Added mailboxes as local data sources (#1022)

app/MindWork AI Studio/Tools/Mail/MailRemovalGuard.cs

@@ -0,0 +1,59 @@
+using AIStudio.Tools.Databases.IndexStore;
+
+namespace AIStudio.Tools.Mail;
+
+/// <summary>
+/// Keeps a sync from removing a large part of a mailbox from the index without asking.
+/// </summary>
+/// <remarks>
+/// Nothing on the server is touched either way. But a mail removed from the index has to be
+/// embedded again should it come back, and much of a mailbox drops out at once for reasons the user
+/// may not have meant: a smaller folder or period was picked, a folder was renamed on the server,
+/// or the server lists a folder as empty for a while. Embedding all of that again costs money with
+/// a cloud provider and hours with a large mailbox, so the sync asks first.
+///
+/// A few mails are not worth a question, however large their share: a mailbox of fifty mails is
+/// embedded again in no time. Nor is a small share of a large mailbox, such as the mails of one day
+/// leaving the period.
+/// </remarks>
+public static class MailRemovalGuard
+{
+    /// <summary>
+    /// How many mails a removal has to reach at least before anybody is asked.
+    /// </summary>
+    public const int MIN_REMOVAL_COUNT = 100;
+
+    /// <summary>
+    /// Which share of the indexed mails a removal has to reach at least before anybody is asked.
+    /// </summary>
+    public const double MIN_REMOVAL_SHARE = 0.2;
+
+    /// <summary>
+    /// Whether a removal is large enough to ask about.
+    /// </summary>
+    /// <param name="indexedCount">How many the index holds.</param>
+    /// <param name="removalCount">How many would go.</param>
+    /// <returns>True when the user has to agree first.</returns>
+    public static bool IsMassRemoval(int indexedCount, int removalCount) => removalCount >= MIN_REMOVAL_COUNT && removalCount >= indexedCount * MIN_REMOVAL_SHARE;
+
+    /// <summary>
+    /// Decides whether a sync may remove what it found gone.
+    /// </summary>
+    /// <remarks>
+    /// The user agrees to a number, not to whatever a sync comes up with: an approval counts only
+    /// for the very count which was held back and shown, cf. ApprovePendingMailRemovalAsync.
+    /// </remarks>
+    /// <param name="indexedCount">How many the index holds.</param>
+    /// <param name="removalCount">How many would go.</param>
+    /// <param name="syncState">How the sync of the mailbox stands, with any removal held back before.</param>
+    /// <returns>The decision.</returns>
+    public static MailRemovalDecision Decide(int indexedCount, int removalCount, MailboxSyncState syncState)
+    {
+        if (!IsMassRemoval(indexedCount, removalCount))
+            return MailRemovalDecision.PROCEED;
+
+        return syncState is { PendingRemovalApprovedUtc: not null } && syncState.PendingRemovalCount == removalCount
+            ? MailRemovalDecision.PROCEED
+            : MailRemovalDecision.HOLD_BACK;
+    }
+}