diff --git a/aoc2019/input/day04.txt b/aoc2019/input/day04.txt
new file mode 100644
index 0000000..6c18726
--- /dev/null
+++ b/aoc2019/input/day04.txt
@@ -0,0 +1 @@
+153517-630395
diff --git a/aoc2019/src/day04.rs b/aoc2019/src/day04.rs
new file mode 100644
index 0000000..0ecc99b
--- /dev/null
+++ b/aoc2019/src/day04.rs
@@ -0,0 +1,79 @@
+use aoc::err;
+use aoc::Result;
+
+const INPUT: &str = include_str!("../input/day04.txt");
+
+pub fn run() -> Result<()> {
+    println!("part 1: {}", part1(INPUT)?);
+
+    Ok(())
+}
+
+fn part1(input: &str) -> Result<usize> {
+    let mut range = input.trim_end().split('-');
+    let begin: usize = range
+        .next()
+        .ok_or_else(|| err!("invalid input: {}", input))?
+        .parse()?;
+    let end: usize = range
+        .next()
+        .ok_or_else(|| err!("invalid input: {}", input))?
+        .parse()?;
+
+    let res = (begin..=end)
+        .map(|n| DigitsIter::new(n).collect::<Vec<_>>())
+        .filter(|digits| digits.windows(2).any(|window| window[0] == window[1]))
+        .filter(|digits| digits.windows(2).all(|window| window[0] <= window[1]))
+        .count();
+
+    Ok(res)
+}
+
+struct DigitsIter {
+    n: usize,
+    div: usize,
+}
+
+impl DigitsIter {
+    fn new(n: usize) -> Self {
+        let mut div = 1;
+        while n >= div * 10 {
+            div *= 10;
+        }
+
+        DigitsIter { n, div }
+    }
+}
+
+impl Iterator for DigitsIter {
+    type Item = usize;
+
+    fn next(&mut self) -> Option<Self::Item> {
+        match self.div {
+            0 => None,
+            _ => {
+                let res = self.n / self.div;
+                self.n %= self.div;
+                self.div /= 10;
+                Some(res)
+            }
+        }
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn part1_provided() {
+        assert_eq!(part1("111111-111111").unwrap(), 1);
+        assert_eq!(part1("223450-223450").unwrap(), 0);
+        assert_eq!(part1("123789-123789").unwrap(), 0);
+    }
+
+    #[test]
+    fn part1_real() {
+        assert_eq!(part1(INPUT).unwrap(), 1729);
+    }
+}
diff --git a/aoc2019/src/lib.rs b/aoc2019/src/lib.rs
index 89345b1..e051fbd 100644
--- a/aoc2019/src/lib.rs
+++ b/aoc2019/src/lib.rs
@@ -1,3 +1,4 @@
 pub mod day01;
 pub mod day02;
 pub mod day03;
+pub mod day04;
diff --git a/aoc2019/src/main.rs b/aoc2019/src/main.rs
index 3685bc6..4fa0ab4 100644
--- a/aoc2019/src/main.rs
+++ b/aoc2019/src/main.rs
@@ -3,9 +3,10 @@ use aoc::Result;
 use aoc2019::day01;
 use aoc2019::day02;
 use aoc2019::day03;
+use aoc2019::day04;
 
 fn main() -> Result<()> {
-    let days: &[fn() -> Result<()>] = &[day01::run, day02::run, day03::run];
+    let days: &[fn() -> Result<()>] = &[day01::run, day02::run, day03::run, day04::run];
 
     aoc::run(days)
 }