day 6: meow

6.rkt

@@ -1,39 +1,32 @@
 #lang curly-fn racket
 
-(require "scripts/aoc.rkt")
+(require "scripts/aoc.rkt" math/number-theory)
 
 ;; solution for day 6
 
-(define (do-race total-time hold-time)
-  (define rem-time (- total-time hold-time))
-  (* rem-time hold-time))
+(define *fudge* 1e-10)
+
+(define (do-race time dist)
+  (match-define (list lower upper) (quadratic-solutions 1 (- time) dist))
+  (~> (- (floor (- upper *fudge*)) (ceiling (+ lower *fudge*)))
+      add1 inexact->exact))
 
 (define (part1 input)
-  (apply
-    *
-    (for/list ([time (in-list (first input))] [dist (in-list (second input))])
-      (for/fold ([cnt 0]) ([hold (in-range time)])
-        (define res (do-race time hold))
-        (if (> res dist)
-          (add1 cnt)
-          cnt)))))
+  (for/product ([time (in-list (first input))]
+                [dist (in-list (second input))])
+    (do-race time dist)))
 
-(define (cvt l)
-  (string->number (apply string-append (map number->string l))))
+(define (number-concat l)
+  (~>> (map number->string l)
+       (apply string-append)
+       string->number))
 
 (define (part2 input)
-  (define time (cvt (first input)))
-  (define dist (cvt (second input)))
-  (for/fold ([cnt 0]) ([hold (in-range time)])
-    (define res (do-race time hold))
-    (if (> res dist)
-      (add1 cnt)
-      cnt)))
+  (apply do-race (map number-concat input)))
 
 (define (parse fname)
-  (match-define (list time-str dist-str) (file->lines fname))
-  (list (map string->number (rest (string-split time-str)))
-        (map string->number (rest (string-split dist-str)))))
+  (map #{map string->number (rest (string-split %))}
+       (file->lines fname)))
 
 ; (dbg (part1 (parse "inputs/6-test1")))
 ; (error)