3 files changed, 41 insertions, 0 deletions
diff --git a/Cargo.lock b/Cargo.lock
index c0c9382..db20100 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -5,3 +5,12 @@ version = 3
 [[package]]
 name = "advent"
 version = "0.1.0"
+dependencies = [
+ "md5",
+]
+
+[[package]]
+name = "md5"
+version = "0.7.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "490cc448043f947bae3cbee9c203358d62dbee0db12107a74be5c30ccfd09771"
diff --git a/Cargo.toml b/Cargo.toml
index bb0c0e0..40a7112 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -5,3 +5,4 @@ authors = ["Reiner Herrmann <reiner@reiner-h.de>"]
 edition = "2021"
 
 [dependencies]
+md5 = "0.7"
diff --git a/src/bin/day4.rs b/src/bin/day4.rs
new file mode 100644
index 0000000..c6963bb
--- /dev/null
+++ b/src/bin/day4.rs
@@ -0,0 +1,31 @@
+fn main() {
+    let input = "ckczppom";
+    println!("4a: {}", find_hash_count(input, "00000"));
+    println!("4b: {}", find_hash_count(input, "000000"));
+}
+
+fn find_hash_count(input: &str, start: &str) -> u32 {
+    let mut i = 0;
+
+    loop {
+        let text = input.to_string() + &i.to_string();
+        let digest = format!("{:?}", md5::compute(&text));
+
+        if digest.starts_with(start) {
+            return i;
+        }
+
+        i += 1;
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test() {
+        assert_eq!(find_hash_count("abcdef", "00000"), 609043);
+        assert_eq!(find_hash_count("pqrstuv", "00000"), 1048970);
+    }
+}