diff --git a/2019/07.rkt b/2019/07.rkt
index f53ee00..be6ef1f 100644
--- a/2019/07.rkt
+++ b/2019/07.rkt
@@ -1,57 +1,60 @@
 #lang br
-(require racket/file rackunit racket/generator)
+(require racket/file rackunit)
 
 (define (string->ints str) (map string->number (string-split str #px",\\s*")))
 (define (string->regs str) (list->vector (string->ints str)))
 
 (define ((binarize proc) x y) (if (proc x y) 1 0))
 
-(define (run str . inputs)
-  (define regs (string->regs str))
-  (define (reg-ref ptr) (vector-ref regs ptr))
-  (define (reg-set! ptr val) (vector-set! regs (reg-ref ptr) val))
-  (define ip (open-input-string (string-join (map ~a inputs) " ")))
-  (let/ec terminate
-    (let loop ([ptr 0])
-      (match-define (list opcode mode-1 mode-2 mode-3)
-        (match (for/list ([c (in-string (~r (reg-ref ptr) #:min-width 5 #:pad-string "0"))])
-                 (string->number (string c)))
-          [(list d4 d3 d2 d1 d0) (cons (+ (* 10 d1) d0)
-                                       (for/list ([mode-val (list d2 d3 d4)]
-                                                  [offset '(1 2 3)])
-                                         (λ (ptr) ((if (zero? mode-val) reg-ref values) (reg-ref (+ ptr offset))))))]))
-      (define next-ptr
-        (match opcode
-          [(or 1 2 7 8) ; 4-arity: add & multiply & compare
-           (reg-set! (+ ptr 3) ((match opcode
-                                  [1 +]
-                                  [2 *]
-                                  [7 (binarize <)]
-                                  [8 (binarize =)]) (mode-1 ptr) (mode-2 ptr)))
-           (+ ptr 4)]
-          [(or 3 4) ; 2-arity: input & output
-           (match opcode
-             [3 (parameterize ([current-input-port ip])
-                  (reg-set! (+ ptr 1) (read)))]
-             [4 (println (mode-1 ptr))])
-           (+ ptr 2)]
-          [(or 5 6) ; 3-arity: jump
-           (if ((match opcode
-                  [5 not]
-                  [6 values]) (zero? (mode-1 ptr)))
-               (mode-2 ptr)
-               (+ ptr 3))]
-          [99 (terminate)]
-          [_ (error 'unknown-opcode)]))
-      (loop next-ptr))))
+(define (make-amp-thread program calling-thd)
+  (thread (λ ()
+            (define regs (string->regs program))
+            (define (reg-ref ptr) (vector-ref regs ptr))
+            (define (reg-set! ptr val) (vector-set! regs (reg-ref ptr) val))
+            (let/ec terminate
+              (let loop ([ptr 0])
+                (match-define (list opcode mode-1 mode-2 mode-3)
+                  (match (for/list ([c (in-string (~r (reg-ref ptr) #:min-width 5 #:pad-string "0"))])
+                                   (string->number (string c)))
+                    [(list d4 d3 d2 d1 d0) (cons (+ (* 10 d1) d0)
+                                                 (for/list ([mode-val (list d2 d3 d4)]
+                                                            [offset '(1 2 3)])
+                                                           (λ (ptr) ((if (zero? mode-val) reg-ref values) (reg-ref (+ ptr offset))))))]))
+                (define next-ptr
+                  (match opcode
+                    [(or 1 2 7 8) ; 4-arity: add & multiply & compare
+                     (reg-set! (+ ptr 3) ((match opcode
+                                            [1 +]
+                                            [2 *]
+                                            [7 (binarize <)]
+                                            [8 (binarize =)]) (mode-1 ptr) (mode-2 ptr)))
+                     (+ ptr 4)]
+                    [(or 3 4) ; 2-arity: input & output
+                     (match opcode
+                       [3 (reg-set! (+ ptr 1) (thread-receive))]
+                       [4 (thread-send calling-thd (mode-1 ptr))])
+                     (+ ptr 2)]
+                    [(or 5 6) ; 3-arity: jump
+                     (if ((match opcode
+                            [5 not]
+                            [6 values]) (zero? (mode-1 ptr)))
+                         (mode-2 ptr)
+                         (+ ptr 3))]
+                    [99 (terminate)]
+                    [_ (error "unknown-opcode" opcode)]))
+                (loop next-ptr))))))
 
-(define-syntax-rule (last-output func)
-  (last (map string->number (string-split (with-output-to-string (λ () func))))))
+(define (make-amp-threads program phases)
+  (for/list ([phase (in-list (if (string? phases) (string->ints phases) phases))])
+            (define thd (make-amp-thread program (current-thread)))
+            (thread-send thd phase)
+            thd))
 
 (define (thruster-signal program phases)
   (for/fold ([input 0])
-            ([phase (in-list (if (string? phases) (string->ints phases) phases))])
-    (last-output (run program phase input))))
+            ([amp (in-list (make-amp-threads program phases))])
+    (thread-send amp input)
+    (thread-receive)))
 
 (check-eq? (thruster-signal "3,15,3,16,1002,16,10,16,1,16,15,15,4,15,99,0,0" "4,3,2,1,0") 43210)
 (check-eq? (thruster-signal "3,23,3,24,1002,24,10,24,1002,23,-1,23,
@@ -59,8 +62,34 @@
 (check-eq? (thruster-signal "3,31,3,32,1002,32,10,32,1001,31,-2,31,1007,31,0,33,
 1002,33,7,33,1,33,31,31,1,32,31,31,4,31,99,0,0,0" "1,0,4,3,2") 65210)
 
+(define amp-program (file->string "07.rktd"))
+
 ;; 1
 (check-eq?
  (apply max (for/list ([phases (in-permutations '(0 1 2 3 4))])
-              (thruster-signal (file->string "07.rktd") phases)))
- 87138)
\ No newline at end of file
+                      (thruster-signal amp-program phases)))
+ 87138)
+
+(define (thruster-signal-cycle program phases)
+  (for/fold ([input 0])
+            ([amp (in-cycle (make-amp-threads program phases))]
+             #:break (not (thread-running? amp)))
+    (thread-send amp input)
+    (thread-receive)))
+
+(check-eq?
+ (thruster-signal-cycle "3,26,1001,26,-4,26,3,27,1002,27,2,27,1,27,26,
+27,4,27,1001,28,-1,28,1005,28,6,99,0,0,5" "9,8,7,6,5")
+ 139629729)
+
+(check-eq?
+ (thruster-signal-cycle "3,52,1001,52,-5,52,3,53,1,52,56,54,1007,54,5,55,1005,55,26,1001,54,
+-5,54,1105,1,12,1,53,54,53,1008,54,0,55,1001,55,1,55,2,53,55,53,4,
+53,1001,56,-1,56,1005,56,6,99,0,0,0,0,10" "9,7,8,5,6")
+ 18216)
+
+;; 2
+(check-eq?
+ (apply max (for/list ([phases (in-permutations '(5 6 7 8 9))])
+                      (thruster-signal-cycle amp-program phases)))
+ 17279674)
\ No newline at end of file