1 files changed, 69 insertions, 0 deletions
diff --git a/2021/day8/08b.clj b/2021/day8/08b.clj
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@@ -0,0 +1,69 @@
+(ns day-8)
+
+(require '[clojure.string :as str])
+(require '[clojure.set :as set])
+
+(def ALPHABET #{\a \b \c \d \e \f \g})
+(defn decodeWith [signal mapping]
+  (cond (every? (fn [x] (contains? mapping x)) signal) (set (map (fn [x] (get mapping x)) signal))
+        :else false))
+
+(def SIGNAL_TO_NUM {#{\a \b \c \e \f \g} 0
+                    #{\c \f} 1
+                    #{\a \c \d \e \g} 2
+                    #{\a \c \d \f \g} 3
+                    #{\b \c \d \f} 4
+                    #{\a \b \d \f \g} 5
+                    #{\a \b \d \e \f \g} 6
+                    #{\a \c \f} 7
+                    #{\a \b \c \d \e \f \g} 8
+                    #{\a \b \c \d \f \g} 9})
+
+(defn decodedArrToNum [arr]
+  (reduce + (map-indexed (fn [idx sig] (* (Math/pow 10 (- (count arr) idx 1)) (SIGNAL_TO_NUM sig))) arr)))
+
+; characters that haven't been mapped (ie the mapping doesnt specify what they actually should be)
+(defn unmappedChars [mapping]
+  (set/difference ALPHABET (keys mapping)))
+
+; characters that haven't been used (ie there is no character in the mapping that results in them)
+(defn unusedChars [mapping]
+  (set/difference ALPHABET (vals mapping)))
+
+(defn validResult [x]
+  (contains? #{#{\a \b \c \e \f \g} #{\c \f} #{\a \c \d \e \g} #{\a \c \d \f \g} #{\b \c \d \f} #{\a \b \d \f \g} #{\a \b \d \e \f \g}#{\a \c \f} #{\a \b \c \d \e \f \g} #{\a \b \c \d \f \g}} x))
+
+(defn tryWith [signals mapping knownValues]
+  (cond
+    (not (every? (fn [[in expected]]
+                   (def attemptedDecode (decodeWith in mapping))
+                   (or (= false attemptedDecode) (= expected attemptedDecode)))
+                 knownValues)) false ; stop considering path if it breaks any known values
+    (not (every? (fn [in]
+                   (def attemptedDecode (decodeWith in mapping))
+                   (or (= false attemptedDecode) (validResult attemptedDecode)))
+                  signals)) false ; stop when any of the fully resolved signals are invalid
+    (every? validResult (map (fn [x] (decodeWith x mapping)) signals)) mapping ; base case - all resolved to valid values
+    :else (let [tryingToMap (first (unmappedChars mapping))]
+            (first (filter (comp true? boolean) (for [candidate (unusedChars mapping)]
+                                                  (do
+                                                    (tryWith signals (assoc mapping tryingToMap candidate) knownValues))))))))
+
+(def KNOWN_VALUES {2 #{\c \f} 4 #{\b \c \d \f} 3 #{\a \c \f} 7 #{\a \b \c \d \e \f \g}})
+
+(defn extractKnownValues [signals]
+  (reduce (fn [m s] (cond (contains? KNOWN_VALUES (count s)) (assoc m s (get KNOWN_VALUES (count s)))
+                          :else m)) {} signals))
+
+(def input (as-> (slurp "./input") x
+             (str/split x #"\n")
+             (map (fn [l]
+                    (map (fn [p] (map set (str/split (str/trim p) #" "))) (str/split l #"\|"))) x)))
+
+(def lineMappings (map (fn [l] (let [signals (set (flatten l))]
+                                 (tryWith signals {} (extractKnownValues signals)))) input))
+(def decodedOutputs (map (fn [l m] (let [signals (second l)]
+                                     (map (fn [x] (decodeWith x m)) signals))) input lineMappings))
+(def outputs (map decodedArrToNum decodedOutputs))
+
+(println (reduce + outputs))