From a588c9b2d1cc6c0c6b1465cb9dd687641bab860f Mon Sep 17 00:00:00 2001
From: Jonathan Chan <jonathanhwchan@gmail.com>
Date: Tue, 3 Dec 2019 21:20:46 -0800
Subject: [PATCH] Day 3: Cleanup-ish.

---
 src/03.rkt | 92 ++++++++++++++++++++++++++++++++++--------------------
 1 file changed, 59 insertions(+), 33 deletions(-)

diff --git a/src/03.rkt b/src/03.rkt
index d117f9b..ab9b8bb 100644
--- a/src/03.rkt
+++ b/src/03.rkt
@@ -13,75 +13,101 @@
          [wire2 (string->wire (second input))])
     (values wire1 wire2)))
 
-;; step-path : boolean -> path -> (number . number) -> hashtable -> procedure -> (number . number)
+;; To store the paths of each wire, we will use a hashtable.
+;; The keys of the hashtable are positions on the grid,
+;;   stored as a pair of numbers (x . y).
+;; The values of the hashtable are points on the grid,
+;;   which contain four pieces of information:
+;;   - first-visited? : Whether the first wire has traversed the point;
+;;   - first-steps : How many steps from the origin the first wire has taken;
+;;   - second-visited?, second-steps: idem but for the second wire.
+(define pos-x car)
+(define pos-y cdr)
+(define (pos x y)
+  (cons x y))
+
+(struct point
+  (first-visited?
+   first-steps
+   second-visited?
+   second-steps))
+
+(define origin (pos 0 0))
+(define default-point (point #f 0 #f 0))
+
+;; step-path : boolean -> path -> hashtable -> (pos . number) -> (pos . number)
 ;;   first-wire? : Indicates whether we're stepping for the first or second wire.
 ;;   path : A pair (s: symbol . n: number), where s indicates the direction we step
 ;;     (R, L, U, D for right (+x), left (-x), up (+y), down (-y))
 ;;     and n indicates the number of units we step in that direction
-;;   position : A pair (x . y) that indicates the current location
-;;   hashtable : A map from positions to ((boolean . number) . (boolean . number))that indicate
-;;     the total number of steps to get to that position and
-;;     whether the first or second wire has visited that position
+;;   pos-count : A pair (pos . number) that indicates the current location and the steps taken
+;;   hashtable : A map from pos to point
 ;; Returns the new position after stepping
-(define (step-path first-wire? path pos-count ht)
+(define (step-path first-wire? path ht pos-count)
   (let* ([position (car pos-count)]
          [count    (cdr pos-count)]
          [x (car position)]
          [y (cdr position)]
-         [update (λ (dir)
-                   (λ (i count)
-                     (let* ([key (match dir
-                                   ['x (cons i y)]
-                                   ['y (cons x i)])]
-                            [value (hash-ref ht key '((#f . 0) . (#f . 0)))]
-                            [new-count (add1 count)]
-                            [new-value
-                             (if first-wire?
-                                 (cons `(#t . ,new-count) (cdr value))
-                                 (cons (car value) `(#t . ,new-count)))])
-                       (hash-set! ht key new-value)
-                       new-count)))])
+         [update (curry
+                  (λ (dir i count)
+                    (let* ([key (match dir
+                                  ['x (pos i y)]
+                                  ['y (pos x i)])]
+                           [value (hash-ref ht key default-point)]
+                           [new-count (add1 count)]
+                           [new-value
+                            (if first-wire?
+                                (point #t new-count
+                                       (point-second-visited? value)
+                                       (point-second-steps value))
+                                (point (point-first-visited? value)
+                                       (point-first-steps value)
+                                       #t new-count))])
+                      (hash-set! ht key new-value)
+                      new-count)))])
     (match path
       [`(R . ,(? number? n))
-       (cons (cons (+ x n) y)
+       (cons (pos (+ x n) y)
              (foldl (update 'x) count (range (add1 x) (add1 (+ x n)))))]
       [`(L . ,(? number? n))
-       (cons (cons (- x n) y)
+       (cons (pos (- x n) y)
              (foldl (update 'x) count (reverse (range (- x n) x))))]
       [`(U . ,(? number? n))
-       (cons (cons x (+ y n))
+       (cons (pos x (+ y n))
              (foldl (update 'y) count (range (add1 y) (add1 (+ y n)))))]
       [`(D . ,(? number? n))
-       (cons (cons x (- y n))
+       (cons (pos x (- y n))
              (foldl (update 'y) count (reverse (range (- y n) y))))])))
 
 (define (step-wire first-wire? wire ht)
-  (foldl (λ (path pos-count) (step-path first-wire? path pos-count ht)) '((0 . 0) . 0) wire))
+  (foldl (λ (path pos-count) (step-path first-wire? path ht pos-count)) `(,origin . 0) wire))
 
-(define-values (part1 intersections)
-  (let* ([hashtable (make-hash '(((0 . 0) . ((#f . 0) . (#f . 0)))))]
+(define-values (part1 intersect-points)
+  (let* ([hashtable (make-hash)]
          [_ (step-wire #t wire1 hashtable)]
          [_ (step-wire #f wire2 hashtable)]
          [hashlist (hash->list hashtable)]
          [intersections
           (filter (λ (kv)
                     (let ([v (cdr kv)])
-                      (and (caar v) (cadr v))))
+                      (and (point-first-visited? v)
+                           (point-second-visited? v))))
                   hashlist)]
          [distances
           (map (λ (kv)
                  (let ([k (car kv)])
-                   (+ (abs (car k)) (abs (cdr k)))))
+                   (+ (abs (pos-x k))
+                      (abs (pos-y k)))))
                intersections)])
     (values (apply min distances)
-            intersections)))
+            (map cdr intersections))))
 
 (define part2
   (let* ([steps
-          (map (λ (kv)
-                 (let ([v (cdr kv)])
-                   (+ (cdar v) (cddr v))))
-               intersections)])
+          (map (λ (v)
+                 (+ (point-first-steps v)
+                    (point-second-steps v)))
+               intersect-points)])
     (apply min steps)))
 
 (show-solution part1 part2)
\ No newline at end of file