day 21 complete

1 files changed, 63 insertions, 0 deletions

diff --git a/2022/day-21/day-21.rkt b/2022/day-21/day-21.rkt
new file mode 100644
index 0000000..612c7f1
--- /dev/null
+++ b/2022/day-21/day-21.rkt
@@ -0,0 +1,63 @@
+#lang racket
+
+(require advent-of-code
+         threading
+         (only-in relation ->number ->symbol))
+
+(struct monkey (name op) #:transparent)
+(struct op (f first second) #:transparent)
+
+(define (parse-monkey str)
+  (match (string-split str " ")
+    [(list (app (curryr string-trim ":") name) name1 (app ->symbol f) name2)
+     (monkey name (op f name1 name2))]
+    [(list (app (curryr string-trim ":") name) (app ->number int))
+     (monkey name (op 'constant int #f))]))
+
+(define raw-monkeys (port->lines (open-aoc-input (find-session) 2022 21 #:cache #true)))
+
+(define monkey-table
+  (for/hash ([m raw-monkeys] #:do [(match-define (monkey name op) (parse-monkey m))])
+    (values name op)))
+
+;; part 1
+(define (evaluate-monkey m-name)
+  (match-define (op f name1 name2) (hash-ref monkey-table m-name))
+  (match f
+    ['constant name1]
+    ['+ (+ (evaluate-monkey name1) (evaluate-monkey name2))]
+    ['- (- (evaluate-monkey name1) (evaluate-monkey name2))]
+    ['* (* (evaluate-monkey name1) (evaluate-monkey name2))]
+    ['/ (/ (evaluate-monkey name1) (evaluate-monkey name2))]))
+
+(evaluate-monkey "root")
+
+;; part 2
+(match-define (op _ branch-1 branch-2) (hash-ref monkey-table "root"))
+
+(define (evaluate-monkey* m-name guess)
+  (match-define (op f name1 name2)
+    (if (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))]
+    ['- (- (evaluate-monkey* name1 guess) (evaluate-monkey* name2 guess))]
+    ['* (* (evaluate-monkey* name1 guess) (evaluate-monkey* name2 guess))]
+    ['/ (/ (evaluate-monkey* name1 guess) (evaluate-monkey* name2 guess))]))
+
+;; part 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))
+                  (define result-2 (evaluate-monkey* branch-2 guess))
+                  (println (~a "Guess: "
+                               guess
+                               " result 1: "
+                               (~r (exact->inexact result-1) #:notation 'exponential)
+                               " result 2: "
+                               (~r (exact->inexact result-2) #:notation 'exponential)
+                               " diff: "
+                               (~r (exact->inexact (- result-1 result-2)) #:notation 'exponential)))]
+            #:when (= result-1 result-2))
+  guess)