Add Day 12 solutions and input

12/1.rkt

+#lang racket
+(struct moon (xpos ypos zpos xvel yvel zvel) #:transparent)
+
+(define (read-moons)
+  (let ([regexres (regexp-match #rx"<x=(-?[0-9]+), y=(-?[0-9]+), z=(-?[0-9]+)>" (current-input-port))])
+    (if (not regexres)
+        empty
+        (let ([coords (map (compose string->number bytes->string/utf-8) (cdr regexres))])
+          (cons(moon (list-ref coords 0) (list-ref coords 1) (list-ref coords 2) 0 0 0) (read-moons))))))
+
+(define (sign n)
+  (if (n . > . 0)
+      1
+      (if (n . = . 0)
+          0
+          -1)))
+
+(define (velchange subject cause)
+  (list
+   (sign ((moon-xpos cause) . - . (moon-xpos subject)))
+   (sign ((moon-ypos cause) . - . (moon-ypos subject)))
+   (sign ((moon-zpos cause) . - . (moon-zpos subject)))))
+
+(define (vecsum l r)
+  (if (empty? l)
+      empty
+      (cons ((car l) . + . (car r)) (vecsum (cdr l) (cdr r)))))
+
+(define (move-moon subject a)
+  (moon (+ (moon-xpos subject) (moon-xvel subject) (list-ref a 0))
+        (+ (moon-ypos subject) (moon-yvel subject) (list-ref a 1))
+        (+ (moon-zpos subject) (moon-zvel subject) (list-ref a 2))
+        (+ (moon-xvel subject) (list-ref a 0))
+        (+ (moon-yvel subject) (list-ref a 1))
+        (+ (moon-zvel subject) (list-ref a 2))))
+
+(define (step-moons moons)
+  (for/list ([moon moons])
+    (move-moon moon
+               (foldl (lambda (other accel) (vecsum accel (velchange moon other)))
+                      '(0 0 0) moons))))
+
+(define (timestep moons steps)
+  (if (= 0 steps)
+      moons
+      (timestep (step-moons moons) (sub1 steps))))
+
+(define (energy moons)
+  (if (empty? moons)
+      0
+      (let ([m (car moons)])
+        (+ (* (+ (abs (moon-xpos m)) (abs (moon-ypos m)) (abs (moon-zpos m)))
+              (+ (abs (moon-xvel m)) (abs (moon-yvel m)) (abs (moon-zvel m))))
+           (energy (cdr moons))))))
+      
+
+(energy (timestep (read-moons) 1000))
\ No newline at end of file

12/2.rkt

+#lang racket
+(struct moon (xpos ypos zpos xvel yvel zvel) #:transparent)
+
+(define (read-moons)
+  (let ([regexres (regexp-match #rx"<x=(-?[0-9]+), y=(-?[0-9]+), z=(-?[0-9]+)>" (current-input-port))])
+    (if (not regexres)
+        empty
+        (let ([coords (map (compose string->number bytes->string/utf-8) (cdr regexres))])
+          (cons(moon (list-ref coords 0) (list-ref coords 1) (list-ref coords 2) 0 0 0) (read-moons))))))
+
+(define (sign n)
+  (if (n . > . 0)
+      1
+      (if (n . = . 0)
+          0
+          -1)))
+
+(define (accel subject cause)
+  (sign ((car cause) . - . (car subject))))
+
+(define (move-moon subject a)
+  (cons (+ (car subject) (cdr subject) a)
+        (+ (cdr subject) a)))
+
+(define (step-moons moons)
+  (for/list ([m moons])
+    (move-moon m
+               (foldl (lambda (other a) (+ a (accel m other)))
+                      0 moons))))
+
+(define (timestep moons init steps)
+  (if (equal? moons init)
+      steps
+      (timestep (step-moons moons) init (add1 steps))))      
+
+(let* ([moons (read-moons)]
+       [xaxis (map (lambda (m) (cons (moon-xpos m) (moon-xvel m))) moons)]
+       [yaxis (map (lambda (m) (cons (moon-ypos m) (moon-yvel m))) moons)]
+       [zaxis (map (lambda (m) (cons (moon-zpos m) (moon-zvel m))) moons)])
+  (lcm
+   (timestep (step-moons xaxis) xaxis 1)
+   (timestep (step-moons yaxis) yaxis 1)
+   (timestep (step-moons zaxis) zaxis 1)))
\ No newline at end of file

12/input

+<x=-4, y=-9, z=-3>
+<x=-13, y=-11, z=0>
+<x=-17, y=-7, z=15>
+<x=-16, y=4, z=2>