Day 9.

lib.rkt

@@ -43,7 +43,7 @@
          number->digits-reverse
          digits->number
 
-         rac
+         snoc
          scanl scanr
          list-ref*
          repeat
@@ -228,9 +228,9 @@
 
 ;; List helpers ;;
 
-;; rac : (listof any) -> any -> (listof any)
+;; snoc : (listof any) -> any -> (listof any)
 ;; Append element to the back of the list.
-(define (rac lst v)
+(define (snoc lst v)
   (append lst (list v)))
 
 ;; scanl : (a -> a -> a) -> (listof a) -> (listof a)

src/09.rkt

@@ -0,0 +1,27 @@
+#lang racket
+
+(require "../lib.rkt")
+
+(define input (map string->number (problem-input 9)))
+
+(define (pair-sum lst target)
+  (for*/or ([i lst]
+            [j lst])
+    (= (+ i j) target)))
+
+(define part1
+  (let loop ([current (take input 25)] [next (list-ref input 25)] [remain (drop input 26)])
+    (if (pair-sum current next)
+        (loop (snoc (rest current) next) (first remain) (rest remain))
+        next)))
+
+(define part2
+  (let loop ([current '()] [remain input] [sum 0])
+    (cond
+      [(< sum part1)
+       (loop (snoc current (first remain)) (rest remain) (+ sum (first remain)))]
+      [(> sum part1)
+       (loop (rest current) remain (- sum (first current)))]
+      [else (+ (apply min current) (apply max current))])))
+
+(show-solution part1 part2)