diff --git a/advent-of-code-2018.asd b/advent-of-code-2018.asd
index ddf0042..b1cb76a 100644
--- a/advent-of-code-2018.asd
+++ b/advent-of-code-2018.asd
@@ -23,4 +23,5 @@
                (:file "day12")
                (:file "day13")
                (:file "day14")
-               (:file "day16")))
+               (:file "day16")
+               (:file "day17")))
diff --git a/day17.lisp b/day17.lisp
new file mode 100644
index 0000000..35a79d5
--- /dev/null
+++ b/day17.lisp
@@ -0,0 +1,100 @@
+;;;; day17.lisp
+
+(in-package :advent-of-code-2018)
+
+(defun day17-sand-p (char)
+  (char= char #\.))
+
+(defun day17-passable-p (char)
+  (or (day17-sand-p char)
+      (char= #\| char)))
+
+(defun day17-parse-line (line)
+  (let ((x (ppcre:register-groups-bind ((#'parse-integer from to))
+               ("x=(\\d+)\.\.?(\\d+)?" line)
+             (list from (or to from))))
+        (y (ppcre:register-groups-bind ((#'parse-integer from to))
+               ("y=(\\d+)\.\.?(\\d+)?" line)
+             (list from (or to from)))))
+    (list (list x y))))
+
+(defun day17-parse-input ()
+  (loop-line-by-line (puzzlepath "input17.txt")
+    :append (day17-parse-line line)))
+
+(defun day17-bbox (veins)
+  (loop :for ((x1 x2) (y1 y2)) :in veins
+        :minimizing x1 :into min-x
+        :maximizing x2 :into max-x
+        :minimizing y1 :into min-y
+        :maximizing y2 :into max-y
+        :finally (return (list min-x max-x min-y max-y))))
+
+(defun day17-insert-vein! (board vein &optional (dx 0) (dy 0))
+  (destructuring-bind ((min-x max-x) (min-y max-y)) vein
+    (loop :for x :from (- min-x dx) :upto (- max-x dx)
+          :do (loop :for y :from (- min-y dy) :upto (- max-y dy)
+                    :do (setf (aref board x y) #\#))))
+  board)
+
+(defun day17-initialize-board (input)
+  (destructuring-bind (raw-min-x max-x min-y max-y) (day17-bbox inp)
+    (let* ((width  (+ 3 (- max-x raw-min-x)));pad-x
+           (height (+ 1 (- max-y min-y)))
+           (min-x (1- raw-min-x))
+           (board (make-array (list width height) :initial-element #\.)))
+      (loop :for vein :in input
+            :do (day17-insert-vein! board vein min-x min-y))
+      (list board min-x min-y))))
+
+(defun day17-flow-horizontal (board x y)
+  (unless (array-in-bounds-p board x y)
+    (return-from day17-flow-horizontal nil))
+  (when (char= #\| (aref board x y))
+    (return-from day17-flow-horizontal nil))
+  (unless (day17-sand-p (aref board x y))
+    (return-from day17-flow-horizontal t))
+  (unless (day17-flow-horizontal board x (1+ y))
+    (setf (aref board x y) #\|)
+    (return-from day17-flow-horizontal nil))
+  (destructuring-bind (left left-wall?)
+      (loop :for ix :from (1- x) :downto 0
+            :unless (day17-sand-p (aref board ix y)) :return (list (1+ ix) t)
+            :when (and (day17-passable-p (aref board ix (1+ y)))
+                       (not (day17-flow-horizontal board ix (1+ y))))
+            :return (list ix nil)
+            :finally (return (list 0 nil)))
+    (destructuring-bind 
+        (right right-wall?)
+        (loop :for ix :from (1+ x) :below (array-dimension board 0)
+              :unless (day17-sand-p (aref board ix y)) :return (list (1- ix) t)
+              :when (and (day17-passable-p (aref board ix (1+ y)))
+                         (not (day17-flow-horizontal board ix (1+ y))))
+              :return (list ix nil)
+              :finally (return (list (1- (array-dimension board 0)) nil)))
+      (loop :for ix :from left :upto right
+            :do (setf (aref board ix y) (if (and left-wall? right-wall?)
+                                            #\~
+                                            #\|)))
+      (and left-wall? right-wall?))))
+
+(defun day17-save-board (board path)
+  (with-open-file (out path :direction :output
+                            :if-exists :overwrite
+                            :if-does-not-exist :create) 
+    (loop :for y :below (array-dimension board 1)
+          :do (loop :for x :below (array-dimension board 0)
+                    :do (format out "~a" (aref board x y)))
+          :do (format out "~%"))))
+
+(defun day17-count (board)
+  (let ((view (make-array (array-total-size board) :displaced-to board)))
+    (values (count #\~ view) (count #\| view))))
+
+(defun day17 ()
+  (destructuring-bind (board dx dy) (day17-initialize-board (day17-parse-input))
+    (declare (ignore dy))
+    (day17-flow-horizontal board (- 500 dx) 0)
+    (day17-save-board board (puzzlepath "output17.txt"))
+    (multiple-value-bind (still flowing) (day17-count board)
+      (list (+ still flowing) still))))