continue day23

aoc-racket.scrbl

@@ -43,4 +43,4 @@ You can install this package (if you haven't already) with
 @include-section[(submod "day20.rkt" doc)]
 @include-section[(submod "day21.rkt" doc)]
 @include-section[(submod "day22.rkt" doc)]
-@include-section[(submod "day23.rkt" doc)]
+@;include-section[(submod "day23.rkt" doc)]

day23.scrbl

@@ -16,18 +16,74 @@
 
 @section{What's the value in register @tt{b} after the program runs?}
 
+The virtual machine has two registers, @tt{a} and @tt{b}, that both start at 0. It also has six instructions:
+
+@itemlist[
+          
+ @item{@tt{hlf r} sets register r to half its current value, then continues.}
+ @item{@tt{tpl r} sets register r to triple its current value, then continues.}
+ @item{@tt{inc r} adds 1 to register r, then continues.}
+ @item{@tt{jmp offset} jumps the instruction that is @tt{offset} steps away. An @tt{offset} can be positive or negative.}
+ @item{@tt{jie r, offset} is like @tt{jmp}, but only jumps if r is even.}
+ @item{@tt{jio r, offset} jumps by @tt{offset} if register r = 1.}
+ ]
+
+Although the virtual machine has the equivalent of functions & variables, the jump instructions add a complication. We can't just evaluate the instructions top to bottom. We have to maintain a list of all the instructions, and a pointer to where we are, so if we get a jump instruction, we can move to the right place.
+
+Because we have to repeatedly update the values of the register, it'll be more convenient to use a hash table (which is designed for this purpose).
 
 @chunk[<day23-setup>
        (require racket rackunit)
        (provide (all-defined-out))
        
-                                             
+       (define registers (make-hash '((a . 0)(b . 0))))
+       
+       (define-syntax-rule (define-reg-updater id thunk)
+         (define (id reg)
+           (hash-update! registers reg thunk)))
+       
+       (define-reg-updater tpl (λ(val) (* 3 val)))
+       (define-reg-updater inc (λ(val) (add1 val)))
+       (define-reg-updater hlf (λ(val) (/ val 2)))
+       
+       (define (jmpf reg num pred)
+         (if (pred (hash-ref registers reg)) num 1))
+       
+       (define-syntax (ins stx)
+         (syntax-case stx (jmp jio jie)
+           [(_ jio reg num)
+            #'(jmpf 'reg num (λ(x) (= 1 x)))]
+           [(_ jie reg num)
+            #'(jmpf 'reg num even?)]
+           [(_ jmp num)
+            #'num]
+           [(_ op reg)
+            #'(op 'reg)]
+           [else #'(void)]))
+
+         (define-syntax (parse-instructions stx)
+           (syntax-case stx ()
+             [(_ input-str)
+              #'(let* ([str (string-replace input-str "," "")]
+                       [lines (string-split str "\n")]
+                       [datums)
+                  42)]))
                             
        ]
 
 @chunk[<day23-q1>
        
        (define (q1 input-str)
+         (define instructions (parse-instructions input-str))
+         (let eval-instruction ([idx 0])
+           (if (>= idx (length instructions))
+               (hash-ref registers 'b)
+               (let* ([inst (list-ref instructions idx)]
+                      [result (inst)]
+                      [next-idx (+ (if (number? result)
+                                       result
+                                       1) idx)])
+                 (eval-instruction next-idx))))
          184)]