1 files changed, 140 insertions, 0 deletions
diff --git a/09/src/main.rs b/09/src/main.rs
new file mode 100644
index 0000000..dd3f769
--- /dev/null
+++ b/09/src/main.rs
@@ -0,0 +1,140 @@
+use advent_lib::prelude::*;
+
+
+fn main() -> Result<()> {
+  let mut args = std::env::args();
+  if args.len() != 2 {
+    eprintln!("Usage: advent input");
+  }
+  let _ = args.next();
+  let filename = args.next().unwrap();
+
+  let input = advent_lib::read_lines_file(&filename)?;
+
+  let mut grid = Vec::new();
+  for line in &input {
+    let mut row = Vec::new();
+    for c in line.chars() {
+      let value: i64 = c.to_digit(10).unwrap() as i64;
+      row.push(value);
+    }
+    grid.push(row);
+  }
+
+  let height = grid.len();
+  let width = grid[0].len();
+
+  let mut sum = 0;
+  let mut low_points: Vec<(usize, usize)> = Vec::new();
+  for y in 0 .. height {
+    for x in 0 .. width {
+      let here: i64 = grid[y][x];
+
+      if x > 0 {
+        let left = grid[y][x - 1];
+        if here >= left {
+          continue;
+        }
+      }
+
+      if x + 1 < width {
+        let right = grid[y][x + 1];
+        if here >= right {
+          continue;
+        }
+      }
+
+      if y > 0 {
+        let up = grid[y - 1][x];
+        if here >= up {
+          continue;
+        }
+      }
+
+      if y + 1 < height {
+        let down = grid[y + 1][x];
+        if here >= down {
+          continue;
+        }
+      }
+
+      let risk = here + 1;
+
+      low_points.push((x, y));
+      sum += risk;
+    }
+  }
+  println!("{}", sum);
+
+  let mut basin_sizes: Vec<i64> = Vec::new();
+  for (x, y) in &low_points {
+    let mut visited_grid = Vec::new();
+    for _ in 0 .. height {
+      let mut visited_row = Vec::new();
+      for _ in 0 .. width {
+        visited_row.push(false);
+      }
+      visited_grid.push(visited_row);
+    }
+
+    let size = basin_size(*x, *y, &grid, &mut visited_grid);
+    basin_sizes.push(size);
+  }
+  basin_sizes.sort();
+
+  let mut result = 1;
+  result *= basin_sizes[basin_sizes.len() - 1];
+  result *= basin_sizes[basin_sizes.len() - 2];
+  result *= basin_sizes[basin_sizes.len() - 3];
+  println!("{}", result);
+
+  Ok(())
+}
+
+
+fn basin_size(x: usize, y: usize, grid: &Vec<Vec<i64>>,
+              mut visited_grid: &mut Vec<Vec<bool>>)
+  -> i64
+{
+  let mut size = 1;
+  visited_grid[y][x] = true;
+
+  let height = grid.len();
+  let width = grid[0].len();
+  let here = grid[y][x];
+
+  if x > 0 {
+    let left_visited = visited_grid[y][x - 1];
+    let left = grid[y][x - 1];
+    if !left_visited && (here < left) && (left != 9) {
+      size += basin_size(x - 1, y, &grid, &mut visited_grid);
+    }
+  }
+
+  if x + 1 < width {
+    let right_visited = visited_grid[y][x + 1];
+    let right = grid[y][x + 1];
+    if !right_visited && (here < right) && (right != 9) {
+      size += basin_size(x + 1, y, &grid, &mut visited_grid);
+    }
+  }
+
+  if y > 0 {
+    let up_visited = visited_grid[y - 1][x];
+    let up = grid[y - 1][x];
+    if !up_visited && (here < up) && (up != 9) {
+      size += basin_size(x, y - 1, &grid, &mut visited_grid);
+    }
+  }
+
+  if y + 1 < height {
+    let down_visited = visited_grid[y + 1][x];
+    let down = grid[y + 1][x];
+    if !down_visited && (here < down) && (down != 9) {
+      size += basin_size(x, y + 1, &grid, &mut visited_grid);
+    }
+  }
+
+  size
+}
+