diff --git a/day12/src/main.rs b/day12/src/main.rs
index e41f1e1..13cbc8a 100644
--- a/day12/src/main.rs
+++ b/day12/src/main.rs
@@ -36,20 +36,123 @@ impl Grid<char> {
 
 #[derive(Debug)]
 struct Pathfinder {
+    current_elevation: char,
+    current_idx: usize,
+    start_idx: usize,
+    end_idx: usize,
     grid: Grid<char>,
     steps: Vec<Direction>,
 }
 
 impl Pathfinder {
     pub fn from_file_str(file_str: &str) -> Self {
-        Pathfinder {
+        let mut pf = Pathfinder {
+            current_elevation: 'a',
+            current_idx: 0,
+            start_idx: 0,
+            end_idx: 0,
             grid: Grid::from_file_str(file_str),
             steps: Vec::new(),
-        }
+        };
+        pf.go_to_start();
+
+        pf
     }
 
     fn find_start_and_end(&self) -> (usize, usize) {
-        (self.grid.find_pos('S').unwrap(), self.grid.find_pos('E').unwrap())
+        (self.start_idx, self.end_idx)
+    }
+
+    fn go_to_start(&mut self) {
+        let start = self.grid.find_pos('S').unwrap();
+        let end = self.grid.find_pos('E').unwrap();
+
+        self.current_idx = start;
+        self.start_idx = start;
+        self.end_idx = end;
+        self.current_elevation = 'a';
+        self.grid.vec[start] = 'a';
+        self.grid.vec[end] = 'z';
+    }
+
+    fn get_index_for_move(&self, from: usize, dir: Direction) -> Option<usize> {
+        let (mut x, mut y) = self.grid.idx_xy(from);
+
+        match dir {
+            Direction::Up => {
+                if y >= 1 {
+                    y -= 1;
+                } else {
+                    return None;
+                }
+            }
+            Direction::Down => {
+                if y < self.grid.num_rows() - 1 {
+                    y += 1;
+                } else {
+                    return None;
+                }
+            }
+            Direction::Left => {
+                if x >= 1 {
+                    x -= 1;
+                } else {
+                    return None;
+                }
+            }
+            Direction::Right => {
+                if x < self.grid.num_cols() - 1 {
+                    x += 1;
+                } else {
+                    return None;
+                }
+            }
+        };
+
+        Some(self.grid.xy_idx(x, y))
+    }
+
+    fn is_valid_move(&self, start: usize, dir: Direction) -> bool {
+        if let Some(end) = self.get_index_for_move(start, dir) {
+            // Is the item within the grid?
+            let start_char = self.grid.get(start);
+            let end_char = self.grid.get(end);
+            if start_char.is_none() || end_char.is_none() {
+                return false;
+            }
+
+            // Is the elevation change 0 or 1?
+            let start_char = *start_char.unwrap();
+            let end_char = *end_char.unwrap();
+            println!("start, end: {}, {}", start_char, end_char);
+            let diff = end_char.to_digit(26).unwrap() - start_char.to_digit(26).unwrap();
+            if diff > 1 || end_char < start_char {
+                return false;
+            }
+
+            let (start_x, start_y) = self.grid.idx_xy(start);
+            let (end_x, end_y) = self.grid.idx_xy(end);
+            let x_diff = usize::abs_diff(end_x,  start_x);
+            let y_diff = usize::abs_diff(end_y, start_y);
+
+            // Have we moved 0 or 1 in a cardinal direction?
+            return match (x_diff, y_diff) {
+                (0, 0) | (0, 1) | (1,0) => true,
+                _ => false,
+            };
+        };
+
+        false
+    }
+
+    fn find_valid_moves(&self, start: usize) -> Vec<usize> {
+        [Direction::Up, Direction::Down, Direction::Left, Direction::Right]
+            .into_iter()
+            .filter(|d| self.is_valid_move(start, *d))
+            .map(|d| self.get_index_for_move(start, d))
+            .filter(|m| m.is_some())
+            .map(|m| m.unwrap())
+            .collect()
     }
 }
 
@@ -75,4 +178,13 @@ mod tests {
 
         assert_eq!((start, end), (0, 21));
     }
+
+    #[test]
+    fn find_valid_moves() {
+        let finder = Pathfinder::from_file_str(get_test_data());
+        let (start, end) = finder.find_start_and_end();
+
+        assert_eq!(finder.find_valid_moves(start), vec![8, 1]);
+        assert_eq!(finder.find_valid_moves(8), vec![16, 9]);
+    }
 }