From 4e2ea7d641ebfa3582a71a5e69ae4856ae15bd3f Mon Sep 17 00:00:00 2001
From: haskal <haskal@awoo.systems>
Date: Wed, 9 Dec 2020 00:54:22 -0500
Subject: [PATCH] day 9: apply optimizations to part 1

---
 9.rkt | 53 ++++++++++++++++++++++++++---------------------------
 1 file changed, 26 insertions(+), 27 deletions(-)

diff --git a/9.rkt b/9.rkt
index 34e5a23..498ec44 100644
--- a/9.rkt
+++ b/9.rkt
@@ -6,39 +6,38 @@
 
 ;; helper functions here
 
-(define (part1 input)
-  (define vec (list->vector input))
-  (let/ec exit
-    (for ([i (in-range 25 (vector-length vec))])
-      (define valid?
-        (for*/first ([a (in-range (- i 25) i)] [b (in-range a i)]
-                     #:when (= (vector-ref vec i) (+ (vector-ref vec a) (vector-ref vec b)))) #t))
-      (unless valid?
-        (exit (vector-ref vec i))))))
+;; linear time sum checker, assuming sorted subrange
+(define (contains-sum? vec start end wanted-sum)
+  (define (helper [a start] [b (sub1 end)])
+    (define cur-sum (+ (vector-ref vec a) (vector-ref vec b)))
+    (cond
+      [(>= a b) #f]
+      [(= cur-sum wanted-sum) #t]
+      [(< cur-sum wanted-sum) (helper (add1 a) b)]
+      [(> cur-sum wanted-sum) (helper a (sub1 b))]))
+  (vector-sort! vec < start end)
+  (helper))
 
-(define (part2 input)
-  (define pt1 10884537)
-  (define pt1-i 501)
-  (let/ec exit
-    (define vec (list->vector input))
-    (define (cb args)
-      (when (= pt1 (apply + args))
-        (exit (+ (apply min args) (apply max args)))))
-    (for* ([ct (in-range 3 100)] [i (in-range 0 (- pt1-i ct))])
-      (define sub (vector-copy vec i (+ i ct)))
-      (cb (vector->list sub)))))
+;; continuously sort the part of the vector below the iterator, and call contains-sum?
+(define (part1 vec)
+  (for*/first ([i (in-range 25 (vector-length vec))] [x (in-value (vector-ref vec i))]
+               #:unless (contains-sum? vec 0 i x))
+    (cons i x)))
 
-(module+ test
-  (require rackunit)
-  ;; tests here
-  (displayln "no tests :("))
+(define (part2 vec pt1-ans)
+  (match-define (cons pt1-i pt1-val) pt1-ans)
+  (for*/first ([ct (in-range 3 100)] [i (in-range 0 (- pt1-i ct))]
+               [sub (in-value (vector-copy vec i (+ i ct)))]
+               #:when (= pt1-val (for/fold ([x 0]) ([v (in-vector sub)]) (+ x v))))
+    (+ (vector-argmin identity sub) (vector-argmax identity sub))))
 
 (module+ main
-  (define input (file->list "inputs/9"))
+  (define input (list->vector (file->list "inputs/9")))
   ;; part 1
-  (answer 9 1 (part1 input))
+  (define pt1 (part1 (vector-copy input)))
+  (answer 9 1 (cdr pt1))
 
   ;; part 2
-  (answer 9 2 (part2 input))
+  (answer 9 2 (part2 (vector-copy input) pt1))
 
   (displayln "meow"))