1 files changed, 13 insertions, 11 deletions
diff --git a/2022/day-21/day-21.rkt b/2022/day-21/day-21.rkt
index 752dfb8..9f08f97 100644
--- a/2022/day-21/day-21.rkt
+++ b/2022/day-21/day-21.rkt
@@ -22,9 +22,7 @@
 ;; part 1
 (define (evaluate-monkey m-name [guess #f])
   (match-define (op f name1 name2)
-    (if (and guess (equal? m-name "humn"))
-        (op 'constant guess #f)
-        (hash-ref monkey-table m-name)))
+    (if (and guess (equal? m-name "humn")) (op 'constant guess #f) (hash-ref monkey-table m-name)))
   (match f
     ['constant name1]
     ['+ (+ (evaluate-monkey name1 guess) (evaluate-monkey name2 guess))]
@@ -38,11 +36,15 @@
 (match-define (op _ branch-1 branch-2) (hash-ref monkey-table "root"))
 
 ;; the branch that doesn't depend on humn
-(define result-2 (evaluate-monkey branch-2))
-
-;; I plugged in numbers for guess to find a suitable starting range and settled on the first one
-;; that I found that got me within a million
-(for/first ([guess (in-naturals 3059361690000)]
-            #:do [(define result-1 (evaluate-monkey branch-1 guess))]
-            #:when (= result-1 result-2))
-  guess)
+(define known-side (evaluate-monkey branch-2))
+
+(for/fold ([lower-bound 0] [upper-bound 1e16] #:result (inexact->exact lower-bound))
+          ([_ (in-naturals)])
+  #:break (= lower-bound upper-bound)
+  (define midpoint (quotient (+ lower-bound upper-bound) 2))
+  (define candidate (evaluate-monkey branch-1 midpoint))
+  (println (~a midpoint " -> " candidate " -> " (- candidate known-side)))
+  (cond
+    [(= candidate known-side) (values midpoint midpoint)]
+    [(> candidate known-side) (values midpoint upper-bound)]
+    [(< candidate known-side) (values lower-bound midpoint)]))