plugeth/trie/arc.go

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2015-07-05 23:19:48 +00:00
// Copyright (c) 2015 Hans Alexander Gugel <alexander.gugel@gmail.com>
//
// Permission is hereby granted, free of charge, to any person obtaining a copy
// of this software and associated documentation files (the "Software"), to deal
// in the Software without restriction, including without limitation the rights
// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
// copies of the Software, and to permit persons to whom the Software is
// furnished to do so, subject to the following conditions:
//
// The above copyright notice and this permission notice shall be included in all
// copies or substantial portions of the Software.
//
// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
// SOFTWARE.
// This file contains a modified version of package arc from
// https://github.com/alexanderGugel/arc
//
// It implements the ARC (Adaptive Replacement Cache) algorithm as detailed in
// https://www.usenix.org/legacy/event/fast03/tech/full_papers/megiddo/megiddo.pdf
package trie
import (
"container/list"
"sync"
)
type arc struct {
p int
c int
t1 *list.List
b1 *list.List
t2 *list.List
b2 *list.List
cache map[string]*entry
mutex sync.Mutex
}
type entry struct {
key hashNode
value node
ll *list.List
el *list.Element
}
// newARC returns a new Adaptive Replacement Cache with the
// given capacity.
func newARC(c int) *arc {
return &arc{
c: c,
t1: list.New(),
b1: list.New(),
t2: list.New(),
b2: list.New(),
cache: make(map[string]*entry, c),
}
}
// Clear clears the cache
func (a *arc) Clear() {
a.mutex.Lock()
defer a.mutex.Unlock()
a.p = 0
a.t1 = list.New()
a.b1 = list.New()
a.t2 = list.New()
a.b2 = list.New()
a.cache = make(map[string]*entry, a.c)
}
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// Put inserts a new key-value pair into the cache.
// This optimizes future access to this entry (side effect).
func (a *arc) Put(key hashNode, value node) bool {
a.mutex.Lock()
defer a.mutex.Unlock()
ent, ok := a.cache[string(key)]
if ok != true {
ent = &entry{key: key, value: value}
a.req(ent)
a.cache[string(key)] = ent
} else {
ent.value = value
a.req(ent)
}
return ok
}
// Get retrieves a previously via Set inserted entry.
// This optimizes future access to this entry (side effect).
func (a *arc) Get(key hashNode) (value node, ok bool) {
a.mutex.Lock()
defer a.mutex.Unlock()
ent, ok := a.cache[string(key)]
if ok {
a.req(ent)
return ent.value, ent.value != nil
}
return nil, false
}
func (a *arc) req(ent *entry) {
if ent.ll == a.t1 || ent.ll == a.t2 {
// Case I
ent.setMRU(a.t2)
} else if ent.ll == a.b1 {
// Case II
// Cache Miss in t1 and t2
// Adaptation
var d int
if a.b1.Len() >= a.b2.Len() {
d = 1
} else {
d = a.b2.Len() / a.b1.Len()
}
a.p = a.p + d
if a.p > a.c {
a.p = a.c
}
a.replace(ent)
ent.setMRU(a.t2)
} else if ent.ll == a.b2 {
// Case III
// Cache Miss in t1 and t2
// Adaptation
var d int
if a.b2.Len() >= a.b1.Len() {
d = 1
} else {
d = a.b1.Len() / a.b2.Len()
}
a.p = a.p - d
if a.p < 0 {
a.p = 0
}
a.replace(ent)
ent.setMRU(a.t2)
} else if ent.ll == nil {
// Case IV
if a.t1.Len()+a.b1.Len() == a.c {
// Case A
if a.t1.Len() < a.c {
a.delLRU(a.b1)
a.replace(ent)
} else {
a.delLRU(a.t1)
}
} else if a.t1.Len()+a.b1.Len() < a.c {
// Case B
if a.t1.Len()+a.t2.Len()+a.b1.Len()+a.b2.Len() >= a.c {
if a.t1.Len()+a.t2.Len()+a.b1.Len()+a.b2.Len() == 2*a.c {
a.delLRU(a.b2)
}
a.replace(ent)
}
}
ent.setMRU(a.t1)
}
}
func (a *arc) delLRU(list *list.List) {
lru := list.Back()
list.Remove(lru)
delete(a.cache, string(lru.Value.(*entry).key))
}
func (a *arc) replace(ent *entry) {
if a.t1.Len() > 0 && ((a.t1.Len() > a.p) || (ent.ll == a.b2 && a.t1.Len() == a.p)) {
lru := a.t1.Back().Value.(*entry)
lru.value = nil
lru.setMRU(a.b1)
} else {
lru := a.t2.Back().Value.(*entry)
lru.value = nil
lru.setMRU(a.b2)
}
}
func (e *entry) setLRU(list *list.List) {
e.detach()
e.ll = list
e.el = e.ll.PushBack(e)
}
func (e *entry) setMRU(list *list.List) {
e.detach()
e.ll = list
e.el = e.ll.PushFront(e)
}
func (e *entry) detach() {
if e.ll != nil {
e.ll.Remove(e.el)
}
}