Possibly wrong implementation of "format_units"

[PaulRBerg]

Description

I was looking to use the format-units function, part of ethers-core::utils. I was expecting the same behaviour as in the canonical ethers.js, but the format_units function behaves differently in ethers-rs.

Actual Behaviour

Take the following Rust code:

use ethers::{prelude::*, utils::format_units};
let eth_in_wei = U256::from_dec_str("1000000000000000000").unwrap();
let eth_in_gwei = format_units(eth_in_wei, 9);
println!("eth_in_gwei: {:?}", eth_in_gwei);

This logs the following:

eth_in_gwei: 111111111111111111

Expected Behaviour

Take the following JavaScript code:

const { ethers } = require("ethers");
let eth_in_wei = "1000000000000000000";
const eth_in_gwei = ethers.utils.formatUnits(eth_in_wei, 9);
console.log({ eth_in_gwei });

This logs the following:

{ eth_in_gwei: '1000000000.0' }

Which is correct, as per the identity 1 eth = 1e9 gwei = 1e18 wei.

Possible Solution

Rewrite the format_units function to divide by 10^(decimals) instead of just decimals.

Environment

Using [email protected]

[gakonst]

This should be fixed in #170 @PaulRBerg!

[sakulstra]

hey just doing my first steps with rust and I think there might still be an issue:

// js
ethers.utils.formatUnits("409030851937682650000",18) ->  "409.03085193768265"

// ethers-rs
format_units(&U256::from_dec_str("409030851937682650000"), 18).to_string(); -> "409"

[x3ccd4828]

@gakonst can you reopen this issue since it's still not correct?

[gakonst]

i think this was addressed in #463?

[x3ccd4828]

I have modified the parse_unit test to show that if you have 18 decimal places some are lost since f64 doesn't have enough bits to hold such a high precision number.

x3ccd4828@617997a

[x3ccd4828]

I can submit a pull request with a fix but it will have to use the rug crate (https://crates.io/crates/rug) to create a float larger than 64bit.

[x3ccd4828]

@gakonst have you had a chance to look at the modified test?

[gakonst]

+1 on the test and fixing it by using rug with default-features = false