Refactor CurrencyToVote (#6896)

* Refactor CurrencyToVote to avoid calls to total_issuance.

* Update frame/support/src/traits.rs

Co-authored-by: Guillaume Thiolliere <gui.thiolliere@gmail.com>

* Some grumbles

* Fix last grumbles.

* Fix comment

* Final fix

Co-authored-by: Guillaume Thiolliere <gui.thiolliere@gmail.com>

substrate/frame/support/src/traits.rs

@@ -23,9 +23,11 @@ use sp_std::{prelude::*, result, marker::PhantomData, ops::Div, fmt::Debug};
 use codec::{FullCodec, Codec, Encode, Decode, EncodeLike};
 use sp_core::u32_trait::Value as U32;
 use sp_runtime::{
-	RuntimeDebug, ConsensusEngineId, DispatchResult, DispatchError, traits::{
+	RuntimeDebug, ConsensusEngineId, DispatchResult, DispatchError,
+	traits::{
 		MaybeSerializeDeserialize, AtLeast32Bit, Saturating, TrailingZeroInput, Bounded, Zero,
-		BadOrigin, AtLeast32BitUnsigned
+		BadOrigin, AtLeast32BitUnsigned, UniqueSaturatedFrom, UniqueSaturatedInto,
+		SaturatedConversion,
 	},
 };
 use crate::dispatch::Parameter;
@@ -1710,6 +1712,73 @@ pub trait Instance: 'static {
     const PREFIX: &'static str ;
 }
 
+/// A trait similar to `Convert` to convert values from `B` an abstract balance type
+/// into u64 and back from u128. (This conversion is used in election and other places where complex
+/// calculation over balance type is needed)
+///
+/// Total issuance of the currency is passed in, but an implementation of this trait may or may not
+/// use it.
+///
+/// # WARNING
+///
+/// the total issuance being passed in implies that the implementation must be aware of the fact
+/// that its values can affect the outcome. This implies that if the vote value is dependent on the
+/// total issuance, it should never ber written to storage for later re-use.
+pub trait CurrencyToVote<B> {
+	/// Convert balance to u64.
+	fn to_vote(value: B, issuance: B) -> u64;
+
+	/// Convert u128 to balance.
+	fn to_currency(value: u128, issuance: B) -> B;
+}
+
+/// An implementation of `CurrencyToVote` tailored for chain's that have a balance type of u128.
+///
+/// The factor is the `(total_issuance / u64::max()).max(1)`, represented as u64. Let's look at the
+/// important cases:
+///
+/// If the chain's total issuance is less than u64::max(), this will always be 1, which means that
+/// the factor will not have any effect. In this case, any account's balance is also less. Thus,
+/// both of the conversions are basically an `as`; Any balance can fit in u64.
+///
+/// If the chain's total issuance is more than 2*u64::max(), then a factor might be multiplied and
+/// divided upon conversion.
+pub struct U128CurrencyToVote;
+
+impl U128CurrencyToVote {
+	fn factor(issuance: u128) -> u128 {
+		(issuance / u64::max_value() as u128).max(1)
+	}
+}
+
+impl CurrencyToVote<u128> for U128CurrencyToVote {
+	fn to_vote(value: u128, issuance: u128) -> u64 {
+		(value / Self::factor(issuance)).saturated_into()
+	}
+
+	fn to_currency(value: u128, issuance: u128) -> u128 {
+		value.saturating_mul(Self::factor(issuance))
+	}
+}
+
+
+/// A naive implementation of `CurrencyConvert` that simply saturates all conversions.
+///
+/// # Warning
+///
+/// This is designed to be used mostly for testing. Use with care, and think about the consequences.
+pub struct SaturatingCurrencyToVote;
+
+impl<B: UniqueSaturatedInto<u64> + UniqueSaturatedFrom<u128>> CurrencyToVote<B> for SaturatingCurrencyToVote {
+	fn to_vote(value: B, _: B) -> u64 {
+		value.unique_saturated_into()
+	}
+
+	fn to_currency(value: u128, _: B) -> B {
+		B::unique_saturated_from(value)
+	}
+}
+
 #[cfg(test)]
 mod tests {
 	use super::*;