day17: add part 3

day17/part3.clj

@@ -0,0 +1,59 @@
+;
+;
+; Put into a lein project so that you can pass Java more RAM (e.g. -Xmx15G)
+; https://www.reddit.com/r/adventofcode/comments/riqtwx/2021_day_17_day_17_part_3/
+;
+;
+
+(require '[clojure.core.reducers :as r])
+
+(def target-area [[20000 -5000] [30000 -10000]])
+(def initial-pos [0 0])
+
+(defn beyond-target? [[[tbx tby] [tex tey]] [x y]]
+  (or (> x tex) (< y tey)))
+
+(defn within-target? [[[tbx tby] [tex tey]] [x y]]
+  (and (>= x tbx) (<= x tex) (<= y tby) (>= y tey)))
+
+(defn apply-velocity [[[px py] [vx vy]]]
+  [[(+ px vx) (+ py vy)] [(cond-> vx (pos? vx) dec) (dec vy)]])
+
+(defn take-last-while [pred coll]
+  (loop [v (first coll) r (rest coll)]
+    (if (not (pred (first r)))
+      v
+      (recur (first r) (rest r)))))
+
+(defn build-path [target vel]
+  (->> (iterate apply-velocity [initial-pos vel])
+       (take-last-while (comp not (partial beyond-target? target) first))
+       (first)))
+
+; Used to determine best x velocity for highest path
+(def tnum-seq (iterate #(do [(apply + %) (inc (second %))]) [0 1]))
+
+(let [[tb te]      target-area
+      lowest-x     (second (last (take-while #(< (first %) (first tb)) tnum-seq)))
+      highest-y    (dec (Math/abs (second te)))]
+  (->> (for [y (range (second te) (inc highest-y))
+             x (range lowest-x (inc (first te)))] [x y])
+       (#(do (println "Generated xy pairs...") %))
+       (#(do (println "Total: " (* (- highest-y (second te)) (- (inc highest-y) lowest-x))) %))
+       (partition 1000000)
+       (#(do (println "Prepared partitions...") %))
+       (reduce
+          (fn [sum nlst]
+            (println sum)
+            (+ sum
+              (r/fold +
+                (r/monoid
+                  (fn [tot xy]
+                    (cond-> tot
+                      (within-target? target-area (build-path target-area xy))
+                      inc))
+                  (constantly 0))
+                (into [] nlst))))
+           0)
+       (println)))
+