day 18

18-snailfish/code.scm

@@ -0,0 +1,147 @@
+(import (srfi :13)) ; string extensions
+
+; returns a new string where all occurrences of `find` have been replaced by `replace`
+(define (string-substitute str find replace)
+  (let [(len-find (string-length find))]
+    (let loop [(str str)]
+      (let [(index (string-contains str find))]
+	(if index
+	  (loop (string-replace str replace index (+ index len-find)))
+	  str)))))
+
+; turns a snailfish number into a regular scheme list
+(define (snailfish-number->list str)
+  ; this feels like cheating x)
+  (get-datum
+    (open-string-input-port
+      (string-substitute str "," " "))))
+
+; loads all the snailfish numbers from the given text port, one by line
+(define (load-numbers port)
+  (let loop [(numbers '())]
+    (if (eof-object? (peek-char port))
+      (reverse numbers)
+      (loop (cons (snailfish-number->list (get-line port)) numbers)))))
+
+; updates literal from snailfish number at given index
+(define (update-literal! number index updater)
+  (let [(i 0)]
+    (let rec [(p number)]
+      (if (null? p)
+	i
+	(if (number? (car p))
+	  (if (= i index)
+	    (begin
+	      (set-car! p (updater (car p)))
+	      (set! i #f)) ; kill recursion
+	    (begin
+	      (set! i (+ i 1))
+	      (rec (cdr p))))
+	  (begin
+	    (rec (car p))
+	    (when i
+	      (rec (cdr p)))))))))
+
+; change literal at given index to be given value
+(define (set-literal! number index value)
+  (update-literal! number index (lambda (v) value)))
+
+; increment literal at given index by given value
+(define (add-literal! number index value)
+  (update-literal! number index (lambda (v) (+ v value))))
+
+; change the car of the cdr of l
+(define (set-cadr! l v)
+  (set-car! (cdr l) v))
+
+; explode snailfish number if possible
+; returns #t if any number has exploded 
+(define (explode! input)
+  (let [(index 0)]
+    (let look [(parent '()) (set-parent! #f) (depth 0) (pair input)]
+      (if (not (number? pair))
+	(if (>= depth 4)
+	  ; explode
+	  (let [(left (car pair))
+		(right (cadr pair))]
+	    (add-literal! input (- index 1) left)
+	    (add-literal! input (+ index 2) right)
+	    (set-parent! parent 0)
+	    (set! index #f)) ; kill recursion
+	  (begin
+	    (look pair set-car! (+ depth 1) (car pair))
+	    (when index
+	      (look pair set-cadr! (+ depth 1) (cadr pair)))))
+	; pair is actually a number
+	(set! index (+ index 1))))
+    (not index)))
+
+; split snailfish number if possible
+; returns #t if any number has been split
+(define (split! input)
+  (let [(index 0)]
+    (let look [(pair input)]
+      (if (number? pair)
+	(if (>= pair 10)
+	  (begin
+	    ; split
+	    (set-literal! input index (list (div pair 2) (- pair (div pair 2))))
+	    (set! index #f) ; kill recursion
+	    (reduce! input))
+	  (set! index (+ index 1)))
+	(begin
+	  (look (car pair))
+	  (when index ; check recursion hasn't been killed
+	    (look (cadr pair))))))
+    (not index)))
+
+; reduce snailfish number according to the rules of the problem
+(define (reduce! input)
+  (if (explode! input)
+    (reduce! input)
+    (if (split! input)
+      (reduce! input)
+      input)))
+
+; adds two snailfish numbers
+; input will be corrupted!
+(define (add! a b)
+  (reduce! (list a b)))
+
+; compute magnitude of snailfish number
+(define (magnitude n)
+  (if (number? n)
+    n
+    (+ (* 3 (magnitude (car n)))
+       (* 2 (magnitude (cadr n))))))
+
+; deep copy given list
+(define (duplicate lst)
+  (if (pair? lst)
+    (cons (duplicate (car lst)) (duplicate (cdr lst)))
+    lst))
+
+; finds the largest magnitude obtained by summing any two numbers from the given list
+(define (largest-magnitude numbers)
+  (let [(largest 0)]
+    (for-each
+      (lambda (n1)
+	(for-each
+	  (lambda (n2)
+	    (let* [(sum (add! (duplicate n1) (duplicate n2)))
+		   (mag (magnitude sum))]
+	      (when (> mag largest)
+		(set! largest mag))))
+	  (remove n1 numbers)))
+	numbers)
+    largest))
+
+(call-with-input-file
+  "input"
+  (lambda (file)
+    (let [(numbers (load-numbers file))]
+      (let [(numbers (duplicate numbers))]
+	(printf "part 1:~% Magnitude of final sum: ~a~%"
+		(magnitude (fold-left add! (car numbers) (cdr numbers)))))
+      (printf "part 2:~% Largest magnitude: ~a~%"
+	      (largest-magnitude numbers)))))

18-snailfish/input

@@ -0,0 +1,100 @@
+[[[[9,5],[9,4]],[[6,5],[7,0]]],4]
+[[[5,2],[[7,2],1]],[[[7,5],[0,8]],[[6,9],[7,3]]]]
+[[[9,7],[0,1]],9]
+[1,[[7,3],[[3,7],[3,2]]]]
+[[9,[[0,8],7]],[[3,1],[[6,6],[9,0]]]]
+[4,[[4,4],[[7,7],1]]]
+[[[[6,2],[5,1]],[[3,3],9]],[7,[[5,7],[5,0]]]]
+[[[[4,8],[4,9]],[1,[9,3]]],[1,[1,[6,1]]]]
+[[[[4,7],[3,4]],[8,3]],[[3,7],[0,[1,8]]]]
+[[[6,[4,8]],[4,5]],[4,[1,3]]]
+[[[0,7],0],[[6,[1,8]],[9,[7,9]]]]
+[[[[4,8],[3,9]],[4,5]],[1,1]]
+[[[4,2],[0,[6,7]]],[[[1,8],2],[8,8]]]
+[[[[1,1],7],5],[[6,[5,6]],[6,[7,5]]]]
+[[[[3,2],5],[[5,3],1]],[[[0,4],[9,6]],9]]
+[[6,[7,6]],9]
+[[[[4,0],[0,1]],7],1]
+[[[[1,3],4],6],[[1,[4,2]],[1,4]]]
+[[[[6,9],[4,1]],[[6,3],[0,8]]],[[4,0],[[3,2],[2,9]]]]
+[[[3,6],[[2,0],[3,2]]],[2,5]]
+[[[[4,3],5],5],[[4,[4,0]],6]]
+[[[[4,0],3],[[3,5],8]],[[8,[4,4]],[[9,9],[4,1]]]]
+[[[2,7],6],1]
+[[[[5,3],[8,4]],[0,0]],4]
+[[[0,[8,1]],0],3]
+[[[6,5],[8,2]],[[[6,9],[6,1]],[9,9]]]
+[0,[[4,9],6]]
+[[9,[[9,9],4]],[[[4,7],1],2]]
+[[8,0],[[[0,7],6],[[6,4],2]]]
+[[1,[[2,4],8]],1]
+[[[[1,3],4],[[1,3],0]],[[[1,2],3],2]]
+[[[[2,1],2],[5,[2,8]]],[2,[[6,0],2]]]
+[[[8,[1,0]],[[6,7],[9,6]]],[[2,[9,7]],5]]
+[[[3,[2,0]],[[3,2],[0,0]]],[[[4,6],[9,4]],[[7,8],[5,1]]]]
+[[3,[[9,9],[7,2]]],[[1,3],[2,[3,2]]]]
+[4,[4,[[9,5],6]]]
+[[[[5,7],7],[[3,4],0]],[[9,[8,2]],[2,3]]]
+[[[[2,1],[5,7]],4],[[[6,3],8],[[1,6],[5,1]]]]
+[[[4,4],[[0,9],[7,8]]],[[2,[2,5]],5]]
+[1,[5,[[3,7],[8,2]]]]
+[[[[9,5],[8,6]],[5,5]],[[[9,2],8],[[9,3],[3,8]]]]
+[0,[[9,5],[[3,7],7]]]
+[[[8,[0,4]],[[2,9],6]],[[6,[8,0]],4]]
+[[0,[3,5]],[[5,[0,1]],[[3,6],7]]]
+[[2,[7,1]],[[[5,0],[7,7]],[[2,3],9]]]
+[[5,[9,[3,9]]],[[8,[3,7]],[[7,6],[3,0]]]]
+[[[4,[2,5]],5],[3,1]]
+[[[[4,3],1],[[5,7],6]],[0,[3,1]]]
+[[8,9],[[[0,7],5],[6,[5,7]]]]
+[[6,8],[[5,8],[[8,2],[6,0]]]]
+[[1,[5,6]],5]
+[[[6,1],[9,[1,2]]],1]
+[[5,[7,[4,8]]],[[4,[2,9]],5]]
+[[[2,2],[[7,1],3]],[[[9,7],[4,6]],[1,[0,1]]]]
+[[3,[6,[4,5]]],2]
+[[[0,2],[[8,1],[0,6]]],[[7,[9,6]],0]]
+[[[[1,0],[5,1]],[[0,6],5]],[[[1,8],8],[[0,2],5]]]
+[[6,[[3,6],6]],[[[9,7],[6,4]],[[9,5],1]]]
+[[[0,[5,6]],[9,0]],[[2,9],9]]
+[1,[[4,[9,3]],0]]
+[[1,0],[[1,9],[4,8]]]
+[[[9,3],[7,0]],[[[5,1],[3,8]],9]]
+[[[3,9],[[5,9],2]],[[7,2],1]]
+[[1,[[3,0],[7,6]]],[7,[8,1]]]
+[0,[6,[[7,1],[1,1]]]]
+[[4,[[5,0],[2,1]]],[[[8,8],[8,1]],7]]
+[[[[9,3],[4,3]],4],[7,5]]
+[[9,[[7,4],[8,3]]],[[[1,9],7],[[1,6],[3,1]]]]
+[[6,9],[5,[0,[5,1]]]]
+[[[8,7],3],[[4,8],[0,7]]]
+[[[[3,1],2],[[1,6],[4,3]]],[0,6]]
+[[5,[[5,4],3]],[[8,8],9]]
+[[5,[3,[4,5]]],[[2,[6,0]],[6,1]]]
+[[[[9,5],3],6],[[8,[1,9]],[[5,2],5]]]
+[[[7,5],[[3,6],4]],[6,[[5,1],[0,1]]]]
+[[1,[[4,8],[1,3]]],7]
+[[4,[[4,0],5]],[[[6,2],7],[[4,8],[4,9]]]]
+[[[[2,3],[0,9]],[7,2]],[4,5]]
+[[[[7,7],[8,0]],[7,7]],[[[6,6],[3,2]],[4,[4,3]]]]
+[[[[8,7],6],[[5,5],0]],[[6,[7,3]],[[4,1],[1,7]]]]
+[[[2,[2,2]],[[5,2],1]],[[9,[9,2]],6]]
+[[[[1,7],6],[[8,8],5]],[6,[1,[1,7]]]]
+[[[[8,6],[3,2]],[[5,2],[2,0]]],[[[8,7],2],[[5,5],2]]]
+[[[8,[9,0]],[[9,5],[7,5]]],[[5,1],[[1,1],[4,6]]]]
+[5,[9,[[0,2],7]]]
+[8,[[0,[4,9]],[[7,4],9]]]
+[[[[2,9],5],[[0,6],[6,6]]],[[0,6],[[4,2],[9,9]]]]
+[7,[[[4,3],3],[[5,4],[6,0]]]]
+[[0,[8,[1,1]]],5]
+[[[1,8],[[4,6],[9,7]]],[[[6,6],[2,6]],[4,3]]]
+[[0,[[7,5],[9,9]]],[[9,7],[6,2]]]
+[[[9,[3,0]],[[1,4],0]],[[1,1],1]]
+[[[0,7],[[3,0],8]],[[6,[8,0]],[[4,5],[4,0]]]]
+[[[[2,9],[4,2]],[5,[9,3]]],[4,[2,[3,4]]]]
+[[[1,[7,3]],[[5,7],0]],[6,[[6,5],2]]]
+[4,5]
+[[7,9],[6,[[6,5],[1,0]]]]
+[[4,[[7,5],8]],[[4,0],[[6,6],[0,4]]]]
+[[[9,[7,7]],[[4,2],7]],4]
+[[0,[0,3]],5]

18-snailfish/simple

@@ -0,0 +1,10 @@
+[[[0,[5,8]],[[1,7],[9,6]]],[[4,[1,2]],[[1,4],2]]]
+[[[5,[2,8]],4],[5,[[9,9],0]]]
+[6,[[[6,2],[5,6]],[[7,6],[4,7]]]]
+[[[6,[0,7]],[0,9]],[4,[9,[9,0]]]]
+[[[7,[6,4]],[3,[1,3]]],[[[5,5],1],9]]
+[[6,[[7,3],[3,2]]],[[[3,8],[5,7]],4]]
+[[[[5,4],[7,7]],8],[[8,3],8]]
+[[9,3],[[9,9],[6,[4,9]]]]
+[[2,[[7,7],7]],[[5,8],[[9,3],[0,2]]]]
+[[[[5,2],5],[8,[3,7]]],[[5,[7,5]],[4,4]]]