (=> list integer)
+  (->> banks (mapcar #'best-joltage-2) (reduce #'+)))
+
+(let ((input (loop for line in (uiop:read-file-lines "3-input")
+                   for chars = (coerce line 'list)
+                   for digit-strings = (mapcar #'string chars)
+                   collect (mapcar #'parse-integer* digit-strings))))
+  (->> input
+       round-1
+       (format t "Round 1: ~a~%"))
+  (->> input
+       round-2
+       (format t "Round 2: ~a~%")))