day4.rkt

#lang racket
(require threading)

(define low 171309)
(define high 643603)

(define (gen-passwords from to)
  (map number->string (range from (+ 1 to))))

(define (increasing? instr)
  (let ([charlist (string->list instr)])
    (eq? (sort charlist char<?) charlist)))

(define (counts lst)
  (define (count-rec lst counts)
    (if (empty? lst)
        counts
        (if (hash-has-key? counts (car lst))
            (count-rec (cdr lst) (hash-update counts (car lst) (λ (x) (+ x 1))))
            (count-rec (cdr lst) (hash-set counts (car lst) 1)))))
  (count-rec lst #hash()))

(define (repeated? instr)
  (~>> (string->list instr)
       (counts)
       (hash-values)
       (filter (λ (x) (> x 1)))
       (length)
       (< 0)))

(define (has-double? instr)
  (~>> (string->list instr)
       (counts)
       (hash-values)
       (filter (λ (x) (= 2 x)))
       (length)
       (< 0)))

(define passwords (gen-passwords low (+ 1 high)))

(define (part1)
  (~>> passwords
       (filter increasing?)
       (filter repeated?)
       (length)))

(define (part2)
  (~>> passwords
       (filter increasing?)
       (filter has-double?)
       (length)))

(writeln "Part 1: ")
(writeln (part1))

(writeln "Part 2: ")
(writeln (part2))