Day 14.

input/14.txt

@@ -0,0 +1,102 @@
+KHSSCSKKCPFKPPBBOKVF
+
+OS -> N
+KO -> O
+SK -> B
+NV -> N
+SH -> V
+OB -> V
+HH -> F
+HP -> H
+BP -> O
+HS -> K
+SN -> B
+PS -> C
+BS -> K
+CF -> H
+SO -> C
+NO -> H
+PP -> H
+SS -> P
+KV -> B
+KN -> V
+CC -> S
+HK -> H
+FN -> C
+OO -> K
+CH -> H
+CP -> V
+HB -> N
+VC -> S
+SP -> F
+BO -> F
+SF -> H
+VO -> B
+FF -> P
+CN -> O
+NP -> H
+KK -> N
+OP -> S
+BH -> F
+CB -> V
+HC -> P
+KH -> V
+OV -> V
+NK -> S
+PN -> F
+VV -> N
+HO -> S
+KS -> C
+FP -> F
+FH -> F
+BB -> C
+FB -> V
+SB -> K
+KP -> B
+FS -> C
+KC -> P
+SC -> C
+VF -> F
+VN -> B
+CK -> C
+KF -> H
+NS -> C
+FV -> K
+HV -> B
+HF -> K
+ON -> S
+CV -> N
+BV -> F
+NB -> N
+NN -> F
+BF -> N
+VB -> V
+VS -> K
+BK -> V
+VP -> P
+PB -> F
+KB -> C
+VK -> O
+NF -> F
+FO -> F
+PH -> N
+VH -> B
+HN -> B
+FK -> K
+PO -> H
+CO -> B
+FC -> V
+OK -> F
+OF -> V
+PF -> F
+BC -> B
+BN -> O
+NC -> K
+SV -> H
+OH -> B
+PC -> O
+OC -> C
+CS -> P
+PV -> V
+NH -> C
+PK -> H

lib.rkt

@@ -89,6 +89,11 @@
 (define (char->number c)
   (- (char->integer c) (char->integer #\0)))
 
+;; char->symbol : char? -> symbol?
+;; Convert a character into the corresponding symbol
+(define (char->symbol c)
+  (string->symbol (list->string (list c))))
+
 ;; string->vector : string? -> (vectorof char?)
 (define (string->vector str)
   (list->vector (string->list str)))

src/14.rkt

@@ -0,0 +1,38 @@
+#lang curly-fn racket
+
+(require "../lib.rkt")
+
+(define-values (template rules left-end right-end)
+  (match-let ([(list `(,template*) rules) (problem-input-grouped-lines 14)])
+    (define template (map char->symbol (string->list template*)))
+    (values (for/hash ([left template]
+                       [right (rest template)])
+              (values (cons left right) 1))
+            (for/hash ([rule rules])
+              (match-let ([(list _ left right insert) (regexp-match #px"(\\w)(\\w) -> (\\w)" rule)])
+                (values (cons (string->symbol left)
+                              (string->symbol right))
+                        (string->symbol insert))))
+            (first template) (last template))))
+
+(define (step template)
+  (for/fold ([polymer (hash)])
+            ([pair (hash-keys template)])
+    (match-let* ([(cons left right) pair]
+                 [insert (hash-ref rules pair)]
+                 [count (hash-ref template pair)]
+                 [polymer (hash-update polymer (cons left insert) #{+ % count} 0)]
+                 [polymer (hash-update polymer (cons insert right) #{+ % count} 0)])
+      polymer)))
+
+(define (count n)
+  (for/fold ([counts (hash right-end 1 left-end 1)]
+             #:result (let ([counts (hash-values counts)])
+                        (/ (- (maximum counts) (minimum counts)) 2)))
+            ([(k v) ((iterate step n) template)])
+    (match-let* ([(cons left right) k]
+                 [counts (hash-update counts left  #{+ % v} 0)]
+                 [counts (hash-update counts right #{+ % v} 0)])
+      counts)))
+
+(show-solution (count 10) (count 40))