From 637477664a19c5a212235b44a83a20fd643854a6 Mon Sep 17 00:00:00 2001
From: Jonathan Chan <jcxz@cs.ubc.ca>
Date: Sat, 12 Dec 2020 00:52:32 -0800
Subject: [PATCH] Refactored Day 12 and added solution using matrices.

---
 src/12-alt.rkt | 53 ++++++++++++++++++++++++++++++++
 src/12.rkt     | 82 +++++++++++++++++---------------------------------
 2 files changed, 81 insertions(+), 54 deletions(-)
 create mode 100644 src/12-alt.rkt

diff --git a/src/12-alt.rkt b/src/12-alt.rkt
new file mode 100644
index 0000000..7a346e8
--- /dev/null
+++ b/src/12-alt.rkt
@@ -0,0 +1,53 @@
+#lang racket
+
+(require math/matrix
+         "../lib.rkt")
+
+(define (parse line)
+  (match line
+    [(regexp #px"([NESWLRF])(\\d+)" (list _ dir amt))
+     (list (string->symbol dir) (string->number amt))]))
+
+(define input (map parse (problem-input 12)))
+
+(define N (col-matrix [0 -1]))
+(define E (col-matrix [1 0]))
+(define S (col-matrix [0 1]))
+(define W (col-matrix [-1 0]))
+(define R90 (matrix [[0 -1] [1 0]]))
+(define L90 (matrix [[0 1] [-1 0]]))
+
+(define (move instr p1 p2 pos dir)
+  (match instr
+    [`(N ,amt) (values (matrix+ pos (matrix-scale N (* amt p1)))
+                       (matrix+ dir (matrix-scale N (* amt p2))))]
+    [`(E ,amt) (values (matrix+ pos (matrix-scale E (* amt p1)))
+                       (matrix+ dir (matrix-scale E (* amt p2))))]
+    [`(S ,amt) (values (matrix+ pos (matrix-scale S (* amt p1)))
+                       (matrix+ dir (matrix-scale S (* amt p2))))]
+    [`(W ,amt) (values (matrix+ pos (matrix-scale W (* amt p1)))
+                       (matrix+ dir (matrix-scale W (* amt p2))))]
+    [`(F ,amt) (values (matrix+ pos (matrix-scale dir amt)) dir)]
+    [(or `(R 90) `(L 270)) (values pos (matrix* R90 dir))]
+    [(or `(L 90) `(R 270)) (values pos (matrix* L90 dir))]
+    [`(,_ 180) (values pos (matrix-scale dir -1))]))
+
+(define part1
+  (match-let-values
+   ([(pos dir)
+     (for/fold ([pos (col-matrix [0 0])]
+                [dir E])
+               ([instr input])
+       (move instr 1 0 pos dir))])
+   (matrix-1norm pos)))
+
+(define part2
+  (match-let-values
+   ([(ship wpt)
+     (for/fold ([ship (col-matrix [0 0])]
+                [wpt (col-matrix [10 -1])])
+               ([instr input])
+       (move instr 0 1 ship wpt))])
+   (matrix-1norm ship)))
+
+(show-solution part1 part2)
diff --git a/src/12.rkt b/src/12.rkt
index 3a45ffe..0f1825b 100644
--- a/src/12.rkt
+++ b/src/12.rkt
@@ -1,7 +1,6 @@
-#lang curly-fn racket
+#lang racket
 
-(require threading
-         "../lib.rkt")
+(require "../lib.rkt")
 
 (define (parse line)
   (match line
@@ -10,58 +9,33 @@
 
 (define input (map parse (problem-input 12)))
 
-(define (turn curr-dir dir angle)
-  (define dirs
-    (if (symbol=? dir 'R)
-        '(N E S W)
-        '(N W S E)))
-  (define index (index-of dirs curr-dir))
-  (list-ref dirs (modulo (+ index (/ angle 90)) 4)))
-
-(define (rotate x y dir amt)
-  (cond
-    [(= amt 180) (list (- x) (- y))]
-    [(= amt 270) (rotate x y (if (symbol=? dir 'R) 'L 'R) 90)]
-    [(symbol=? dir 'R) (list (- y) x)]
-    [(symbol=? dir 'L) (list y (- x))]))
-
-(define (move instr x y dir)
+(define (move instr p1 p2 x y dx dy)
   (match instr
-    [`(N ,amt) (values x (- y amt) dir)]
-    [`(E ,amt) (values (+ x amt) y dir)]
-    [`(S ,amt) (values x (+ y amt) dir)]
-    [`(W ,amt) (values (- x amt) y dir)]
-    [`(F ,amt) (move `(,dir ,amt) x y dir)]
-    [`(,lr ,amt) (values x y (turn dir lr amt))]))
+    [`(N ,amt) (values x (- y (* amt p1)) dx (- dy (* amt p2)))]
+    [`(E ,amt) (values (+ x (* amt p1)) y (+ dx (* amt p2)) dy)]
+    [`(S ,amt) (values x (+ y (* amt p1)) dx (+ dy (* amt p2)))]
+    [`(W ,amt) (values (- x (* amt p1)) y (- dx (* amt p2)) dy)]
+    [`(F ,amt) (values (+ x (* dx amt)) (+ y (* dy amt)) dx dy)]
+    [(or '(R 90) '(L 270)) (values x y (- dy) dx)]
+    [(or '(L 90) '(R 270)) (values x y dy (- dx))]
+    [`(,_ 180) (values x y (- dx) (- dy))]))
 
-(define (move* instr x-ship y-ship x-wpt y-wpt)
-  (match instr
-    [`(N ,amt) (values x-ship y-ship x-wpt (- y-wpt amt))]
-    [`(E ,amt) (values x-ship y-ship (+ x-wpt amt) y-wpt)]
-    [`(S ,amt) (values x-ship y-ship x-wpt (+ y-wpt amt))]
-    [`(W ,amt) (values x-ship y-ship (- x-wpt amt) y-wpt)]
-    [`(F ,amt) (values (+ x-ship (* x-wpt amt)) (+ y-ship (* y-wpt amt)) x-wpt y-wpt)]
-    [`(,lr ,amt)
-     (match-let ([(list x-wpt y-wpt) (rotate x-wpt y-wpt lr amt)])
-       (values x-ship y-ship x-wpt y-wpt))]))
+(define part1
+  (match-let-values
+   ([(x y dx dy)
+     (for/fold ([x  0] [y  0]
+                [dx 1] [dy 0])
+               ([instr input])
+       (move instr 1 0 x y dx dy))])
+   (+ (abs x) (abs y))))
 
-(define (part1)
-  (define-values (x y dir)
-    (for/fold ([x 0]
-               [y 0]
-               [dir 'E])
-              ([instr input])
-      (move instr x y dir)))
-  (+ (abs x) (abs y)))
+(define part2
+  (match-let-values
+   ([(x-ship y-ship x-wpt y-wpt)
+     (for/fold ([x-ship 0] [y-ship 0]
+                [x-wpt 10] [y-wpt -1])
+               ([instr input])
+       (move instr 0 1 x-ship y-ship x-wpt y-wpt))])
+   (+ (abs x-ship) (abs y-ship))))
 
-(define (part2)
-  (define-values (x-ship y-ship x-wpt y-wpt)
-    (for/fold ([x-ship 0]
-               [y-ship 0]
-               [x-wpt 10]
-               [y-wpt -1])
-              ([instr input])
-      (move* instr x-ship y-ship x-wpt y-wpt)))
-  (+ (abs x-ship) (abs y-ship)))
-
-(show-solution (part1) (part2))
+(show-solution part1 part2)