Refactor out MaxPossibleReward, fix staking arithmetic

core/sr-primitives/src/curve.rs

@@ -23,7 +23,9 @@ use core::ops::Sub;
 #[derive(PartialEq, Eq, primitives::RuntimeDebug)]
 pub struct PiecewiseLinear<'a> {
 	/// Array of points. Must be in order from the lowest abscissas to the highest.
-	pub points: &'a [(Perbill, Perbill)]
+	pub points: &'a [(Perbill, Perbill)],
+	/// The maximum value that can be returned.
+	pub maximum: Perbill,
 }
 
 fn abs_sub<N: Ord + Sub<Output=N> + Clone>(a: N, b: N) -> N where {
@@ -135,7 +137,8 @@ fn test_calculate_for_fraction_times_denominator() {
 			(Perbill::from_parts(0_000_000_000), Perbill::from_parts(0_500_000_000)),
 			(Perbill::from_parts(0_500_000_000), Perbill::from_parts(1_000_000_000)),
 			(Perbill::from_parts(1_000_000_000), Perbill::from_parts(0_000_000_000)),
-		]
+		],
+		maximum: Perbill::from_parts(1_000_000_000),
 	};
 
 	pub fn formal_calculate_for_fraction_times_denominator(n: u64, d: u64) -> u64 {

node/runtime/src/lib.rs

@@ -241,7 +241,7 @@ impl session::historical::Trait for Runtime {
 srml_staking_reward_curve::build! {
 	const REWARD_CURVE: PiecewiseLinear<'static> = curve!(
 		min_inflation: 0_025_000,
-		max_inflation: 0_100_000,   // 10% - must be equal to MaxReward below.
+		max_inflation: 0_100_000,
 		ideal_stake: 0_500_000,
 		falloff: 0_050_000,
 		max_piece_count: 40,
@@ -253,8 +253,6 @@ parameter_types! {
 	pub const SessionsPerEra: sr_staking_primitives::SessionIndex = 6;
 	pub const BondingDuration: staking::EraIndex = 24 * 28;
 	pub const RewardCurve: &'static PiecewiseLinear<'static> = &REWARD_CURVE;
-	pub const MaxReward: Perbill = Perbill::from_percent(10);
-	// ^^^ 10% - must be equal to max_inflation, above.
 }
 
 impl staking::Trait for Runtime {
@@ -269,7 +267,6 @@ impl staking::Trait for Runtime {
 	type BondingDuration = BondingDuration;
 	type SessionInterface = Self;
 	type RewardCurve = RewardCurve;
-	type MaxPossibleReward = MaxReward;
 }
 
 parameter_types! {

srml/staking/reward-curve/src/lib.rs

@@ -323,6 +323,14 @@ fn compute_points(input: &INposInput) -> Vec<(u32, u32)> {
 fn generate_piecewise_linear(points: Vec<(u32, u32)>) -> TokenStream2 {
 	let mut points_tokens = quote!();
 
+	let max = points.iter()
+		.map(|&(_, x)| x)
+		.max()
+		.unwrap_or(0)
+		.checked_mul(1_000)
+		// clip at 1.0 for sanity only since it'll panic later if too high.
+		.unwrap_or(1_000_000_000);
+
 	for (x, y) in points {
 		let error = || panic!(format!(
 			"Generated reward curve approximation doesn't fit into [0, 1] -> [0, 1] \
@@ -346,6 +354,7 @@ fn generate_piecewise_linear(points: Vec<(u32, u32)>) -> TokenStream2 {
 	quote!(
 		_sr_primitives::curve::PiecewiseLinear::<'static> {
 			points: & [ #points_tokens ],
+			maximum: _sr_primitives::Perbill::from_parts(#max),
 		}
 	)
 }

srml/staking/src/inflation.rs

@@ -32,13 +32,17 @@ pub fn compute_total_payout<N>(
 	npos_token_staked: N,
 	total_tokens: N,
 	era_duration: u64
-) -> N where N: SimpleArithmetic + Clone
-{
+) -> (N, N) where N: SimpleArithmetic + Clone {
 	// Milliseconds per year for the Julian year (365.25 days).
 	const MILLISECONDS_PER_YEAR: u64 = 1000 * 3600 * 24 * 36525 / 100;
 
-	Perbill::from_rational_approximation(era_duration as u64, MILLISECONDS_PER_YEAR)
-		* yearly_inflation.calculate_for_fraction_times_denominator(npos_token_staked, total_tokens)
+	let portion = Perbill::from_rational_approximation(era_duration as u64, MILLISECONDS_PER_YEAR);
+	let payout = portion * yearly_inflation.calculate_for_fraction_times_denominator(
+		npos_token_staked,
+		total_tokens.clone(),
+	);
+	let maximum = portion * (yearly_inflation.maximum * total_tokens);
+	(payout, maximum)
 }
 
 #[cfg(test)]
@@ -59,26 +63,31 @@ mod test {
 	#[test]
 	fn npos_curve_is_sensible() {
 		const YEAR: u64 = 365 * 24 * 60 * 60 * 1000;
+
+		// check maximum inflation.
+		// not 10_000 due to rounding error.
+		assert_eq!(super::compute_total_payout(&I_NPOS, 0, 100_000u64, YEAR).1, 9_993);
+
 		//super::I_NPOS.calculate_for_fraction_times_denominator(25, 100)
-		assert_eq!(super::compute_total_payout(&I_NPOS, 0, 100_000u64, YEAR), 2_498);
-		assert_eq!(super::compute_total_payout(&I_NPOS, 5_000, 100_000u64, YEAR), 3_248);
-		assert_eq!(super::compute_total_payout(&I_NPOS, 25_000, 100_000u64, YEAR), 6_246);
-		assert_eq!(super::compute_total_payout(&I_NPOS, 40_000, 100_000u64, YEAR), 8_494);
-		assert_eq!(super::compute_total_payout(&I_NPOS, 50_000, 100_000u64, YEAR), 9_993);
-		assert_eq!(super::compute_total_payout(&I_NPOS, 60_000, 100_000u64, YEAR), 4_379);
-		assert_eq!(super::compute_total_payout(&I_NPOS, 75_000, 100_000u64, YEAR), 2_733);
-		assert_eq!(super::compute_total_payout(&I_NPOS, 95_000, 100_000u64, YEAR), 2_513);
-		assert_eq!(super::compute_total_payout(&I_NPOS, 100_000, 100_000u64, YEAR), 2_505);
+		assert_eq!(super::compute_total_payout(&I_NPOS, 0, 100_000u64, YEAR).0, 2_498);
+		assert_eq!(super::compute_total_payout(&I_NPOS, 5_000, 100_000u64, YEAR).0, 3_248);
+		assert_eq!(super::compute_total_payout(&I_NPOS, 25_000, 100_000u64, YEAR).0, 6_246);
+		assert_eq!(super::compute_total_payout(&I_NPOS, 40_000, 100_000u64, YEAR).0, 8_494);
+		assert_eq!(super::compute_total_payout(&I_NPOS, 50_000, 100_000u64, YEAR).0, 9_993);
+		assert_eq!(super::compute_total_payout(&I_NPOS, 60_000, 100_000u64, YEAR).0, 4_379);
+		assert_eq!(super::compute_total_payout(&I_NPOS, 75_000, 100_000u64, YEAR).0, 2_733);
+		assert_eq!(super::compute_total_payout(&I_NPOS, 95_000, 100_000u64, YEAR).0, 2_513);
+		assert_eq!(super::compute_total_payout(&I_NPOS, 100_000, 100_000u64, YEAR).0, 2_505);
 
 		const DAY: u64 = 24 * 60 * 60 * 1000;
-		assert_eq!(super::compute_total_payout(&I_NPOS, 25_000, 100_000u64, DAY), 17);
-		assert_eq!(super::compute_total_payout(&I_NPOS, 50_000, 100_000u64, DAY), 27);
-		assert_eq!(super::compute_total_payout(&I_NPOS, 75_000, 100_000u64, DAY), 7);
+		assert_eq!(super::compute_total_payout(&I_NPOS, 25_000, 100_000u64, DAY).0, 17);
+		assert_eq!(super::compute_total_payout(&I_NPOS, 50_000, 100_000u64, DAY).0, 27);
+		assert_eq!(super::compute_total_payout(&I_NPOS, 75_000, 100_000u64, DAY).0, 7);
 
 		const SIX_HOURS: u64 = 6 * 60 * 60 * 1000;
-		assert_eq!(super::compute_total_payout(&I_NPOS, 25_000, 100_000u64, SIX_HOURS), 4);
-		assert_eq!(super::compute_total_payout(&I_NPOS, 50_000, 100_000u64, SIX_HOURS), 7);
-		assert_eq!(super::compute_total_payout(&I_NPOS, 75_000, 100_000u64, SIX_HOURS), 2);
+		assert_eq!(super::compute_total_payout(&I_NPOS, 25_000, 100_000u64, SIX_HOURS).0, 4);
+		assert_eq!(super::compute_total_payout(&I_NPOS, 50_000, 100_000u64, SIX_HOURS).0, 7);
+		assert_eq!(super::compute_total_payout(&I_NPOS, 75_000, 100_000u64, SIX_HOURS).0, 2);
 
 		const HOUR: u64 = 60 * 60 * 1000;
 		assert_eq!(
@@ -87,7 +96,7 @@ mod test {
 				2_500_000_000_000_000_000_000_000_000u128,
 				5_000_000_000_000_000_000_000_000_000u128,
 				HOUR
-			),
+			).0,
 			57_038_500_000_000_000_000_000
 		);
 	}

srml/staking/src/lib.rs

@@ -524,10 +524,6 @@ pub trait Trait: system::Trait {
 
 	/// The NPoS reward curve to use.
 	type RewardCurve: Get<&'static PiecewiseLinear<'static>>;
-
-	/// The maximum possible reward (in proportion of total issued tokens) that can be paid in one
-	/// reward cycle.
-	type MaxPossibleReward: Get<Perbill>;
 }
 
 /// Mode of era-forcing.
@@ -1203,7 +1199,7 @@ impl<T: Trait> Module<T> {
 			let validator_len: BalanceOf<T> = (validators.len() as u32).into();
 			let total_rewarded_stake = Self::slot_stake() * validator_len;
 
-			let total_payout = inflation::compute_total_payout(
+			let (total_payout, max_payout) = inflation::compute_total_payout(
 				&T::RewardCurve::get(),
 				total_rewarded_stake.clone(),
 				T::Currency::total_issuance(),
@@ -1220,16 +1216,14 @@ impl<T: Trait> Module<T> {
 				}
 			}
 
-			let total_reward = total_imbalance.peek();
-			// assert!(total_reward <= total_payout)
-
-			let max_reward = T::MaxPossibleReward::get() * T::Currency::total_issuance();
-			let rest_reward = max_reward.saturating_sub(total_reward);
+			// assert!(total_imbalance.peek() == total_payout)
+			let total_payout = total_imbalance.peek();
 
-			Self::deposit_event(RawEvent::Reward(total_reward, rest_reward));
+			let rest = max_payout.saturating_sub(total_payout);
+			Self::deposit_event(RawEvent::Reward(total_payout, rest));
 
 			T::Reward::on_unbalanced(total_imbalance);
-			T::RewardRemainder::on_unbalanced(T::Currency::issue(rest_reward));
+			T::RewardRemainder::on_unbalanced(T::Currency::issue(rest));
 		}
 
 		// Increment current era.

srml/staking/src/mock.rs

@@ -192,7 +192,6 @@ parameter_types! {
 	pub const SessionsPerEra: SessionIndex = 3;
 	pub const BondingDuration: EraIndex = 3;
 	pub const RewardCurve: &'static PiecewiseLinear<'static> = &I_NPOS;
-	pub const MaxReward: Perbill = Perbill::from_percent(10);
 }
 impl Trait for Test {
 	type Currency = balances::Module<Self>;
@@ -206,7 +205,6 @@ impl Trait for Test {
 	type BondingDuration = BondingDuration;
 	type SessionInterface = Self;
 	type RewardCurve = RewardCurve;
-	type MaxPossibleReward = MaxReward;
 }
 
 pub struct ExtBuilder {
@@ -434,14 +432,12 @@ pub fn start_era(era_index: EraIndex) {
 }
 
 pub fn current_total_payout_for_duration(duration: u64) -> u64 {
-	let res = inflation::compute_total_payout(
+	inflation::compute_total_payout(
 		<Test as Trait>::RewardCurve::get(),
 		<Module<Test>>::slot_stake() * 2,
 		Balances::total_issuance(),
 		duration,
-	);
-
-	res
+	).0
 }
 
 pub fn reward_all_elected() {