data←⊃⎕NGET 'input.txt' 1
)copy dfns iotag
interval←(iotag⍥⊃)(,⍤(∘.,))(iotag⍥(⊃∘⌽))
set←∪⊃,/{∪⊃,/2 interval/↓2↑⍤1⊢((⌈3÷⍨⍴s),3)⍴⊢s←⊃(//)',>-'⎕VFI ⍵}¨data
⍝ Part 1
∇n←sim map;sand;d;dl;dr;m
 n←0
next:
 sand←500 0
loop:
 d←0 1+sand
 dl←¯1 1+sand
 dr←1 1+sand
 m←~(d dl dr)∊map
 :If 0<+/m
     sand←⊃m/(d dl dr)
     →((⊃⌽sand)>164)/abyss
 :Else
     map←map,⊂sand
     n+←1
     →next
 :EndIf
 →loop
abyss:
∇
;;; lisp part2
(ql:quickload :fset)

(load "data.lisp")
(defparameter *map*
  (reduce #'fset:with `(,(fset:empty-set) ,@+data+)))

(defparameter *test*
  (fset:set '(498 . 4) '(498 . 5)
            '(498 . 6) '(497 . 6) '(496 . 6) '(503 . 4) '(502 . 4) '(502 . 5) '(502 . 6) '(502 . 7) '(502 . 8)
            '(502 . 9) '(501 . 9) '(500 . 9) '(499 . 9) '(498 . 9) '(497 . 9) '(496 . 9) '(495 . 9) '(494 . 9)))

(defun sim (map limit)
  (let ((n 1)
        (x 500)
        (y 0))
    (loop
      (if (= (1+ y) limit)
          (progn
            (setq map (fset:with map (cons x y)))
            (incf n)
            (setq x 500
                  y 0))
          (if (fset:member? (cons x (1+ y)) map)
              (if (fset:member? (cons (- x 1) (1+ y)) map)
                  (if (fset:member? (cons (1+ x) (1+ y)) map)
                      (progn (when (and (= x 500) (= y 0))
                               (return-from sim n))
                             (setq map (fset:with map (cons x y)))
                             (incf n)
                             (setq x 500
                                   y 0))
                      (progn (incf x)
                             (incf y)))
                  (progn
                    (decf x)
                    (incf y)))
              (incf y))))))