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package memoization
import (
"container/list"
"crypto/sha256"
"fmt"
"sync"
)
// Hasher is an interface that requires a Hash method. The Hash method is
// expected to return a string representation of the hash of the object.
type Hasher interface {
Hash() string
}
// entry is a struct that holds a key-value pair. It is used as an element
// in the evictionList of the MemoCache.
type entry[T any] struct {
key string
value T
}
// MemoCache is a struct that represents a cache with a set capacity. It
// uses an LRU (Least Recently Used) eviction policy. It is safe for
// concurrent use.
type MemoCache[H Hasher, T any] struct {
capacity int
mutex sync.Mutex
cache map[string]*list.Element // The cache holding the results
evictionList *list.List // A list to keep track of the order for LRU
hashableItems map[string]T // This map keeps track of the original hashable items (optional)
}
// NewMemoCache is a function that creates a new MemoCache with a given
// capacity. It returns a pointer to the created MemoCache.
func NewMemoCache[H Hasher, T any](capacity int) *MemoCache[H, T] {
return &MemoCache[H, T]{
capacity: capacity,
cache: make(map[string]*list.Element),
evictionList: list.New(),
hashableItems: make(map[string]T),
}
}
// Capacity is a method that returns the capacity of the MemoCache.
func (m *MemoCache[H, T]) Capacity() int {
return m.capacity
}
// Size is a method that returns the current size of the MemoCache. It is
// the number of items currently stored in the cache.
func (m *MemoCache[H, T]) Size() int {
m.mutex.Lock()
defer m.mutex.Unlock()
return m.evictionList.Len()
}
// Get is a method that returns the value associated with the given
// hashable item in the MemoCache. If there is no corresponding value, the
// method returns nil.
func (m *MemoCache[H, T]) Get(h H) (T, bool) {
m.mutex.Lock()
defer m.mutex.Unlock()
hashedKey := h.Hash()
if element, found := m.cache[hashedKey]; found {
m.evictionList.MoveToFront(element)
return element.Value.(*entry[T]).value, true
}
var result T
return result, false
}
// Set is a method that sets the value for the given hashable item in the
// MemoCache. If the cache is at capacity, it evicts the least recently
// used item before adding the new item.
func (m *MemoCache[H, T]) Set(h H, value T) {
m.mutex.Lock()
defer m.mutex.Unlock()
hashedKey := h.Hash()
if element, found := m.cache[hashedKey]; found {
m.evictionList.MoveToFront(element)
element.Value.(*entry[T]).value = value
return
}
// Check if the cache is at capacity
if m.evictionList.Len() >= m.capacity {
// Evict the least recently used item from the cache
toEvict := m.evictionList.Back()
if toEvict != nil {
evictedEntry := m.evictionList.Remove(toEvict).(*entry[T])
delete(m.cache, evictedEntry.key)
delete(m.hashableItems, evictedEntry.key) // if you're keeping track of original items
}
}
// Add the value to the cache and the evictionList
newEntry := &entry[T]{
key: hashedKey,
value: value,
}
element := m.evictionList.PushFront(newEntry)
m.cache[hashedKey] = element
m.hashableItems[hashedKey] = value // if you're keeping track of original items
}
// HString is a type that implements the Hasher interface for strings.
type HString string
// Hash is a method that returns the hash of the string.
func (h HString) Hash() string {
return fmt.Sprintf("%x", sha256.Sum256([]byte(h)))
}
// HInt is a type that implements the Hasher interface for integers.
type HInt int
// Hash is a method that returns the hash of the integer.
func (h HInt) Hash() string {
return fmt.Sprintf("%x", sha256.Sum256([]byte(fmt.Sprintf("%d", h))))
}
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