1
  2
  3
  4
  5
  6
  7
  8
  9
 10
 11
 12
 13
 14
 15
 16
 17
 18
 19
 20
 21
 22
 23
 24
 25
 26
 27
 28
 29
 30
 31
 32
 33
 34
 35
 36
 37
 38
 39
 40
 41
 42
 43
 44
 45
 46
 47
 48
 49
 50
 51
 52
 53
 54
 55
 56
 57
 58
 59
 60
 61
 62
 63
 64
 65
 66
 67
 68
 69
 70
 71
 72
 73
 74
 75
 76
 77
 78
 79
 80
 81
 82
 83
 84
 85
 86
 87
 88
 89
 90
 91
 92
 93
 94
 95
 96
 97
 98
 99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
// This file is part of Substrate.

// Copyright (C) 2019-2021 Parity Technologies (UK) Ltd.
// SPDX-License-Identifier: Apache-2.0

// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
// 	http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

//! The imbalance trait type and its associates, which handles keeps everything adding up properly
//! with unbalanced operations.

use crate::traits::misc::{SameOrOther, TryDrop};
use sp_runtime::traits::Saturating;
use sp_std::ops::Div;

mod on_unbalanced;
mod signed_imbalance;
mod split_two_ways;
pub use on_unbalanced::OnUnbalanced;
pub use signed_imbalance::SignedImbalance;
pub use split_two_ways::SplitTwoWays;

/// A trait for a not-quite Linear Type that tracks an imbalance.
///
/// Functions that alter account balances return an object of this trait to
/// express how much account balances have been altered in aggregate. If
/// dropped, the currency system will take some default steps to deal with
/// the imbalance (`balances` module simply reduces or increases its
/// total issuance). Your module should generally handle it in some way,
/// good practice is to do so in a configurable manner using an
/// `OnUnbalanced` type for each situation in which your module needs to
/// handle an imbalance.
///
/// Imbalances can either be Positive (funds were added somewhere without
/// being subtracted elsewhere - e.g. a reward) or Negative (funds deducted
/// somewhere without an equal and opposite addition - e.g. a slash or
/// system fee payment).
///
/// Since they are unsigned, the actual type is always Positive or Negative.
/// The trait makes no distinction except to define the `Opposite` type.
///
/// New instances of zero value can be created (`zero`) and destroyed
/// (`drop_zero`).
///
/// Existing instances can be `split` and merged either consuming `self` with
/// `merge` or mutating `self` with `subsume`. If the target is an `Option`,
/// then `maybe_merge` and `maybe_subsume` might work better. Instances can
/// also be `offset` with an `Opposite` that is less than or equal to in value.
///
/// You can always retrieve the raw balance value using `peek`.
#[must_use]
pub trait Imbalance<Balance>: Sized + TryDrop + Default {
	/// The oppositely imbalanced type. They come in pairs.
	type Opposite: Imbalance<Balance>;

	/// The zero imbalance. Can be destroyed with `drop_zero`.
	fn zero() -> Self;

	/// Drop an instance cleanly. Only works if its `self.value()` is zero.
	fn drop_zero(self) -> Result<(), Self>;

	/// Consume `self` and return two independent instances; the first
	/// is guaranteed to be at most `amount` and the second will be the remainder.
	fn split(self, amount: Balance) -> (Self, Self);

	/// Consume `self` and return two independent instances; the amounts returned will be in
	/// approximately the same ratio as `first`:`second`.
	///
	/// NOTE: This requires up to `first + second` room for a multiply, and `first + second` should
	/// fit into a `u32`. Overflow will safely saturate in both cases.
	fn ration(self, first: u32, second: u32) -> (Self, Self)
	where
		Balance: From<u32> + Saturating + Div<Output = Balance>,
	{
		let total: u32 = first.saturating_add(second);
		if total == 0 {
			return (Self::zero(), Self::zero())
		}
		let amount1 = self.peek().saturating_mul(first.into()) / total.into();
		self.split(amount1)
	}

	/// Consume self and add its two components, defined by the first component's balance,
	/// element-wise to two pre-existing Imbalances.
	///
	/// A convenient replacement for `split` and `merge`.
	fn split_merge(self, amount: Balance, others: (Self, Self)) -> (Self, Self) {
		let (a, b) = self.split(amount);
		(a.merge(others.0), b.merge(others.1))
	}

	/// Consume self and add its two components, defined by the ratio `first`:`second`,
	/// element-wise to two pre-existing Imbalances.
	///
	/// A convenient replacement for `split` and `merge`.
	fn ration_merge(self, first: u32, second: u32, others: (Self, Self)) -> (Self, Self)
	where
		Balance: From<u32> + Saturating + Div<Output = Balance>,
	{
		let (a, b) = self.ration(first, second);
		(a.merge(others.0), b.merge(others.1))
	}

	/// Consume self and add its two components, defined by the first component's balance,
	/// element-wise into two pre-existing Imbalance refs.
	///
	/// A convenient replacement for `split` and `subsume`.
	fn split_merge_into(self, amount: Balance, others: &mut (Self, Self)) {
		let (a, b) = self.split(amount);
		others.0.subsume(a);
		others.1.subsume(b);
	}

	/// Consume self and add its two components, defined by the ratio `first`:`second`,
	/// element-wise to two pre-existing Imbalances.
	///
	/// A convenient replacement for `split` and `merge`.
	fn ration_merge_into(self, first: u32, second: u32, others: &mut (Self, Self))
	where
		Balance: From<u32> + Saturating + Div<Output = Balance>,
	{
		let (a, b) = self.ration(first, second);
		others.0.subsume(a);
		others.1.subsume(b);
	}

	/// Consume `self` and an `other` to return a new instance that combines
	/// both.
	fn merge(self, other: Self) -> Self;

	/// Consume self to mutate `other` so that it combines both. Just like `subsume`, only with
	/// reversed arguments.
	fn merge_into(self, other: &mut Self) {
		other.subsume(self)
	}

	/// Consume `self` and maybe an `other` to return a new instance that combines
	/// both.
	fn maybe_merge(self, other: Option<Self>) -> Self {
		if let Some(o) = other {
			self.merge(o)
		} else {
			self
		}
	}

	/// Consume an `other` to mutate `self` into a new instance that combines
	/// both.
	fn subsume(&mut self, other: Self);

	/// Maybe consume an `other` to mutate `self` into a new instance that combines
	/// both.
	fn maybe_subsume(&mut self, other: Option<Self>) {
		if let Some(o) = other {
			self.subsume(o)
		}
	}

	/// Consume self and along with an opposite counterpart to return
	/// a combined result.
	///
	/// Returns `Ok` along with a new instance of `Self` if this instance has a
	/// greater value than the `other`. Otherwise returns `Err` with an instance of
	/// the `Opposite`. In both cases the value represents the combination of `self`
	/// and `other`.
	fn offset(self, other: Self::Opposite) -> SameOrOther<Self, Self::Opposite>;

	/// The raw value of self.
	fn peek(&self) -> Balance;
}