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Day 12: Part 1, with a Part 2 that is too slow :(

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Jonathan Chan 2019-12-11 23:28:53 -08:00
parent bdf1332d62
commit 301f30268e
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#lang racket
(require racket/set
"../lib.rkt")
(define moons-pos
'((-9 -1 -1)
( 2 9 5)
(10 18 -12)
(-6 15 -7)))
(define moons-vel
'((0 0 0)
(0 0 0)
(0 0 0)
(0 0 0)))
(define example
'((-1 0 2)
( 2 -10 -7)
( 4 -8 8)
( 3 5 -1)))
(define positions (mutable-set))
(define (less-than->number n1 n2)
(cond
[(< n1 n2) 1]
[(= n1 n2) 0]
[(> n1 n2) -1]))
;; Calculate the velocity on the first moon
;; caused by the second moon
(define (velocity moon-on moon-by)
(list (less-than->number (first moon-on) (first moon-by))
(less-than->number (second moon-on) (second moon-by))
(less-than->number (third moon-on) (third moon-by))))
(define (<+> v1 v2)
(zip + v1 v2))
;; Calculate velocities for all moons
(define (velocities moons vels)
(zip <+> vels
(map (λ (moon)
(foldl (λ (other v)
(<+> v (velocity moon other)))
'(0 0 0) moons))
moons)))
(define (energy moons vels)
(sum (zip (λ (moon vel)
(* (sum (map abs moon))
(sum (map abs vel))))
moons vels)))
(define (step moons vels)
(let* ([vels (velocities moons vels)]
[moons (zip <+> moons vels)])
(values moons vels)))
(define (step-n n moons vels)
(if (zero? n)
(values moons vels)
(let-values ([(moons vels) (step moons vels)])
(step-n (sub1 n) moons vels))))
(define part1
(let-values ([(moons vels)
(step-n 1000 moons-pos moons-vel)])
(energy moons vels)))
(define (part2-nogood)
(let loop ([steps 0]
[moons moons-pos]
[vels moons-vel])
(if (set-member? positions moons)
steps
(let*-values ([(moons* vels*)
(step moons vels)])
(set-add! positions moons)
(loop (add1 steps) moons* vels*)))))
(show-solution part1 #f)