2015 day 4

2015/Cargo.lock

@@ -3,3 +3,12 @@
 [[package]]
 name = "aoc2015"
 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"

2015/Cargo.toml

@@ -8,3 +8,4 @@ edition = "2018"
 name = "aoc"
 
 [dependencies]
+md5 = "0.7.0"

2015/src/bin/day4.rs

@@ -0,0 +1,30 @@
+use aoc::input;
+
+fn main() {
+    let lines = input::read_lines();
+    let mut key = lines[0].to_string();
+    let key_len = key.len();
+
+    // Part 1
+    let mut n = 0;
+    loop {
+        key.replace_range(key_len.., &n.to_string());
+        let hash = md5::compute(key.as_bytes());
+        if &hash[..2] == [0, 0] && hash[2] < 0x10 {
+            break
+        }
+        n += 1
+    }
+    println!("Found coin with n {}.", n);
+
+    // Part 2
+    loop {
+        key.replace_range(key_len.., &n.to_string());
+        let hash = md5::compute(key.as_bytes());
+        if &hash[..3] == [0, 0, 0] {
+            break
+        }
+        n += 1
+    }
+    println!("Found coin with n {}.", n);
+}