Day 2

2.rkt

@@ -0,0 +1,61 @@
+#lang racket
+
+(define input (map string->number (string-split
+                                   (string-trim (file->string "input.2.txt")) ",")))
+
+(define pos1-replacement 12)
+(define pos2-replacement 2)
+
+(define mutated-input (list-set (list-set input 1 pos1-replacement) 2 pos2-replacement))
+
+(define op-add 1)
+(define op-mul 2)
+(define op-exit 99)
+
+(define (intcode-arith op tape pos)
+  (let ([lhs (list-ref tape (+ pos 1))]
+        [rhs (list-ref tape (+ pos 2))]
+        [dst (list-ref tape (+ pos 3))])
+    (intcode-eval (list-set tape dst (op (list-ref tape lhs) (list-ref tape rhs)))
+                  (+ pos 4))))
+
+(define intcode-add ((curry intcode-arith) +))
+(define intcode-mul ((curry intcode-arith) *))
+
+(define (intcode-eval tape pos)
+  (let ([op (list-ref tape pos)])
+    (cond [(= op op-add) (intcode-add tape pos)]
+          [(= op op-mul) (intcode-mul tape pos)]
+          [(= op op-exit) (first tape)]
+          [else (error "invalid operator")])))
+
+;; Part 1
+(displayln (intcode-eval mutated-input 0))
+
+;; if i were less lazy i'd make a proper symbolic execution engine for this but...
+
+(define wanted-output 19690720)
+
+(define (brute-force i j)
+  (cond [(= wanted-output (intcode-eval (list-set (list-set input 1 i) 2 j) 0)) (list i j)]
+        [(>= i 100) (brute-force 0 (add1 j))]
+        [else (brute-force (add1 i) j)]))
+
+;; Part 2
+(displayln (brute-force 0 0))
+
+;; Okay now with actual symbolic execution
+
+(define symbolic-globals '(1 2))
+
+(define-struct symbvar (idx))
+(define-struct constr-arith (op lhs rhs dest))
+(define-struct constr-oneof (vals))
+(define-struct state (constrs nextvar))
+
+(define initial-state (make-state '() 0))
+
+(define (create-symbvar state)
+  (values (make-symbvar (state-nextvar state))
+          (make-state (state-constrs state) (add1 (state-nextvar state)))))
+