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    //! mCaptcha is a proof of work based Denaial-of-Service attack protection system.
    //! This is is a WASM library that you can embed in your frontend code to protect your
    //! service.
    //!
    //! A commercial managed solution is in the works but I'd much rather prefer
    //! folks host their own instances as it will make the more decentralized and free.
    //!
    //! ## Workflow:
    //! mCaptcha workflow in the frontend is simple.
    //! 1. Call service to get a proof of work(PoW) configuration
    //! 2. Call into mCaptcha to get PoW
    //! 3. Send PoW to your service

    use wasm_bindgen::prelude::*;

    // When the `wee_alloc` feature is enabled, use `wee_alloc` as the global
    // allocator.
    #[cfg(feature = "wee_alloc")]
    #[global_allocator]
    static ALLOC: wee_alloc::WeeAlloc = wee_alloc::WeeAlloc::INIT;

    use pow_sha256::PoW;

    #[wasm_bindgen]
    pub fn gen_pow(difficulty_factor: u32, secret: String) -> String {
        let difficulty = u128::max_value() - u128::max_value() / difficulty_factor as u128;
        let a = PoW::prove_work(&secret.as_bytes().to_vec(), difficulty).unwrap();
        let payload = serde_json::to_string(&a).unwrap();
        payload
    }

    #[cfg(test)]
    mod tests {
        use super::*;

        #[test]
        fn it_works() {
            let payload = gen_pow(500, "MFsqLMZId629Dh2hrtux2Qdn3gBzCaSt".into());
            assert_eq!("{\"nonce\":312,\"result\":\"340175381422106372296624206295814425082\",\"_spook\":null}",    &payload );

            let payload = gen_pow(1_000, "MFsqLMZId629Dh2hrtux2Qdn3gBzCaSt".into());
            assert_eq!("{\"nonce\":312,\"result\":\"340175381422106372296624206295814425082\",\"_spook\":null}", &payload);

            let payload = gen_pow(2_000, "MFsqLMZId629Dh2hrtux2Qdn3gBzCaSt".into());
            assert_eq!(&payload, "{\"nonce\":312,\"result\":\"340175381422106372296624206295814425082\",\"_spook\":null}");

            let payload = gen_pow(100_000, "MFsqLMZId629Dh2hrtux2Qdn3gBzCaSt".into());
            assert_eq!(&payload, "{\"nonce\":59930,\"result\":\"340281433562218714678373578791487113813\",\"_spook\":null}");

            let payload = gen_pow(1_000_000, "MFsqLMZId629Dh2hrtux2Qdn3gBzCaSt".into());

            assert_eq!(&payload,"{\"nonce\":1902451,\"result\":\"340282308726676882310449308394036800665\",\"_spook\":null}");
        }
    }