Added PowerPoint templates to the Slide Assistant (#1055)

Co-authored-by: Thorsten Sommer <SommerEngineering@users.noreply.github.com>

app/MindWork AI Studio/Tools/PowerPoint/StreamedSlideCounter.cs

@@ -0,0 +1,50 @@
+namespace AIStudio.Tools.PowerPoint;
+
+/// <summary>
+/// Counts the slides an answer has started while it still streams in, without reading it as JSON.
+/// </summary>
+/// <remarks>
+/// A slide starts with the key of its layout, or with the key of the template slide it is taken
+/// from. JSON escapes the quotation marks within a text, so no text of a slide looks like such a
+/// key. The answer grows by thousands of characters, and the count runs with every rendering, so
+/// each count starts where the last one ended. One counter belongs to one answer.
+/// </remarks>
+public sealed class StreamedSlideCounter
+{
+    private static readonly string[] SLIDE_KEYS = ["\"layout\"", "\"template_slide\""];
+
+    private int countedLength;
+    private int countedSlides;
+
+    /// <summary>
+    /// Counts the slides which the answer has started so far.
+    /// </summary>
+    /// <param name="answerSoFar">The answer as far as it has streamed in, which only grows.</param>
+    /// <returns>How many slides the answer has started.</returns>
+    public int Count(string answerSoFar)
+    {
+        // An answer which got shorter was replaced; it is counted anew:
+        if (answerSoFar.Length < this.countedLength)
+        {
+            this.countedLength = 0;
+            this.countedSlides = 0;
+        }
+
+        foreach (var key in SLIDE_KEYS)
+        {
+            //
+            // A key which the last count saw only in part ends after the end of what it counted,
+            // so the count goes back by less than one key:
+            //
+            var index = Math.Max(0, this.countedLength - key.Length + 1);
+            while ((index = answerSoFar.IndexOf(key, index, StringComparison.Ordinal)) >= 0)
+            {
+                this.countedSlides++;
+                index += key.Length;
+            }
+        }
+
+        this.countedLength = answerSoFar.Length;
+        return this.countedSlides;
+    }
+}