diff --git a/src/day19.rs b/src/day19.rs
new file mode 100644
index 0000000..855b637
--- /dev/null
+++ b/src/day19.rs
@@ -0,0 +1,53 @@
+use std::collections::HashSet;
+use aoc_runner_derive::{aoc, aoc_generator};
+
+type Transformation = (String, String);
+
+#[aoc_generator(day19)]
+fn parse(input: &str) -> (Vec<Transformation>, String) {
+    let (a, b) = input.split_once("\n\n").unwrap();
+    let t = a.lines().map(|line| {
+        let (lhs, rhs) = line.split_once(" => ").unwrap();
+        (lhs.to_string(), rhs.to_string())
+    }).collect();
+    (t, b.to_string())
+}
+
+#[aoc(day19, part1)]
+fn part1((transformations, start): &(Vec<Transformation>, String)) -> usize {
+    let all: HashSet<String> = transformations.iter()
+        .flat_map(|(from, to)| {
+            let start_indexes = starts(start, from);
+            start_indexes.into_iter()
+                .map(|si| {
+                    let mut s = start.clone();
+                    s.replace_range(si..si+from.len(), to);
+                    s
+                })
+        }).collect();
+
+    all.len()
+}
+
+fn starts(a: &str, pattern: &str) -> Vec<usize> {
+    let mut start = 0;
+    let mut result = vec![];
+    while let Some(index) = a[start..].find(pattern) {
+        result.push(start + index);
+        start += index + 1;
+    }
+    result
+}
+
+#[cfg(test)]
+mod test {
+    #[test]
+    fn part1() {
+        let input = "H => HO
+H => OH
+O => HH
+
+HOH";
+        assert_eq!(super::part1(& super::parse(input)), 4);
+    }
+}
\ No newline at end of file
diff --git a/src/lib.rs b/src/lib.rs
index 1380dfb..5598cfc 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -11,5 +11,6 @@ mod day15;
 mod day16;
 mod day17;
 mod day18;
+mod day19;
 
 aoc_lib! { year = 2015 }