【Rust】文字列の繰り返し

文字列の繰り返しは repeat()　を使用する

fn main() {
    let mut difficulty = 4;
    let str: String = "0".to_string();
    let target = str.repeat(difficulty);
    println!("{}", target);
}

$ cargo run
Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.29s
Running `target/debug/crypt`
0000

リファクタリングする。

fn main() {
    let now = time::Instant::now();
    let previous_hash = "b9b9ee9ffc95fa4956b63b6043a99d0a8f04e0e52e687fc1958d3c6dff885f01";
    let mut num = rand::thread_rng().gen_range(0..1000000);
    let mut hash_num = format!("{}{}", previous_hash, num.to_string());
    let mut header = Sha256::digest(hash_num);
    let mut target: String  = (&hex::encode(header)[..4]).to_string();
    
    let mut cnt = 1;
    println!("count: {} {:x}", cnt, header);

    let mut difficulty = 4;
    let str: String = "0".to_string();
    let difficulty_str = str.repeat(difficulty);

    while target != difficulty_str {
        println!("count: {} {:x}", cnt, header);
        num = rand::thread_rng().gen_range(0..1000000);
        hash_num = format!("{}{}", previous_hash, num.to_string());
        header = Sha256::digest(hash_num);
        target = (&hex::encode(header)[..4]).to_string();
        cnt += 1;
    }
    println!("count: {} {:x}", cnt, header);
    println!("{:?}", now.elapsed());
}

$ cargo run
…
count: 60678 0000b49915dc1fbb5fc8660d285674dbbe9e8d06f31fc5040b6044b4fb3da86a
2.082390487s

問題ありません。なるほどね。これで、difficultyの値を自動調整できるようになりました。