`CurrencyToVote` to `PowerToVote`

[aurexav]

not finish in #94

[aurexav]

will be fix in #218

[aurexav]

impl PowerToVoteHandler {
	fn factor() -> Power {
		(TOTAL_POWER / u64::max_value() as Power).max(1)
	}
}

impl Convert<Power, u64> for PowerToVoteHandler {
	fn convert(x: Power) -> u64 {
		(x / Self::factor()) as u64
	}
}

impl Convert<u128, Power> for PowerToVoteHandler {
	fn convert(x: u128) -> Power {
		x * Self::factor()
	}
}

TOTAL_POWER is 1_000_000_000.
So, Power : Votes = 1 : 1.
Actually, Power is Votes.

1. remove the CurrencyToVoteHandler
2. new CurrencyToPowerHandler

And we can refactoring the Power from u128 to u32.

const TOTAL_POWER: u128 = 1_000_000_000;
let u32_max = u32::max_value();
assert_eq!(u32_max, 4_294_967_296);
assert!((u32_max as u128) > TOTAL_POWER);

[aurexav]

@hackfisher,

Handler Design:

// darwinia-support lib.rs
pub trait CurrencyPowerConvertor<Currency, Power> {
	fn to_currency(power: Power, pool: Currency) -> Currency;

	fn to_power(currency: Currency, pool: Currency) -> Power;
}

// runtime impl.rs
use crate::TOTAL_POWER;
use node_primitives::{Balance, Power};

pub struct CurrencyPowerHandler;

impl<Currency> CurrencyPowerConvertor<Currency, Power> for CurrencyPowerHandler 
where
	Currency: Bounded + SimpleArithmetic + TryInto<Balance> + TryFrom<Balance>,
{
	fn to_currency(power: Power, pool: Currency) -> Currency {
		Currency::saturated_from(
			Perquintill::from_rational_approximation(
				power,
				TOTAL_POWER / 2,
			) * pool.saturated_into::<Balance>().max(1)
		)
	}
	
	fn to_power(currency: Currency, pool: Currency) -> Power {
		Perquintill::from_rational_approximation(
			currency.saturated_into::<Balance>(),
			pool.saturated_into::<Balance>().max(1),
		) * (TOTAL_POWER / 2)
	}
}

Pool is required to calculate Power, can not impl Convert, we have to modify the phragmen module.

[hackfisher]

@hackfisher,

Handler Design:

// darwinia-support lib.rs
pub trait CurrencyPowerConvertor<Currency, Power> {
	fn to_currency(power: Power, pool: Currency) -> Currency;

	fn to_power(currency: Currency, pool: Currency) -> Power;
}

// runtime impl.rs
use crate::TOTAL_POWER;
use node_primitives::{Balance, Power};

pub struct CurrencyPowerHandler;

impl<Currency> CurrencyPowerConvertor<Currency, Power> for CurrencyPowerHandler 
where
	Currency: Bounded + SimpleArithmetic + TryInto<Balance> + TryFrom<Balance>,
{
	fn to_currency(power: Power, pool: Currency) -> Currency {
		Currency::saturated_from(
			Perquintill::from_rational_approximation(
				power,
				TOTAL_POWER / 2,
			) * pool.saturated_into::<Balance>().max(1)
		)
	}
	
	fn to_power(currency: Currency, pool: Currency) -> Power {
		Perquintill::from_rational_approximation(
			currency.saturated_into::<Balance>(),
			pool.saturated_into::<Balance>().max(1),
		) * (TOTAL_POWER / 2)
	}
}

Pool is required to calculate Power, can not impl Convert, we have to modify the phragmen module.

This design makes sense.

[aurexav]

deps on #262,

if #262 fixed, we can ignore the converter

---

The converter is use for preventing overflow in phragmen.
I suggest to keep the currency to power function in staking, staking logic should not in runtime.