137 lines
5.0 KiB
Go
137 lines
5.0 KiB
Go
// Copyright 2018 The go-ethereum Authors
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// This file is part of the go-ethereum library.
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//
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// The go-ethereum library is free software: you can redistribute it and/or modify
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// it under the terms of the GNU Lesser General Public License as published by
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// the Free Software Foundation, either version 3 of the License, or
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// (at your option) any later version.
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//
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// The go-ethereum library is distributed in the hope that it will be useful,
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// but WITHOUT ANY WARRANTY; without even the implied warranty of
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// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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// GNU Lesser General Public License for more details.
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//
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// You should have received a copy of the GNU Lesser General Public License
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// along with the go-ethereum library. If not, see <http://www.gnu.org/licenses/>.
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/*
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Package lookup defines feed lookup algorithms and provides tools to place updates
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so they can be found
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*/
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package lookup
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import (
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"context"
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"time"
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)
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const maxuint64 = ^uint64(0)
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// LowestLevel establishes the frequency resolution of the lookup algorithm as a power of 2.
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const LowestLevel uint8 = 0 // default is 0 (1 second)
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// HighestLevel sets the lowest frequency the algorithm will operate at, as a power of 2.
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// 31 -> 2^31 equals to roughly 38 years.
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const HighestLevel = 31
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// DefaultLevel sets what level will be chosen to search when there is no hint
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const DefaultLevel = HighestLevel
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//Algorithm is the function signature of a lookup algorithm
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type Algorithm func(ctx context.Context, now uint64, hint Epoch, read ReadFunc) (value interface{}, err error)
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// Lookup finds the update with the highest timestamp that is smaller or equal than 'now'
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// It takes a hint which should be the epoch where the last known update was
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// If you don't know in what epoch the last update happened, simply submit lookup.NoClue
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// read() will be called on each lookup attempt
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// Returns an error only if read() returns an error
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// Returns nil if an update was not found
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var Lookup Algorithm = LongEarthAlgorithm
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// TimeAfter must point to a function that returns a timer
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// This is here so that tests can replace it with
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// a mock up timer factory to simulate time deterministically
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var TimeAfter = time.After
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// ReadFunc is a handler called by Lookup each time it attempts to find a value
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// It should return <nil> if a value is not found
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// It should return <nil> if a value is found, but its timestamp is higher than "now"
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// It should only return an error in case the handler wants to stop the
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// lookup process entirely.
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type ReadFunc func(ctx context.Context, epoch Epoch, now uint64) (interface{}, error)
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// NoClue is a hint that can be provided when the Lookup caller does not have
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// a clue about where the last update may be
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var NoClue = Epoch{}
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// getBaseTime returns the epoch base time of the given
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// time and level
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func getBaseTime(t uint64, level uint8) uint64 {
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return t & (maxuint64 << level)
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}
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// Hint creates a hint based only on the last known update time
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func Hint(last uint64) Epoch {
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return Epoch{
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Time: last,
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Level: DefaultLevel,
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}
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}
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// GetNextLevel returns the frequency level a next update should be placed at, provided where
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// the last update was and what time it is now.
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// This is the first nonzero bit of the XOR of 'last' and 'now', counting from the highest significant bit
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// but limited to not return a level that is smaller than the last-1
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func GetNextLevel(last Epoch, now uint64) uint8 {
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// First XOR the last epoch base time with the current clock.
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// This will set all the common most significant bits to zero.
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mix := (last.Base() ^ now)
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// Then, make sure we stop the below loop before one level below the current, by setting
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// that level's bit to 1.
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// If the next level is lower than the current one, it must be exactly level-1 and not lower.
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mix |= (1 << (last.Level - 1))
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// if the last update was more than 2^highestLevel seconds ago, choose the highest level
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if mix > (maxuint64 >> (64 - HighestLevel - 1)) {
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return HighestLevel
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}
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// set up a mask to scan for nonzero bits, starting at the highest level
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mask := uint64(1 << (HighestLevel))
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for i := uint8(HighestLevel); i > LowestLevel; i-- {
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if mix&mask != 0 { // if we find a nonzero bit, this is the level the next update should be at.
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return i
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}
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mask = mask >> 1 // move our bit one position to the right
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}
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return 0
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}
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// GetNextEpoch returns the epoch where the next update should be located
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// according to where the previous update was
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// and what time it is now.
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func GetNextEpoch(last Epoch, now uint64) Epoch {
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if last == NoClue {
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return GetFirstEpoch(now)
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}
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level := GetNextLevel(last, now)
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return Epoch{
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Level: level,
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Time: now,
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}
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}
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// GetFirstEpoch returns the epoch where the first update should be located
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// based on what time it is now.
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func GetFirstEpoch(now uint64) Epoch {
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return Epoch{Level: HighestLevel, Time: now}
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}
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var worstHint = Epoch{Time: 0, Level: 63}
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var trace = func(id int32, formatString string, a ...interface{}) {
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//fmt.Printf("Step ID #%d "+formatString+"\n", append([]interface{}{id}, a...)...)
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}
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