Official Go implementation of the Ethereum protocol
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go-ethereum/les/utils/expiredvalue.go

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// Copyright 2020 The go-ethereum Authors
// This file is part of the go-ethereum library.
//
// The go-ethereum library is free software: you can redistribute it and/or modify
// it under the terms of the GNU Lesser General Public License as published by
// the Free Software Foundation, either version 3 of the License, or
// (at your option) any later version.
//
// The go-ethereum library is distributed in the hope that it will be useful,
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
// GNU Lesser General Public License for more details.
//
// You should have received a copy of the GNU Lesser General Public License
// along with the go-ethereum library. If not, see <http://www.gnu.org/licenses/>.
package utils
import (
"math"
"github.com/ethereum/go-ethereum/common/mclock"
)
// ExpiredValue is a scalar value that is continuously expired (decreased
// exponentially) based on the provided logarithmic expiration offset value.
//
// The formula for value calculation is: base*2^(exp-logOffset). In order to
// simplify the calculation of ExpiredValue, its value is expressed in the form
// of an exponent with a base of 2.
//
// Also here is a trick to reduce a lot of calculations. In theory, when a value X
// decays over time and then a new value Y is added, the final result should be
// X*2^(exp-logOffset)+Y. However it's very hard to represent in memory.
// So the trick is using the idea of inflation instead of exponential decay. At this
// moment the temporary value becomes: X*2^exp+Y*2^logOffset_1, apply the exponential
// decay when we actually want to calculate the value.
//
// e.g.
// t0: V = 100
// t1: add 30, inflationary value is: 100 + 30/0.3, 0.3 is the decay coefficient
// t2: get value, decay coefficient is 0.2 now, final result is: 200*0.2 = 40
type ExpiredValue struct {
Base, Exp uint64 // rlp encoding works by default
}
// ExpirationFactor is calculated from logOffset. 1 <= Factor < 2 and Factor*2^Exp
// describes the multiplier applicable for additions and the divider for readouts.
// If logOffset changes slowly then it saves some expensive operations to not calculate
// them for each addition and readout but cache this intermediate form for some time.
// It is also useful for structures where multiple values are expired with the same
// Expirer.
type ExpirationFactor struct {
Exp uint64
Factor float64
}
// ExpFactor calculates ExpirationFactor based on logOffset
func ExpFactor(logOffset Fixed64) ExpirationFactor {
return ExpirationFactor{Exp: logOffset.ToUint64(), Factor: logOffset.Fraction().Pow2()}
}
// Value calculates the expired value based on a floating point base and integer
// power-of-2 exponent. This function should be used by multi-value expired structures.
func (e ExpirationFactor) Value(base float64, exp uint64) float64 {
return base / e.Factor * math.Pow(2, float64(int64(exp-e.Exp)))
}
// value calculates the value at the given moment.
func (e ExpiredValue) Value(logOffset Fixed64) uint64 {
offset := Uint64ToFixed64(e.Exp) - logOffset
return uint64(float64(e.Base) * offset.Pow2())
}
// add adds a signed value at the given moment
func (e *ExpiredValue) Add(amount int64, logOffset Fixed64) int64 {
integer, frac := logOffset.ToUint64(), logOffset.Fraction()
factor := frac.Pow2()
base := factor * float64(amount)
if integer < e.Exp {
base /= math.Pow(2, float64(e.Exp-integer))
}
if integer > e.Exp {
e.Base >>= (integer - e.Exp)
e.Exp = integer
}
if base >= 0 || uint64(-base) <= e.Base {
// This is a temporary fix to circumvent a golang
// uint conversion issue on arm64, which needs to
// be investigated further. FIXME
e.Base = uint64(int64(e.Base) + int64(base))
return amount
}
net := int64(-float64(e.Base) / factor)
e.Base = 0
return net
}
// addExp adds another ExpiredValue
func (e *ExpiredValue) AddExp(a ExpiredValue) {
if e.Exp > a.Exp {
a.Base >>= (e.Exp - a.Exp)
}
if e.Exp < a.Exp {
e.Base >>= (a.Exp - e.Exp)
e.Exp = a.Exp
}
e.Base += a.Base
}
// subExp subtracts another ExpiredValue
func (e *ExpiredValue) SubExp(a ExpiredValue) {
if e.Exp > a.Exp {
a.Base >>= (e.Exp - a.Exp)
}
if e.Exp < a.Exp {
e.Base >>= (a.Exp - e.Exp)
e.Exp = a.Exp
}
if e.Base > a.Base {
e.Base -= a.Base
} else {
e.Base = 0
}
}
// LinearExpiredValue is very similar with the expiredValue which the value
// will continuously expired. But the different part is it's expired linearly.
type LinearExpiredValue struct {
Offset uint64 // The latest time offset
Val uint64 // The remaining value, can never be negative
Rate mclock.AbsTime `rlp:"-"` // Expiration rate(by nanosecond), will ignored by RLP
}
// value calculates the value at the given moment. This function always has the
// assumption that the given timestamp shouldn't less than the recorded one.
func (e LinearExpiredValue) Value(now mclock.AbsTime) uint64 {
offset := uint64(now / e.Rate)
if e.Offset < offset {
diff := offset - e.Offset
if e.Val >= diff {
e.Val -= diff
} else {
e.Val = 0
}
}
return e.Val
}
// add adds a signed value at the given moment. This function always has the
// assumption that the given timestamp shouldn't less than the recorded one.
func (e *LinearExpiredValue) Add(amount int64, now mclock.AbsTime) uint64 {
offset := uint64(now / e.Rate)
if e.Offset < offset {
diff := offset - e.Offset
if e.Val >= diff {
e.Val -= diff
} else {
e.Val = 0
}
e.Offset = offset
}
if amount < 0 && uint64(-amount) > e.Val {
e.Val = 0
} else {
e.Val = uint64(int64(e.Val) + amount)
}
return e.Val
}
// Expirer changes logOffset with a linear rate which can be changed during operation.
// It is not thread safe, if access by multiple goroutines is needed then it should be
// encapsulated into a locked structure.
// Note that if neither SetRate nor SetLogOffset are used during operation then LogOffset
// is thread safe.
type Expirer struct {
logOffset Fixed64
rate float64
lastUpdate mclock.AbsTime
}
// SetRate changes the expiration rate which is the inverse of the time constant in
// nanoseconds.
func (e *Expirer) SetRate(now mclock.AbsTime, rate float64) {
dt := now - e.lastUpdate
if dt > 0 {
e.logOffset += Fixed64(logToFixedFactor * float64(dt) * e.rate)
}
e.lastUpdate = now
e.rate = rate
}
// SetLogOffset sets logOffset instantly.
func (e *Expirer) SetLogOffset(now mclock.AbsTime, logOffset Fixed64) {
e.lastUpdate = now
e.logOffset = logOffset
}
// LogOffset returns the current logarithmic offset.
func (e *Expirer) LogOffset(now mclock.AbsTime) Fixed64 {
dt := now - e.lastUpdate
if dt <= 0 {
return e.logOffset
}
return e.logOffset + Fixed64(logToFixedFactor*float64(dt)*e.rate)
}
// fixedFactor is the fixed point multiplier factor used by Fixed64.
const fixedFactor = 0x1000000
// Fixed64 implements 64-bit fixed point arithmetic functions.
type Fixed64 int64
// Uint64ToFixed64 converts uint64 integer to Fixed64 format.
func Uint64ToFixed64(f uint64) Fixed64 {
return Fixed64(f * fixedFactor)
}
// float64ToFixed64 converts float64 to Fixed64 format.
func Float64ToFixed64(f float64) Fixed64 {
return Fixed64(f * fixedFactor)
}
// toUint64 converts Fixed64 format to uint64.
func (f64 Fixed64) ToUint64() uint64 {
return uint64(f64) / fixedFactor
}
// fraction returns the fractional part of a Fixed64 value.
func (f64 Fixed64) Fraction() Fixed64 {
return f64 % fixedFactor
}
var (
logToFixedFactor = float64(fixedFactor) / math.Log(2)
fixedToLogFactor = math.Log(2) / float64(fixedFactor)
)
// pow2Fixed returns the base 2 power of the fixed point value.
func (f64 Fixed64) Pow2() float64 {
return math.Exp(float64(f64) * fixedToLogFactor)
}