/// <summary>
/// Returns the index of the entry to replace in a given set of the <see cref="Cache{TItem, UKey, VData}"/>; this could be an empty entry or an entry that should be replaced
/// </summary>
/// <param name="startIndex">The starting index of the set</param>
/// <param name="endIndex">The ending index of the set</param>
/// <returns>An index inclusively between <c>startIndex</c> and <c>endIndex</c> that should be replaced</returns>
protected int GetReplacementIndex(int startIndex, int endIndex)
{
// readlock needed
cacheLock.EnterReadLock();
try
{
// first check if we have an empty block to use
int ans = GetFirstEmptyIndex(startIndex, endIndex);
// if so use that for the put
if (ans > -1)
{
return(ans);
}
// otherwise we'll need to use our IReplacementAlgorithm to figure out which block to use
// Using ArraySegment wrapped in a ReadOnlyCollection we can referencially pass the set
// we need for full inspection quickly without increasing memory footprint too much, since
// we're not copying elements, but we don't risk the consumer trying to edit the cache (that's our job)
ans = _replacementFinder.GetReplacementIndex(
new ReadOnlyCollection <TItem>(
(IList <TItem>) new ArraySegment <TItem>(_cacheArray, startIndex, endIndex - startIndex + 1)));
// Make sure replacement strategy didn't do something crazy
if (ans < 0 || ans > (endIndex - startIndex))
{
throw new ArgumentException(string.Format("IReplacementAlgorithm ({0}) returned invalid index value.", _replacementFinder.GetType().FullName), "replacementAlgo");
}
// determine index in full cache
return(startIndex + ans);
}
finally
{ cacheLock.ExitReadLock(); }
}