diff --git a/2018/day17/src/main.rs b/2018/day17/src/main.rs
index 6ddd9be..242356f 100644
--- a/2018/day17/src/main.rs
+++ b/2018/day17/src/main.rs
@@ -3,18 +3,34 @@ use std::fs::File;
 use std::io::{BufRead, BufReader};
 use std::ops::Range;
 
-const MapSize: usize = 1800;
-type Map = [[char; MapSize]; MapSize];
+const MAP_SIZE: usize = 1800;
+const FOUNTAIN: char = '+';
+const EMPTY_SPACE: char = ' ';
+const WALL_SPACE: char = '#';
+const WATER: char = '~';
+const FALLING_WATER: char = '|';
+type Map = Vec<Vec<char>>;
 
-fn find_range_in_line(line: &str) -> (Range<u16>, Range<u16>) {
+#[derive(Debug, PartialEq)]
+struct Coord {
+    x: usize,
+    y: usize,
+}
+
+struct MapBounds {
+    min_y: usize,
+    max_y: usize,
+}
+
+fn find_range_in_line(line: &str) -> (Range<usize>, Range<usize>) {
     let mut parts = line.split(", ");
     let mut left_parts = parts.nth(0).unwrap().split("=");
     let left_coord_type: &str = left_parts.nth(0).unwrap();
-    let left_number: u16 = left_parts.nth(0).unwrap().parse().unwrap();
+    let left_number: usize = left_parts.nth(0).unwrap().parse().unwrap();
     let mut right_parts = parts.nth(0).unwrap().split("=");
     let mut right_numbers = right_parts.nth(1).unwrap().split("..");
-    let right_number_1: u16 = right_numbers.nth(0).unwrap().parse().unwrap();
-    let right_number_2: u16 = right_numbers.nth(0).unwrap().parse().unwrap();
+    let right_number_1: usize = right_numbers.nth(0).unwrap().parse().unwrap();
+    let right_number_2: usize = right_numbers.nth(0).unwrap().parse().unwrap();
     let left_range = Range { start: left_number, end: left_number + 1 };
     let right_range = Range { start: right_number_1, end: right_number_2 + 1 };
     if left_coord_type == "x" {
@@ -24,22 +40,169 @@ fn find_range_in_line(line: &str) -> (Range<u16>, Range<u16>) {
     }
 }
 
-fn build_map_from_file(filename: &str) -> Map {
+fn print_map(map: &Map, min_x: usize, max_x: usize, min_y: usize, max_y: usize) {
+    for y in min_y..=max_y {
+        for x in min_x..=max_x {
+            print!("{}", map[x][y]);
+        }
+        println!();
+    }
+}
+
+fn build_map_from_file(filename: &str) -> (Map, MapBounds) {
+    let mut min_y: usize = MAP_SIZE;
+    let mut max_y: usize = 0;
+    let mut m = Vec::<Vec<char>>::with_capacity(MAP_SIZE);
+    for _ in 0..MAP_SIZE {
+        let mut n = Vec::<char>::with_capacity(MAP_SIZE);
+        for _ in 0..MAP_SIZE {
+            n.push(EMPTY_SPACE);
+        }
+        m.push(n);
+    }
     let lines: Vec<String> = BufReader::new(File::open(filename).unwrap())
         .lines()
         .map(|line| line.unwrap())
         .collect();
-    let mut min_y = MapSize;
-    let mut max_y = 0;
-    let mut m: Map = [[' '; MapSize]; MapSize];
-    m[500][0] = '+';
+    m[500][0] = FOUNTAIN;
     for line in lines.iter() {
         let (x_range, y_range) = find_range_in_line(line.as_str());
+        for x in x_range {
+            for y in y_range.clone() {
+                if y < min_y {
+                    min_y = y;
+                }
+                if y > max_y {
+                    max_y = y;
+                }
+                m[x][y] = '#';
+            }
+        }
     }
-    m
+    (m, MapBounds { min_y, max_y })
+}
+
+fn pour_over(m: &mut Map, start_x: usize, start_y: usize) {
+    let mut frontier: VecDeque<Coord> = VecDeque::new();
+    frontier.push_back(Coord { x: start_x, y: start_y });
+
+    while let Some(coord) = frontier.pop_front() {
+        match m[coord.x][coord.y] {
+            FOUNTAIN => frontier.push_back(Coord { x: coord.x, y: coord.y + 1 }),
+            EMPTY_SPACE => {
+                m[coord.x][coord.y] = FALLING_WATER;
+                if coord.y + 1 < MAP_SIZE {
+                    match m[coord.x][coord.y + 1] {
+                        WALL_SPACE => frontier.push_back(Coord { x: coord.x, y: coord.y }),
+                        EMPTY_SPACE => frontier.push_back(Coord { x: coord.x, y: coord.y + 1 }),
+                        FALLING_WATER => frontier.push_back(Coord { x: coord.x, y: coord.y }),
+                        WATER => {
+                            if m[coord.x + 1][coord.y] == WALL_SPACE || m[coord.x - 1][coord.y] == WALL_SPACE {
+                                frontier.push_back(Coord { x: coord.x, y: coord.y });
+                            } else if m[coord.x + 2][coord.y] == WALL_SPACE || m[coord.x - 2][coord.y] == WALL_SPACE {
+                                frontier.push_back(Coord { x: coord.x, y: coord.y });
+                            } else if m[coord.x + 1][coord.y] == FALLING_WATER || m[coord.x - 1][coord.y] == FALLING_WATER {
+                                frontier.push_back(Coord { x: coord.x, y: coord.y });
+                            }
+                        },
+                        _ => {},
+                    }
+                }
+            },
+            WALL_SPACE => {},
+            FALLING_WATER => {
+                let mut found_down = false;
+                // go left as far as possible
+                for x in (0..coord.x).rev() {
+                    match m[x][coord.y] {
+                        EMPTY_SPACE => {
+                            if m[x][coord.y + 1] == EMPTY_SPACE {
+                                found_down = true;
+                                let c = Coord { x: x, y: coord.y };
+                                if !frontier.contains(&c) {
+                                    frontier.push_back(c);
+                                }
+                                break;
+                            } else {
+                                m[x][coord.y] = WATER;
+                            }
+                        },
+                        WATER => {},
+                        FALLING_WATER => {},
+                        _ => break,
+                    }
+                }
+                // go right as far as possible
+                for x in (coord.x + 1)..MAP_SIZE {
+                    match m[x][coord.y] {
+                        EMPTY_SPACE => {
+                            if m[x][coord.y + 1] == EMPTY_SPACE {
+                                found_down = true;
+                                let c = Coord { x: x, y: coord.y };
+                                if !frontier.contains(&c) {
+                                    frontier.push_back(c);
+                                }
+                                break;
+                            } else {
+                                m[x][coord.y] = WATER;
+                            }
+                        },
+                        WATER => {},
+                        FALLING_WATER => {},
+                        _ => break,
+                    }
+                }
+                // if no down, then push the square up
+                if !found_down {
+                    let c = Coord { x: coord.x, y: coord.y - 1 };
+                    if !frontier.contains(&c) {
+                        frontier.push_back(c);
+                    }
+                }
+            },
+            WATER => {},
+            _ => panic!("invalid map square at {:#?}", coord),
+        }
+    }
+}
+
+fn count_water(m: &Map, bounds: &MapBounds) -> usize {
+    let mut squares: usize = 0;
+    for y in bounds.min_y..=bounds.max_y {
+        for x in 0..MAP_SIZE {
+            if m[x][y] == WATER || m[x][y] == FALLING_WATER {
+                squares += 1;
+            }
+        }
+    }
+    squares
+}
+
+fn count_resting_water(m: &Map, bounds: &MapBounds) -> usize {
+    let mut squares: usize = 0;
+    for y in bounds.min_y..=bounds.max_y {
+        for x in 0..MAP_SIZE {
+            if m[x][y] == WATER {
+                squares += 1;
+            }
+            if m[x][y] == FALLING_WATER {
+                if m[x - 1][y] == WATER || m[x + 1][y] == WATER {
+                    squares += 1;
+                }
+            }
+        }
+    }
+    squares
 }
 
 fn main() {
+    let (mut m, bounds) = build_map_from_file("input");
+    pour_over(&mut m, 500, 0);
+    print_map(&m, 0, MAP_SIZE - 1, 0, MAP_SIZE - 1);
+    let water_squares = count_water(&m, &bounds);
+    println!("found {} water squares", water_squares);
+    let resting_squares = count_resting_water(&m, &bounds);
+    println!("found {} resting water squares", resting_squares);
 }
 
 #[cfg(test)]
@@ -52,4 +215,22 @@ mod tests {
         assert_eq!(x_range, (100..101));
         assert_eq!(y_range, (100..103));
     }
+
+    #[test]
+    fn test_backwards_range_from_line() {
+        let line = "y=99, x=1..5";
+        let (x_range, y_range) = find_range_in_line(line);
+        assert_eq!(x_range, (1..6));
+        assert_eq!(y_range, (99..100));
+    }
+
+    #[test]
+    fn test_sample_data() {
+        let (mut m, bounds) = build_map_from_file("test-input");
+        pour_over(&mut m, 500, 0);
+        let water_squares = count_water(&m, &bounds);
+        assert_eq!(water_squares, 57);
+        let resting_squares = count_resting_water(&m, &bounds);
+        assert_eq!(resting_squares, 29);
+    }
 }