day 12

2020/12.rkt

@@ -0,0 +1,42 @@
+#lang br
+(require racket/file rackunit racket/dict)
+
+(define insts
+  (for/list ([ln (file->lines "12.rktd")])
+    (match-define (list _ name dist) (regexp-match  #px"(\\D+)(\\d+)" ln))
+    (cons (string->symbol name) (string->number dist))))
+
+(define left-turn +i)
+(define right-turn -i)
+(define north +i)
+(define east (* north right-turn))
+(define south (* east right-turn))
+(define west (* south right-turn))
+
+(define (manhattan-dist pos) (+ (abs (real-part pos)) (abs (imag-part pos))))
+
+(define (solve insts [use-waypoint? #false])
+  (for/fold ([pos 0]
+             [facing 1]
+             [waypoint 10+i]
+             #:result (manhattan-dist pos))
+            ([(name dist) (in-dict insts)])
+    (case name
+      [(F)
+       (define ref-point (if use-waypoint? waypoint facing))
+       (values (+ pos (* ref-point dist)) facing waypoint)]
+      [(N E S W)
+       (define direction (match name
+                           ['S south]
+                           ['N north]
+                           ['E east]
+                           ['W west]))
+       (define next-pos (if use-waypoint? pos (+ pos (* direction dist))))
+       (values next-pos facing (+ waypoint (* direction dist)))]
+      [(L R)
+       (define rotation
+         (expt (if (eq? name 'L) left-turn right-turn) (/ dist 90)))
+       (values pos (* facing rotation) (* waypoint rotation))])))
+
+(check-equal? (solve insts) 1496)
+(check-equal? (solve insts #t) 63843)

2020/12.rktd

@@ -0,0 +1,784 @@
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