ADDED   day15.rkt
Index: day15.rkt
==================================================================
--- day15.rkt
+++ day15.rkt
@@ -0,0 +1,152 @@
+#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-cols g))
+       (< py (grid-rows 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)
+          (list (posn (- x 1) y)
+                (posn x (+ y 1))
+                (posn (+ x 1) y)
+                (posn 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))])))