Day 4, part 1 solved.

day_4/Cargo.toml

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+[package]
+name = "day_4"
+version = "0.1.0"
+authors = ["JPFrancoia <[email protected]>"]
+edition = "2018"
+
+# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
+
+[dependencies]
+md5 = "0.7.0"

day_4/src/main.rs

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+use std::{fs, u32};
+use md5;
+
+
+fn main() {
+    let key = fs::read_to_string("input.txt").unwrap();
+    let key = key.trim();
+
+    println!("Key: {}", key);
+
+    let mut counter = 0;
+
+    for n in 1..u32::MAX {
+        //println!("{}", n);
+        let input = format!("{}{}", key, n);
+        let digest = format!("{:x}", md5::compute(input.as_bytes()));
+
+        ////For debug
+        //if n == 609043 {
+            //println!("{}", digest);
+            //println!("{}", digest.starts_with("00000"));
+            //break
+        //}
+
+        if digest.starts_with("00000") {
+            println!("{}", digest);
+            counter = n;
+            break
+        }
+    }
+
+    println!("Suffix: {}", counter);
+}