Day 12.

2020-12-11 22:15:48 -08:00 · 2020-12-11 22:15:48 -08:00 · 417a6c7ddd
parent 9ea5f8de27
commit 417a6c7ddd
2 changed files with 840 additions and 0 deletions
--- a/input/12.txt
+++ b/input/12.txt
@ -0,0 +1,773 @@
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--- a/src/12.rkt
+++ b/src/12.rkt
@ -0,0 +1,67 @@
+#lang curly-fn racket
+
+(require threading
+         "../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 (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)
+  (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))]))
+
+(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)
+  (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)
+  (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))