XFESystem

namespace XFESystem.Desktop;

/// <summary>UI-owned diagnostic state. Unchanged readbacks allocate nothing;
/// the cache never retains a window or its complete large source.</summary>
public sealed class EditorReadbackTracker
{
    public const int MaximumPrefixCharacters = 8192;
    private bool hasState;
    private int windowId, sourceLength;
    private string path = "", prefix = "";

    public bool TryUpdate(int id, string location, string content, out string sourcePrefix)
    {
        ArgumentNullException.ThrowIfNull(location);
        ArgumentNullException.ThrowIfNull(content);
        int prefixLength = Math.Min(content.Length, MaximumPrefixCharacters);
        if (hasState && id == windowId && location == path && content.Length == sourceLength &&
            content.AsSpan(0, prefixLength).SequenceEqual(prefix.AsSpan()))
        {
            sourcePrefix = prefix;
            return false;
        }

        // Copy only after the comparison, independently of the original string.
        // Even a closed large editor therefore leaves at most this bounded prefix.
        prefix = content.AsSpan(0, prefixLength).ToString();
        path = location;
        windowId = id;
        sourceLength = content.Length;
        hasState = true;
        sourcePrefix = prefix;
        return true;
    }

    public void Reset()
    {
        hasState = false;
        windowId = sourceLength = 0;
        path = prefix = "";
    }
}