Days 1 and 2.

input/01.txt

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+1974

src/01.rkt

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+#lang racket
+
+(require "../lib.rkt")
+
+(define input (map string->number (problem-input 1)))
+
+(define part1
+  (let loop ([entries input]
+             [ht (hash)])
+    (let ([entry (first entries)])
+      (if (hash-has-key? ht entry)
+          (* entry (hash-ref ht entry))
+          (loop (rest entries) (hash-set ht (- 2020 entry) entry))))))
+
+(define part2
+  (for*/first ([i input]
+               [j (cdr input)]
+               [k (cddr input)]
+               #:when (= (+ i j k) 2020))
+    (* i j k)))
+
+(show-solution part1 part2)

src/02.rkt

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+#lang racket
+
+(require "../lib.rkt")
+
+(define (parse line)
+  (match-let ([(list min max char pass)
+               (regexp-split #rx"[- ]" (regexp-replace #rx": " line " "))])
+    (list (string->number min)
+          (string->number max)
+          (string-ref char 0)
+          (string->list pass))))
+
+(define input (map parse (problem-input 2)))
+
+(define part1
+  (length (filter (λ (entry)
+                    (match-let* ([(list min max char pass) entry]
+                                 [occurrences (length (filter (∂ char=? char) pass))])
+                      (<= min occurrences max)))
+                  input)))
+
+(define part2
+  (length (filter (λ (entry)
+                    (match-let* ([(list min max char pass) entry])
+                      (xor (char=? (list-ref pass (sub1 min)) char)
+                           (char=? (list-ref pass (sub1 max)) char))))
+                  input)))
+
+(show-solution part1 part2)