🎄⌨️ Advent of Code 2018

#lang debug racket/base

(require racket/match
         racket/function
         racket/list
         racket/vector
         racket/set
         threading)

;; Good ol’ positions.
(struct posn (x y) #:transparent)

;; The concept of “reading order” is an important one in this puzzle. We can use
;; this comparison function with `sort` to ensure a list of positions is sorted
;; according to how you’d encounter them reading left-to-right, top to bottom.
;; This is where I mention that this program views 0,0 as “top left”.
(define (posn<? p1 p2)
  (match-define (posn x1 y1) p1)
  (match-define (posn x2 y2) p2)
  (or (< y1 y2)
      (and (<= y1 y2)
           (<= x1 x2))))

;; Keeping track of elves and gnomes. We’ll have separate lists for each group.
;; Making this a subtype of posn means we can pass a fighter to any function
;; that expects a posn.
(struct fighter (hp) #:super struct:posn #:transparent)

;; “The grid…a digital frontier. I tried to picture clusters of information as
;; they moved through the computer. What did they look like? …I kept dreaming
;; of a world I thought I’d never see. And then one day…I got in.”
;;   https://youtu.be/QBYr0k8dOtw?t=24
(struct grid (vec rows cols) #:transparent)

;; Our “grid” is, behind the scenes, a one-dimensional vector with length ROWS*COLS.
;; This function translates an x,y pair
(define (coords->index g x y)
  (+ (* (grid-cols g) y) x))

;; Create a grid from a list of strings each representing a row, filling each
;; spot with the corresponding character in the string
(define (lines->grid line-strs)
  (define row-count (length line-strs))
  (define col-count (string-length (first line-strs)))
  (grid (apply vector-append
               (map list->vector
                    (map string->list line-strs)))
        row-count
        col-count))

(define test-map
  (lines->grid
   '("#######"
     "#E..G.#"
     "#...#.#"
     "#.G.#G#"
     "#######")))

;; Grids and Positions: put them together

;; Reference the value at given position in a grid
(define (grid-ref g p)
  (match-define (posn x y) p)
  (vector-ref (grid-vec g) (coords->index g x y)))

;; Change the value at given position
(define (grid-mark! g pos v)
  (match-define (posn x y) pos)
  (vector-set! (grid-vec g) (coords->index g x y) v))

;; Used to determine if a fighter could move into a given spot.
;; Anything besides "." counts as an obstruction (incl. other fighters)
(define (grid-clear-at? g p)
  (equal? (grid-ref g p) #\.))

;; Make a blank grid of the same dimensions, for use in making “path grids” (see
;; further below)
(define (copy-blank-grid g)
  (match-define (grid _ rows cols) g)
  (grid (make-vector (* rows cols) #f) rows cols))

;; (For debugging) Represent the grid as a square of single-character values
(define (display-grid g [g2 #f])
  (define grid-size (* (grid-cols g) (grid-rows g)))
  (display
   (apply string-append
          (for/fold ([lst '()]
                     #:result (reverse (cons "\n" lst)))
                    ([val (in-vector (grid-vec g))]
                     [i (in-naturals 1)])
            (define ch
              (cond [(number? val) (number->string (modulo val 10))]
                    [(boolean? val) "-"]
                    [(string? val) val]
                    [else (format "~a" val)]))
            (cond [(and (equal? 0 (modulo i (grid-cols g)))
                        (< i grid-size))
                   (cons "\n" (cons ch lst))]
                  [else (cons ch lst)])))))

;; Is point p inside grid g? Film at 11
(define (inside-grid? g p)
  (match-define (posn px py) p)
  (and (>= px 0)
       (>= py 0)
       (< px (grid-rows g))
       (< py (grid-cols g))))

;; Get a list of a positions neighboring points, ensuring none are out of bounds
(define (neighbor-coords g pos)
  (match-define (posn x y) pos)
  (filter (curry inside-grid? g)
          (map (lambda (lst) (apply posn lst))
               `((,(- x 1) ,y)
                 (,x ,(+ y 1))
                 (,(+ x 1) ,y)
                 (,x ,(- y 1))))))

;; Get all the EMPTY neighboring points of a given spot OR list of spots.
;; If a (listof posn?) is passed, ensures the returned list does not include
;; any of the original positions.
(define (free-neighbors-at world pos)
  (cond [(posn? pos)
         (~> (neighbor-coords world pos)
             (filter (curry grid-clear-at? world) _))]
        [(list? pos)
         (~> (map (curry neighbor-coords world) pos)
             flatten
             (filter (curry grid-clear-at? world) _)
             (set-subtract pos)
             remove-duplicates)]))

;; “Path grids” are a specific use of grids where points are marked with integers
;; indicated their distance from an origin point.
;; A point has been checked when it is not equal to #false.
(define (not-yet-checked? pmap pos)
  (not (grid-ref pmap pos)))

;; Find the most direct path(s) to a fighter from an end-position
(define (path-grid world f end-pos)
  (define result-grid (copy-blank-grid world))
  (define goal-pts (free-neighbors-at world f))
  (grid-mark! result-grid end-pos 0)
  
  (let loop ([pts-to-check (list end-pos)]
             [i 1])
    (define new-coords (~> (free-neighbors-at world pts-to-check)
                           (filter (curry not-yet-checked? result-grid) _)))
    (for-each (lambda (p) (grid-mark! result-grid p i)) new-coords)
    (cond
      [(not (empty? (set-intersect new-coords goal-pts))) result-grid]
      [(empty? new-coords) #f]
      [else (loop new-coords (+ 1 i))])))