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Last active February 18, 2021 16:15
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A simple, thread-safe lookup table for Actions that generate guaranteed unique keys.
using System;
using System.Collections;
using System.Collections.Generic;
using System.Threading;
///<summary> A simple lookup table that generates guaranteed unique keys. Thread Safe. </summary>
public class ActionLookupTable<E>{
private long lastTimeStamp=-1;
Dictionary<long,Action<E>> table = new Dictionary<long, Action<E>>();
///<returns> A guaranteed unique long for the given Action. Use this to Get or Pull the Action.</returns>
public long Put(Action<E> action){
long id=GetUniqueTick();
lock(table){
table.Add(id,action);
}
return id;
}
public Action<E> Get(long id){
Action<E> action;
lock(table){
if(table.TryGetValue(id,out action)){
return action;
}
}
return null;
}
///<summary>Removes and Returns the Action for the given id.</summary>
///<returns> Action or null if invalid key</returns>
public Action<E> Pull(long id){
Action<E> action;
lock(table){
if(table.TryGetValue(id,out action)){
table.Remove(id);
return action;
}
}
return null;
}
//Adapted from https://docs.microsoft.com/en-us/dotnet/api/system.threading.interlocked.compareexchange?view=netframework-4.8
private long GetUniqueTick(){
long initialValue ,newValue;
do{
initialValue =lastTimeStamp;
long now = DateTime.UtcNow.Ticks;
newValue = Math.Max(now,initialValue +1);
// CompareExchange compares lastTimeStamp to initialValue. If
// they are not equal, then another thread has updated the
// running total since this loop started. CompareExchange
// does not update lastTimeStamp. CompareExchange returns the
// contents of lastTimeStamp, which do not equal initialValue,
// so the loop executes again.
}while(Interlocked.CompareExchange(ref lastTimeStamp,newValue,initialValue )!=initialValue );
// If no other thread updated the running total, then
// lastTimeStamp and initialValue are equal when CompareExchange
// compares them, and newValue is stored in lastTimeStamp.
// CompareExchange returns the value that was in lastTimeStamp
// before the update, which is equal to initialValue, so the
// loop ends.
// The function returns newValue, not lastTimeStamp, because
// lastTimeStamp could be changed by another thread between
// the time the loop ends and the function returns.
return newValue;
}
}
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