diff --git a/.gitignore b/.gitignore
new file mode 100644
index 0000000..a4cb4f1
--- /dev/null
+++ b/.gitignore
@@ -0,0 +1,3 @@
+**/*.bak
+**/*.rkt~
+**/compiled/
diff --git a/lib.rkt b/lib.rkt
new file mode 100644
index 0000000..d1355f2
--- /dev/null
+++ b/lib.rkt
@@ -0,0 +1,301 @@
+#lang racket
+
+(require
+  (only-in data/queue
+           make-queue
+           enqueue!)
+  (only-in 2htdp/batch-io
+           read-lines))
+
+(provide problem-input
+         show-solution
+
+         make-vector-grid
+         lists->vectors
+         vectors->lists
+         hash->vectors
+         show-list-grid
+         show-vector-grid
+         show-hash-grid
+
+         ∘ ∂ $ %
+         uncurry
+
+         sum
+         != nchar=?
+         nzero?
+         negate
+         pos-or-zero
+         number->digits
+         number->digits-reverse
+         digits->number
+
+         rac
+         scanl scanr
+         list-ref*
+         repeat
+         chunks-of
+         transpose
+         list->queue
+
+         vector-first
+         vector-last
+         vector-ref*
+         vector-grid-ref*
+         vector-set!*
+         hash->vector
+         vector->hash)
+
+
+;; Function helpers ;;
+(define ∘ compose)
+(define ∂ curry)
+
+;; uncurry : (a1 -> ... -> an -> b) -> ((listof a) -> b)
+(define uncurry
+  (curry apply))
+(define $ uncurry)
+
+
+;; IO helpers ;;
+
+;; problem-input : number? -> (listof string?)
+;; Return contents of input file input/xx.txt as lines of strings.
+(define (problem-input n)
+  (let* ([filename (~a n #:min-width 2 #:align 'right #:left-pad-string "0")]
+         [path     (string-append "../input/" filename ".txt")])
+    (read-lines path)))
+
+;; show-solution : a -> b -> void
+;; Print part1 and part2 on separate lines.
+(define (show-solution part1 part2)
+  (printf "Part 1: ~a\nPart 2: ~a\n" part1 part2))
+
+
+;; Grid helpers ;;
+;; A grid of values might be stored in three different ways:
+;; - As a hashtable from positions (number . number) to values; or
+;; - As a vector of vectors of values; or
+;; - As a list of lists of values.
+
+;; make-vector-grid : number -> number -> number -> vector-grid
+(define (make-vector-grid width height [default 0])
+  (build-vector height (λ (_) (make-vector width default))))
+
+;; lists->vectors : list-grid -> vector-grid
+(define (lists->vectors list-grid)
+  (list->vector (map list->vector list-grid)))
+
+;; vectors->lists : vector-grid -> list-grid
+(define (vectors->lists vector-grid)
+  (map vector->list (vector->list vector-grid)))
+
+;; hash->vectors : hash-grid -> number -> vector-grid
+;; Where the position is not in the hash-grid,
+;; the vector-grid takes on the default value.
+(define (hash->vectors hash-grid [default 0])
+  (let* ([keys (hash-keys hash-grid)]
+         [xs (map car keys)]
+         [ys (map cdr keys)]
+         [min-x (apply min xs)]
+         [min-y (apply min ys)]
+         [width  (add1 (- (apply max xs) min-x))]
+         [height (add1 (- (apply max ys) min-y))]
+         [vector-grid (make-vector-grid width height default)])
+    (hash-for-each
+     hash-grid (λ (pos val)
+                 (let ([x (- (car pos) min-x)]
+                       [y (- (cdr pos) min-y)])
+                   (vector-set! (vector-ref vector-grid y) x val))))
+    vector-grid))
+
+;; show-list-grid : (hashof (value => char)) -> list-grid -> void
+(define (show-list-grid char-hash list-grid)
+  (for-each
+   displayln
+   (map (∘ list->string
+           (∂ map (∂ hash-ref char-hash)))
+        list-grid)))
+
+;; show-vector-grid : (hashof (value => char)) -> vector-grid -> void
+(define (show-vector-grid char-hash vector-grid)
+  (show-list-grid char-hash (vectors->lists vector-grid)))
+
+;; show-hash-grid : (hashof (value => char)) -> hash-grid -> number -> void
+(define (show-hash-grid char-hash hash-grid [default 0])
+  (show-vector-grid char-hash (hash->vectors hash-grid default)))
+
+
+;; Char helpers ;;
+
+;; nchar=? : char -> char -> boolean
+(define (nchar=? c1 c2)
+  (not (char=? c1 c2)))
+
+
+;; Number helpers ;;
+
+;; sum : (listof number) -> number
+(define (sum ns) (apply + ns))
+
+;; != : number -> number -> boolean
+(define (!= n1 n2)
+  (not (= n1 n2)))
+
+;; nzero? : number -> boolean
+(define (nzero? n)
+  (not (zero? n)))
+
+;; negate : number -> number
+(define (negate n)
+  (- 0 n))
+
+;; pos-or-zero : number -> number
+(define (pos-or-zero n)
+  (if (negative? n) 0 n))
+
+;; % : number -> number -> number
+(define % modulo)
+
+;; number->digits-reverse : number -> (listof number)
+;; Return the digits of the given number in reverse order (i.e. RTL)
+(define (number->digits-reverse n)
+  (if (< n 10)
+      (list n)
+      (cons (remainder n 10)
+            (number->digits-reverse (quotient n 10)))))
+
+;; number->digits : number -> (listof number)
+;; Return the digits of the given number (LTR)
+(define (number->digits n)
+  (reverse (number->digits-reverse n)))
+
+;; digits->number : (listof number) -> number
+;; Return the given digits as a number
+(define (digits->number ns)
+  (let loop ([n 0] [ns ns])
+    (if (empty? ns) n
+        (loop (+ (* n 10) (car ns)) (cdr ns)))))
+
+
+;; List helpers ;;
+
+;; rac : (listof any) -> any -> (listof any)
+;; Append element to the back of the list.
+(define (rac lst v)
+  (append lst (list v)))
+
+;; scanl : (a -> a -> a) -> (listof a) -> (listof a)
+;; foldl that accumulates partial results in a list
+(define (scanl f init lst)
+  (reverse
+   (foldl (λ (v lst)
+            (cons (f v (first lst)) lst))
+          (list init) lst)))
+
+;; scanr : (a -> a -> a) -> (listof a) -> (listof a)
+;; foldr that accumulates partial results in a list
+(define (scanr f init lst)
+  (reverse
+   (foldr (λ (v lst)
+            (cons (f v (first lst)) lst))
+          (list init) lst)))
+
+;; list-ref* : (listof a) -> number -> a -> a
+;; Same as list-ref, except a default value is provided
+;; if the index is beyond the length of the list.
+(define (list-ref* lst pos failure-result)
+  (if (>= pos (length lst))
+      failure-result
+      (list-ref lst pos)))
+
+;; repeat : number -> (listof any) -> (listof any)
+(define (repeat m lst)
+  (if (zero? m) '()
+      (append lst (repeat (sub1 m) lst))))
+
+;; chunks-of : (listof any) -> nonzero? -> (listof (listof any))
+;; Partitions a list into lists of the given size in order,
+;; with the final list possibly being smaller
+;; e.g. '(1 2 3 4 5) 2 => '((1 2) (3 4) (5))
+(define (chunks-of lst size)
+  (if (< (length lst) size) lst
+      (cons (take lst size)
+            (chunks-of (drop lst size) size))))
+
+;; transpose : (listof (listof any)) -> (listof (listof any))
+;; Turns a list of lists into a list of lists of
+;; the first elements of the lists, ..., the nth elements
+;; where n is the length of the shortest list.
+;; In short, it transposes a list of rows into a list of columns.
+;; e.g. '((1 2 3 4)          '((1 5 8)
+;;        (5 6 7)         =>   (2 6 9)
+;;        (8 9 10 11 12))      (3 7 10))
+(define (transpose lists)
+  (let* ([min-len (apply min (map length lists))]
+         [lists (map (λ (lst) (take lst min-len)) lists)])
+    (apply map list lists)))
+
+;; list->queue : (listof a) -> (queueof a)
+;; Creates a queue and adds elements of list in order
+(define (list->queue lst)
+  (let ([Q (make-queue)])
+    (for-each (∂ enqueue! Q) lst)
+    Q))
+
+
+;; Vector helpers ;;
+
+;; vector-first : (vectorof any) -> any
+(define (vector-first vec)
+  (vector-ref vec 0))
+
+;; vector-last : (vectorof any) -> any
+(define (vector-last vec)
+  (vector-ref vec (sub1 (vector-length vec))))
+
+;; vector-ref* : (vectorof any) -> number -> any -> any
+;; Same as list-ref, except a default value is provided
+;; if the index is beyond the length of the list.
+(define (vector-ref* vec pos failure-result)
+  (if (>= pos (vector-length vec))
+      failure-result
+      (vector-ref vec pos)))
+
+;; vector-grid-ref* : (vectorof (vectorof any)) -> (list number number) -> any -> any
+;; Given coordinates (x, y), in the yth vector, find the xth element.
+;; If either x or y are beyond the indices of the vectors,
+;; return the default value provided.
+(define (vector-grid-ref* grid coord failure-result)
+  (match-let ([(list x y) coord]
+              [y-len (vector-length grid)])
+    (if (or (< y 0) (>= y y-len))
+        failure-result
+        (let* ([row (vector-ref grid y)]
+               [x-len (vector-length row)])
+          (if (or (< x 0) (>= x x-len))
+              failure-result
+              (vector-ref row x))))))
+
+;; vector-set!* : (vectorof any) -> number -> any -> (vectorof any)
+;; Set the value at given index in a new vector, then return that vector
+;; If the index is beyond the indices of the vector,
+;; a vector that can accomodate that index is returned,
+;; with all the original elements and the element at the index set
+(define (vector-set!* vec pos v)
+  (let ([new-vec (make-vector (max (vector-length vec) (add1 pos)))])
+    (vector-copy! new-vec 0 vec)
+    (vector-set! new-vec pos v)
+    new-vec))
+
+;; hash->vector : (hashof (number => a)) -> (vectorof a)
+;; Convert an intmap into a mutable vector
+(define (hash->vector hash [default 0])
+  (let ([length (add1 (apply max (hash-keys hash)))])
+    (build-vector length (λ (i) (hash-ref hash i default)))))
+
+;; vector->hash : (vectorof a) -> (hashof (number => a))
+;; Convert a vector into an immutable intmap
+(define (vector->hash vec)
+  (let ([kvs (map cons (range (vector-length vec)) (vector->list vec))])
+    (make-immutable-hash kvs)))
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