diff --git a/src/day10/example.txt b/src/day10/example.txt
new file mode 100644
index 0000000..cada9b3
--- /dev/null
+++ b/src/day10/example.txt
@@ -0,0 +1,8 @@
+89010123
+78121874
+87430965
+96549874
+45678903
+32019012
+01329801
+10456732
diff --git a/src/day10/input.txt b/src/day10/input.txt
new file mode 100644
index 0000000..ff85d75
--- /dev/null
+++ b/src/day10/input.txt
@@ -0,0 +1,52 @@
+0145677654325490845691012345621876560340100123012398
+1238988998216781932782121234010983401259230034563387
+4323589867205432801543210945125672378768341459654456
+1014678454102301765456901876034561769897652368776301
+6765521543201512350307872962120120850785677879789210
+0896430439810487461217965871298438941454980968654321
+1765012126723596543478014560167569032363211457256762
+2872121035434654302569123441455678121071102340145895
+3961230345985783211098710332334599432980098743232434
+4550145456676890100167000146721087641001217650141325
+9649056765489910651254121035890014550732309654210016
+8738769894676328762343236124301123669845498763227807
+7129787923565439651456347833210008778996787120156998
+1013496014567010340987656944782119211087898031343210
+1012345002198321232876501855693024302332196541034101
+1234567123023450901965432761054435678445087670123256
+0144898454910767845670301622169546589556764581214347
+4343732367867893034981243213678696765698873294309838
+1234351078754702121893214504589787894780987101456789
+0943765469843213210721303698921009683211074560167678
+7856874348765434785630410787652118701202983076298987
+6765989219210125698540521236543025654343892189347567
+5667867804321087582101638347432134509856782345456498
+1054876965691296443492789858901213212790101276012387
+2123965476780125356783498767654300103685230983401456
+3278954587821034219870567658967876234576541092560845
+2567543296930761006721498743478965987655612451076921
+1056230145945852345012345412661234554567803367987830
+2340167034876945123211296401780109693069954298756101
+7887658123985231034500787345691218782178769110343232
+6992349032100112985011216217885011071078978021232349
+5801239844301101676720105606976522362567987876541458
+4321023765432432369838234745983439453454376965430167
+3087610321396565458949549836112378321043105302301298
+2198565410187076327658678921001065439652234211017657
+3233478903216189014547664567632104508701230322928943
+4542369894103273223014523498549812012349821498834012
+7651423765764784132123210210038763676256734567765423
+8960314349845695041012396342121054985109875498656701
+4871005256736786780169487653434873014018766787567892
+5654196149821677893278565694565963223321051096450943
+6743287032120563034567684787696954101438142345321056
+7899180129061432125675893256787845698589233239885469
+3458098938778743210986710143876034787670132178596378
+2167347845609654331235430782932128236101056017687267
+6056256741012103220543521691047659145692347012570167
+7890165432343210110652434598798578036785498743456898
+6784567876758701328701223123623457629876901234347107
+5413218965869232499899810034510256510267892301298256
+4303409954978149581234745218760105100126765410789340
+3212567823019058670365634309451234981237898323870121
+4321016012108767621256565678321015676546767654965432
diff --git a/src/day10/mod.rs b/src/day10/mod.rs
new file mode 100644
index 0000000..e6180c5
--- /dev/null
+++ b/src/day10/mod.rs
@@ -0,0 +1,118 @@
+use std::vec;
+
+struct Grid<'a> {
+    string: &'a str,
+    width: usize,
+}
+
+impl<'a> Grid<'a> {
+    fn new(string: &'a str) -> Self {
+        let width = string.find('\n').unwrap();
+        Self { string, width }
+    }
+
+    fn width(&self) -> usize {
+        self.width
+    }
+
+    fn height(&self) -> usize {
+        self.string.len() / (self.width() + 1)
+    }
+
+    fn table<T: Clone>(&self, fill: T) -> Table<T> {
+        Table::new(self.width(), self.height(), fill)
+    }
+
+    fn index(&self, i: usize) -> (isize, isize) {
+        let x = (i % (self.width() + 1)) as isize;
+        let y = (i / (self.width() + 1)) as isize;
+        (x, y)
+    }
+
+    fn get(&self, x: isize, y: isize) -> Option<&str> {
+        let i: usize = y.try_into().ok()?;
+        let j: usize = x.try_into().ok()?;
+        if j >= self.width() {
+            return None;
+        }
+        let k = i * (self.width() + 1) + j;
+        self.string.get(k..k + 1)
+    }
+}
+
+struct Table<T> {
+    width: usize,
+    elems: Vec<T>,
+}
+
+impl<T: Clone> Table<T> {
+    fn new(width: usize, height: usize, fill: T) -> Self {
+        let elems = vec![fill; width * height];
+        Self { width, elems }
+    }
+}
+
+impl<T> Table<T> {
+    fn get_mut(&mut self, x: isize, y: isize) -> Option<&mut T> {
+        let i: usize = y.try_into().ok()?;
+        let j: usize = x.try_into().ok()?;
+        if j >= self.width {
+            return None;
+        }
+        self.elems.get_mut(i * self.width + j)
+    }
+}
+
+pub fn puzzle1(input: &str) -> usize {
+    let grid = Grid::new(input);
+    input
+        .chars()
+        .enumerate()
+        .filter(|&(_, c)| c == '0')
+        .map(|(i, _)| {
+            let mut visited = grid.table(false);
+            let mut stack = vec![(0, grid.index(i))];
+            while let Some((n, (x, y))) = stack.pop() {
+                if grid.get(x, y).and_then(|s| s.parse().ok()) != Some(n) {
+                    continue;
+                }
+                let here = visited.get_mut(x, y).unwrap();
+                if *here {
+                    continue;
+                }
+                *here = true;
+                stack.push((n + 1, (x + 1, y)));
+                stack.push((n + 1, (x, y - 1)));
+                stack.push((n + 1, (x - 1, y)));
+                stack.push((n + 1, (x, y + 1)));
+            }
+            let mut score = 0;
+            for y in 0..(grid.height() as isize) {
+                for x in 0..(grid.width() as isize) {
+                    if grid.get(x, y) == Some("9") && *visited.get_mut(x, y).unwrap() {
+                        score += 1;
+                    }
+                }
+            }
+            score
+        })
+        .sum()
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    const EXAMPLE: &str = include_str!("example.txt");
+    const INPUT: &str = include_str!("input.txt");
+
+    #[test]
+    fn test_puzzle1_example() {
+        assert_eq!(puzzle1(EXAMPLE), 36);
+    }
+
+    #[test]
+    fn test_puzzle1_input() {
+        assert_eq!(puzzle1(INPUT), 667);
+    }
+}
diff --git a/src/main.rs b/src/main.rs
index 7cc69e9..c635358 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -9,6 +9,7 @@ mod day06;
 mod day07;
 mod day08;
 mod day09;
+mod day10;
 
 use std::{env, fs};
 
@@ -45,6 +46,8 @@ fn main() {
         (9, 1) => day09::puzzle1(&input).to_string(),
         (9, 2) => day09::puzzle2(&input).to_string(),
 
+        (10, 1) => day10::puzzle1(&input).to_string(),
+
         _ => panic!("no puzzle {} for day {}", puzzle, day),
     };
     println!("{}", answer.trim_end());