m1emi1em.dev
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(D4P2): Somewhat optimized rewrite (~6.5s -> ~255ms)
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src/aoc2025/day4.clj
+59
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src/aoc2025/day4.clj
···
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(defn- parse-file [fname]
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(->> fname util/read-file util/gridify))
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(def paper? (partial = \@))
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(defn- -get-removable [grid]
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(let [height (-> grid count)
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width (-> grid first count)
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; Laziest optimization in existence but it makes part2 run in 6.5s instead of like 19s so yay
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(def get-removable (memoize -get-removable))
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(defn- part1 [grid]
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(-> grid get-removable count))
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-
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(defn- remove-paper [grid] (reduce #(assoc-in %1 %2 \.) grid (get-removable grid)))
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(defn- valid-adj [[y x :as pos] width height]
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(letfn [(valid-coord? [[cy cx]]
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(and (util/between? cy -1 height >)
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(util/between? cx -1 width >)))]
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(->> pos util/adj+
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(filter valid-coord?))))
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(defn- part2 [grid]
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(->> grid
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(iterate remove-paper)
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(take-while #(> (-> % get-removable count) 0))
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(map (comp count get-removable))
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(reduce +)))
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(let [grid-height (-> grid count)
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grid-width (-> grid first count)
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#_(First thing is to get a list of only the spots with paper on them)
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all-paper-spots
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(set
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(for [idy (range grid-height)
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idx (range grid-width)
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:let [pos [idy idx]]
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:when (paper? (get-in grid pos))]
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pos))
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#_(For each paper spot, find all adjacent spots. Make a map of format {position [adjacent spots with paper]})
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paper-spots-to-adjacent-paper-spots
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(into {}
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(for [pos all-paper-spots
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:let [adjacents (->> (valid-adj pos grid-width grid-height)
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(filter all-paper-spots))]]
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[pos adjacents]))
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removable? (fn [[_ v]] (-> v count (< 4)))
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any-removable? (fn [m] (some (fn [[_ v]] (removable? [nil v])) m))
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#_(Deletes a paper spot and any references to it by its neighbors)
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remove-a-paper
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(fn [paper-spots [spot neighbors]]
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(-> (reduce #(update %1 %2 (partial remove #{spot})) paper-spots neighbors)
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(dissoc spot)))
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#_(Takes the current paper spots and returns a list of paper spots with all paper currently eligible to be remove removed)
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paper-iterator
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(fn [paper-spots]
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(let [to-remove (filter removable? paper-spots)]
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(reduce remove-a-paper paper-spots to-remove)))
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starting-paper-count (count all-paper-spots)
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final-state
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(->> paper-spots-to-adjacent-paper-spots
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(iterate paper-iterator)
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(split-with any-removable?)
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second
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first)
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final-paper-count (-> final-state keys count)]
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(- starting-paper-count final-paper-count)))
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(defn- part1 [grid]
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(-> grid get-removable count))
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(defn solve
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([] (solve "04.txt"))
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(->> (parse-file fname) ((juxt part1 part2)))))
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(solve "04-test.txt") ; [13 43]
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