move 2021 days to folder; update README

1 files changed, 41 insertions, 0 deletions

diff --git a/year2021/day24/partboth.clj b/year2021/day24/partboth.clj
new file mode 100644
index 0000000..9548ec3
--- /dev/null
+++ b/year2021/day24/partboth.clj
@@ -0,0 +1,41 @@
+(require '[clojure.string :as str])
+
+; The given program operates on a base-26 stack, either pushing or
+; (conditionally) popping values. This code extracts those operations
+; from the input, then works through them to determine the highest
+; and lowest values that produce an output of zero.
+; See the .md file in this directory for more info.
+
+(def program (->> (slurp "./in")
+                  str/split-lines
+                  (map #(str/split % #"\s+"))
+                  (partition 18)
+                  (map (fn [step]
+                    (if (= "1" (last (nth step 4)))
+                      [:push (read-string (last (nth step 15)))]
+                      [:pop (read-string (last (nth step 5)))])
+                    ))))
+
+(defn program-index [prog] (- 14 (count prog)))
+
+(loop [numbers (vec (repeat 2 (vec (repeat 14 0))))
+       stack '()
+       prog program]
+  (if-let [step (first prog)]
+    (if (= :push (first step))
+      (recur numbers
+             (conj stack [(program-index prog) (second step)])
+             (rest prog))
+      (let [i2     (program-index prog)
+            [i1 v] (first stack)
+            diff   (+ v (second step))]
+        (recur
+          [(if (neg? diff)
+             (-> (first numbers) (assoc i1 9) (assoc i2 (+ 9 diff)))
+             (-> (first numbers) (assoc i2 9) (assoc i1 (- 9 diff))))
+           (if (neg? diff)
+             (-> (second numbers) (assoc i2 1) (assoc i1 (- 1 diff)))
+             (-> (second numbers) (assoc i1 1) (assoc i2 (+ 1 diff))))]
+          (rest stack) (rest prog))))
+  (println (map str/join numbers))))
+