150 lines
3.0 KiB
Go
150 lines
3.0 KiB
Go
package dataext
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import (
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"sync"
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)
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//
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// This is an LRU (Least-Recently-Used) cache based on a double linked list
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// All the work we do below is to have a cache where we can easily remove the least-used element
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// (to ensure that the cache is never bigger than maxsize items)
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//
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// The cache algorithm the following properties:
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// - Memory footprint is O(n), with neglible overhead
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// - The cache is multi-threading safe (sync.Mutex)
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// - Inserts are O(1)
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// - Gets are O(1)
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// - Re-Shuffles are O(1) (= an element is removed on Insert because teh cache was full)
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//
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// There are also a bunch of unit tests to ensure that the cache is always in a consistent state
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//
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type LRUMap[TData any] struct {
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maxsize int
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lock sync.Mutex
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cache map[string]*cacheNode[TData]
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lfuHead *cacheNode[TData]
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lfuTail *cacheNode[TData]
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}
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type cacheNode[TData any] struct {
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key string
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data TData
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parent *cacheNode[TData]
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child *cacheNode[TData]
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}
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func NewLRUMap[TData any](size int) *LRUMap[TData] {
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if size <= 2 && size != 0 {
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panic("Size must be > 2 (or 0)")
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}
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return &LRUMap[TData]{
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maxsize: size,
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lock: sync.Mutex{},
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cache: make(map[string]*cacheNode[TData], size+1),
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lfuHead: nil,
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lfuTail: nil,
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}
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}
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func (c *LRUMap[TData]) Put(key string, value TData) {
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if c.maxsize == 0 {
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return // cache disabled
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}
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c.lock.Lock()
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defer c.lock.Unlock()
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node, exists := c.cache[key]
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if exists {
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// key already in data: only update LFU and value
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c.moveNodeToTop(node)
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node.data = value
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return
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}
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// key does not exist: insert into map and add to top of LFU
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node = &cacheNode[TData]{
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key: key,
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data: value,
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parent: nil,
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child: c.lfuHead,
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}
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if c.lfuHead == nil && c.lfuTail == nil { // special case - previously the cache was empty (head == tail == nil)
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c.lfuHead = node
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c.lfuTail = node
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} else {
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c.lfuHead = node
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node.child.parent = node
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}
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c.cache[key] = node
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if len(c.cache) > c.maxsize { // maxsize is always > 2
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tail := c.lfuTail
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delete(c.cache, tail.key)
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c.lfuTail = tail.parent
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c.lfuTail.child = nil
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tail.parent = nil
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tail.child = nil
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}
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}
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func (c *LRUMap[TData]) TryGet(key string) (TData, bool) {
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if c.maxsize == 0 {
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return *new(TData), false // cache disabled
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}
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c.lock.Lock()
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defer c.lock.Unlock()
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val, ok := c.cache[key]
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if !ok {
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return *new(TData), false
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}
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c.moveNodeToTop(val)
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return val.data, ok
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}
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func (c *LRUMap[TData]) moveNodeToTop(node *cacheNode[TData]) {
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// (only called in critical section !)
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if c.lfuHead == node { // fast case
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return
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}
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// Step 1 unlink
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parent := node.parent
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child := node.child
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if parent != nil {
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parent.child = child
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}
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if child != nil {
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child.parent = parent
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}
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if node == c.lfuHead {
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c.lfuHead = node.child
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}
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if node == c.lfuTail {
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c.lfuTail = node.parent
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}
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// Step 2 re-insert at top
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node.parent = nil
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node.child = c.lfuHead
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c.lfuHead = node
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if node.child != nil {
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node.child.parent = node
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}
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}
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func (c *LRUMap[TData]) Size() int {
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c.lock.Lock()
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defer c.lock.Unlock()
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return len(c.cache)
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}
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