Count the whole conversation against the model's window

app/MindWork AI Studio/Chat/ConversationTokens.cs

@@ -0,0 +1,64 @@
+using AIStudio.Models;
+
+namespace AIStudio.Chat;
+
+/// <summary>
+/// What a conversation costs, as far as the app can count it.
+/// </summary>
+/// <remarks>
+/// Three separate statements, and keeping them apart is the point. How many tokens were counted is
+/// one; what the model's window is, if anybody has written it down, is the second; and how much of
+/// the conversation could not be counted at all is the third. Folding any of them into the others
+/// would turn a gap into a number somebody reads as a fact.
+/// </remarks>
+public readonly record struct ConversationTokens
+{
+    /// <summary>
+    /// The answer when nothing could be counted, which is what a broken tokenizer leaves behind.
+    /// </summary>
+    /// <remarks>
+    /// Deliberately not a zero. A conversation of no tokens and a conversation nobody could measure
+    /// look the same as a number and are not the same thing, so the display shows nothing at all
+    /// rather than claiming an empty chat.
+    /// </remarks>
+    public static readonly ConversationTokens UNAVAILABLE = new();
+
+    /// <summary>
+    /// Whether anything could be counted.
+    /// </summary>
+    public bool IsKnown { get; init; }
+
+    /// <summary>
+    /// How many tokens the counted parts of the conversation take.
+    /// </summary>
+    public int Tokens { get; init; }
+
+    /// <summary>
+    /// Whether the number is an estimate rather than the model's own count.
+    /// </summary>
+    /// <remarks>
+    /// True whenever the built-in tokenizer did the counting, which is the normal case: a model's
+    /// own tokenizer is only used where somebody configured one for their provider. Two tokenizers
+    /// disagree by a few percent on ordinary prose and by a lot more on code or a language they were
+    /// not trained on, so the number is shown as an approximation unless we counted with the
+    /// tokenizer the model itself uses.
+    /// </remarks>
+    public bool IsEstimate { get; init; }
+
+    /// <summary>
+    /// How much the model reads, where anybody has stated it.
+    /// </summary>
+    public ContextWindow Window { get; init; }
+
+    /// <summary>
+    /// How many images travel along which nobody can count.
+    /// </summary>
+    /// <remarks>
+    /// Every vendor charges images differently -- OpenAI by tiles of the scaled image, Anthropic by
+    /// its area, Google by tiles of another size -- and none of those numbers can be had from the
+    /// file without decoding it first. So they are reported as a number of images instead of being
+    /// guessed at, or worse, counted as the base64 text they are sent as: that text is two to three
+    /// orders of magnitude longer than what any vendor charges for the picture.
+    /// </remarks>
+    public int UncountedImages { get; init; }
+}