Refactoring day 22 and 23.

src/22.rkt

@@ -9,18 +9,27 @@
 
 ;; A shuffle operation (technique) is
 ;; an affine transformation on a card's index.
-;; Applying the transformation (m, o) to i yields (m*i + o).
-;; We can compose transformations and only keep track
-;; of the multiple and offset factors (modulo some len).
 ;; The identity transformation is I = (1, 0).
+;; Applying the transformation (m, o) to i yields (m*i + o).
 
 (struct affine (multiple offset) #:transparent)
 
+(define I (affine 1 0))
+
 (define (apply-affine len mo i)
   (match-let ([(affine m o) mo])
     (% (+ (* m i) o) len)))
 
-(define I (affine 1 0))
+;; We can compose transformations and only keep track
+;; of the multiple and offset factors (modulo some len);
+;; <> composes two transformations as if the second were
+;; applied first and the first applied last, so
+;; (m1 , o1) <> (m2, o2) = (m1 * m2, m1 * o2 + o1).
+(define (<> len a1 a2)
+  (match-let ([(affine m1 o1) a1]
+              [(affine m2 o2) a2])
+    (affine (% (* m1 m2) len)
+            (% (+ (* m1 o2) o1) len))))
 
 ;; Applying the transformation (m, o) n times is the same as
 ;; applying the transformation (m^n + o*(m^n - 1)/(m - 1)),
@@ -31,6 +40,13 @@
                [o* (% (* o (sub1 m^n) (modular-inverse (sub1 m) len)) len)])
     (affine m^n o*)))
 
+;; Inverting the transformation (m, o) is the same as
+;; the transformation (m^-1, -o*m^-1)
+(define (affine-invert mo len)
+  (match-let* ([(affine m o) mo]
+               [m^-1 (modular-inverse m len)])
+    (affine m^-1 (* -1 o m^-1))))
+
 ;; All shuffling transformation techniques are modulo the number of cards.
 ;; deal into new stack: reversing the order of the cards,
 ;;   corresponding to the transformation i → -1*i + (length - 1)
@@ -39,56 +55,30 @@
 ;; deal with increment N: placing a card every n steps,
 ;;   corresponding to the transformation i → n*i
 
-(define (DINS len mo)
+(define (DINS len)
-  (match-let ([(affine m o) mo])
+  (affine -1 (sub1 len)))
-    (affine (% (* m -1) len)
-            (% (- (sub1 len) o) len))))
 
-(define (CNC len n mo)
+(define (CNC len n)
-  (match-let ([(affine m o) mo])
+  (affine 1 (* n -1)))
-    (affine m (% (- o n) len))))
 
-(define (DWIN len n mo)
+(define (DWIN len n)
-  (match-let ([(affine m o) mo])
+  (affine n 0))
-    (affine (% (* m n) len)
-            (% (* o n) len))))
-
-;; The corresponding inverse transformations are:
-;;   DINS: -1*i + (length - 1) ← i
-;;   CNC: i + n ← i
-;;   DWIN: n^-1*i ← i
-;; where ·^-1 is the modular multiplicative inverse
-
-(define (inverse-DINS len mo)
-  (DINS len mo))
-
-(define (inverse-CNC len n mo)
-  (CNC len (* n -1) mo))
-
-(define (inverse-DWIN len n mo)
-  (DWIN len (modular-inverse n len) mo))
 
 ;; Shuffling combines all transformations in order.
-;; Inverse shuffling combines all inverse transformations in reverse order.
+;; Inverse shuffling is simply the inverse transformation.
 ;; We begin with the identity transformation, I = (1, 0).
 
-(define (parse len T mo)
+(define (parse len T)
   (match T
-    ["deal into new stack" (DINS len mo)]
+    ["deal into new stack" (DINS len)]
-    [(string-append "cut " s) (CNC len (string->number s) mo)]
+    [(string-append "cut " s) (CNC len (string->number s))]
-    [(string-append "deal with increment " s) (DWIN len (string->number s) mo)]))
+    [(string-append "deal with increment " s) (DWIN len (string->number s))]))
-
-(define (inverse-parse len T mo)
-  (match T
-    ["deal into new stack" (inverse-DINS len mo)]
-    [(string-append "cut " s) (inverse-CNC len (string->number s) mo)]
-    [(string-append "deal with increment " s) (inverse-DWIN len (string->number s) mo)]))
 
 (define (shuffle len)
-  (foldl (∂ parse len) I input))
+  (foldl (λ (T mo) (<> len (parse len T) mo)) I input))
 
 (define (inverse-shuffle len)
-  (foldr (∂ inverse-parse len) I input))
+  (affine-invert (shuffle len) len))
 
 (define part1
   (let ([len 10007])

src/23.rkt

@@ -9,7 +9,7 @@
   (string->program (car (problem-input 23))))
 
 (define network
-  (build-vector 50 (λ (n) (resume-with-input (exec input) n))))
+  (build-vector 50 (λ (n) (resume-with-input (resume-with-input (exec input) n) -1))))
 
 (define packets
   (build-vector 50 (λ (_) (make-queue))))
@@ -32,7 +32,7 @@
             (type-case state st
               [in (resume)
                   (if (queue-empty? input)
-                      (vector-set! network i (resume -1))
+                      (void)
                       (let* ([x (dequeue! input)]
                              [y (dequeue! input)]
                              [st (resume-with-input (resume x) y)])