day 10

day10.rkt

@@ -0,0 +1,53 @@
+#lang racket
+
+(define data (map string->list (file->lines "day10.txt")))
+
+(define pairs (list (list #\{ #\}) (list #\( #\)) (list #\[ #\]) (list #\< #\>)))
+(define opening (map first pairs))
+(define closing (map second pairs))
+
+(define (go input stack) ; returns the incomplete list OR the first broken bracket if corrupted
+  (if (= 0 (length input)) ; if we've run out of input..
+      stack ; we've got an incomplete chunk. return the stack
+      (let ([x (first input)] [xs (rest input)]) ; grab the first element of the input
+        (if (member x opening) ; if it is an opening bracket
+            (go xs (cons x stack)) ; push it on the stack
+            (let* ([y (first stack)] ; otherwise, it's a closing bracket. grab the top of the stack..
+                   [matching (second (assoc y pairs))]) ; and find its matching closing bracket
+              (if (equal? matching x) ; if they match..
+                  (go xs (rest stack)) ; remove it and recurse
+                  x)))))) ; if they don't match, return the offending bracket
+
+(define part1
+  (let* ([res (map (λ (x) (go x '())) data)]
+        [bads (filter (λ (x) (not (list? x))) res)] ; filter out the lists - they're the incomplete chunks
+        [costs (list (list #\) 3) (list #\] 57) (list #\} 1197) (list #\> 25137))]
+        [points (map (λ (x) (second (assoc x costs))) bads)]
+        [total (apply + points)])
+    total))
+
+part1
+
+; PART 2
+
+(define (complete input stack) ; find the necessary closing brackets to complete the chunk
+  (if (= 0 (length input))
+      stack
+      (let* ([x (first input)]
+             [matching (second (assoc x pairs))])
+        (complete (rest input) (cons matching stack)))))
+
+(define (score chunk)
+  (let ([costs (list (list #\) 1) (list #\] 2) (list #\} 3) (list #\> 4))])
+  (foldl (λ (x acc) (+ (second (assoc x costs)) (* 5 acc))) 0 chunk)))
+
+(define part2
+  (let* ([res (map (λ (x) (go x '())) data)]
+         [rev (reverse res)] ; reverse so we can use `first` & `cons`
+         [incompletes (filter (λ (x) (list? x)) res)] ; filter out the lists - they're the incomplete chunks
+         [completing (map reverse (map (λ (x) (complete x '())) incompletes))] ; un-reverse it
+         [scores (map score completing)]
+         [sorted (sort scores <)])
+    (list-ref sorted (/ (- (length sorted) 1) 2)))) ; find the median scoring chunk
+
+part2

day10.test.txt

@@ -0,0 +1,10 @@
+[({(<(())[]>[[{[]{<()<>>
+[(()[<>])]({[<{<<[]>>(
+{([(<{}[<>[]}>{[]{[(<()>
+(((({<>}<{<{<>}{[]{[]{}
+[[<[([]))<([[{}[[()]]]
+[{[{({}]{}}([{[{{{}}([]
+{<[[]]>}<{[{[{[]{()[[[]
+[<(<(<(<{}))><([]([]()
+<{([([[(<>()){}]>(<<{{
+<{([{{}}[<[[[<>{}]]]>[]]

day10.txt

@@ -0,0 +1,98 @@
+{<{[<((<[<({<{(){}}(()())>[<<><>>((){})]}(<<{}{}>(<><>)>))(<[(<>{}){()()}]{{{}[]}(()())}>[
+((<{[<([<<{[<(()())([]())>)<<<{}{}>{[][]}>{<<>>(()[])}>}<(((()[])[{}<>])[<[]{}>{{}[]}])[[({}{})
+({(<[([((<(<[(<>{}){()[]}]<[[]<>]<(){}]>><{(<>())[<>{}]}{(<>[])(<>{})}>)([(<(){}>[(){}]){{()[]}{<>{}}}][<<[]
+{{{[(([{{{[{[(()())[<>[]]]{([]{}){()<>}}}]{{{((){})[[][]]}[[{}{}]<[]()>]}(<(<>()){{}{}>>([{}()
+[<{{{[[(<[(([<{}[]>{()()}][[<>{}]]))]([<<{{}[]}>>([(<>[])<(){}>][[{}{}](()())])]{[{[<><>]}((<>{})[
+{<[<[[<<(<<<<(<>[])(<>[])>{([]())}>[{{[]()}}<({}())[<>()]>]>{[((<>{})[[]<>])([()<>]{<>()})](((<>()
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+{(<([{{{{(((({{}[]}))){[(({}<>))({<><>}<[][]])][<<[]{}>[{}{}]>{<[]>{[]{}}}]})({[{([]<>)([]<>)}
+{([{{{(([<[[[<(){}>{()[]}][([]<>)<[]{}>]]<{(<>())[[]()]}[{(){}}[()<>]]>]([({[]{}}{()[])){{<>()}{()[]}}])>[<[
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+<<[(<([<(({[{({}{})}<{()<>}{<>{}}>][([[]()](()<>))]}[[({{}()}<[]{}>)[<[]()>[()()]]]((<{}{}
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