struct Block {
    hash: String,
    data: String,
    prev_hash: String,
    nonce: u64,
}

struct Blockchain {
    blocks: Vec<Block>,
}

struct ProofOfWork {
    block: Block,
    target: String,
}

fn validate(&self) -> bool {
      let mut hasher = Sha256::new();
      let data = self.prepare_data(self.block.nonce);
      hasher.update(data);

      let hash: String = hasher
          .finalize()
          .iter()
          .map(|x| format!("{:02x}", x))
          .collect();
      hash.starts_with(&self.target)
  }