ethers-uint-rs 0.9.5

Large fixed-size integer arithmetic
Documentation 
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// Copyright 2020 Parity Technologies
//
// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
// option. This file may not be copied, modified, or distributed
// except according to those terms.

#[macro_use]
extern crate ethers_uint_rs;

construct_uint! {
    pub struct U256(4);
}

fn main() {
    // Example modular arithmetic using bigint U256 primitives

    // imagine the field 0..p
    // where the p is defined below
    // (it's a prime!)
    let p = U256::from_dec_str(
        "38873241744847760218045702002058062581688990428170398542849190507947196700873",
    )
    .expect("p to be a good number in the example");

    // then, on this field,
    // (p-1) + (p+1) = 0

    // (p - 1) mod p
    let p_minus_1 = (p - 1) % p;
    // (p + 1) mod p
    let p_plus_1 = (p + 1) % p;
    // ((p - 1) mod p + (p + 1) mod p) mod p
    let sum = (p_minus_1 + p_plus_1) % p;
    assert_eq!(sum, 0.into());

    // on this field,
    // (p-1) + (p-1) = p-2
    let p_minus_1 = (p - 1) % p;
    let sum = (p_minus_1 + p_minus_1) % p;
    assert_eq!(sum, p - 2);

    // on this field,
    // (p-1) * 3 = p-3
    let p_minus_1 = (p - 1) % p;

    // multiplication is a series of additions
    let multiplicator = 3;
    let mul = {
        let mut result = p_minus_1;
        for _ in 0..multiplicator - 1 {
            result = (p_minus_1 + result) % p;
        }
        result
    };

    assert_eq!(mul, p - 3);
}