namespace AIStudio.Provider;

/// <summary>
/// What a provider said one request actually cost, in tokens.
/// </summary>
/// <remarks>
/// The counterpart to what the app counts for itself: the app estimates what the next request will
/// cost, while this is what the provider charged for the last one. Two different statements, and
/// this one is the only exact one of the two.
///
/// Nothing here says "unknown" with a zero. The default value of this type is unknown, which is the
/// right answer for every provider which reports nothing, and a counted request can never cost zero
/// prompt tokens because the factory below refuses to build one.
/// </remarks>
public readonly record struct TokenUsage
{
    /// <summary>
    /// The usage of a request nobody reported anything about.
    /// </summary>
    public static readonly TokenUsage UNKNOWN = new();

    /// <summary>
    /// Whether a provider reported anything at all. When false, the numbers are meaningless.
    /// </summary>
    public bool IsKnown { get; private init; }

    /// <summary>
    /// What everything sent to the model cost: the conversation so far, its attachments, the system
    /// prompt, and whatever tools were offered.
    /// </summary>
    public int PromptTokens { get; private init; }

    /// <summary>
    /// What the model wrote in answer.
    /// </summary>
    public int CompletionTokens { get; private init; }

    /// <summary>
    /// What the whole exchange cost, which is what the next request carries as its history.
    /// </summary>
    public int TotalTokens => this.PromptTokens + this.CompletionTokens;

    /// <summary>
    /// States what a provider reported.
    /// </summary>
    /// <remarks>
    /// A completion of zero tokens is a real answer: a model which was cut off before writing
    /// anything still cost its prompt. A prompt of zero is not, because there is no request
    /// without one, and a provider sending it means we read the wrong field.
    /// </remarks>
    /// <param name="promptTokens">What the request carried. Has to be greater than zero.</param>
    /// <param name="completionTokens">What the answer cost. Zero or more.</param>
    /// <returns>The usage.</returns>
    public static TokenUsage Of(int promptTokens, int completionTokens)
    {
        ArgumentOutOfRangeException.ThrowIfNegativeOrZero(promptTokens);
        ArgumentOutOfRangeException.ThrowIfNegative(completionTokens);

        return new()
        {
            IsKnown = true,
            PromptTokens = promptTokens,
            CompletionTokens = completionTokens,
        };
    }

    /// <summary>
    /// States what a provider reported, or unknown when it reported nothing usable.
    /// </summary>
    /// <remarks>
    /// For the reading side, where the numbers come out of somebody else's JSON: a missing field, a
    /// null, or a zero prompt all mean the same thing there, and none of them is worth an exception.
    /// </remarks>
    /// <param name="promptTokens">What the request carried, as the provider stated it.</param>
    /// <param name="completionTokens">What the answer cost, as the provider stated it.</param>
    /// <returns>The usage, or UNKNOWN.</returns>
    public static TokenUsage OfReported(int? promptTokens, int? completionTokens) =>
        promptTokens is > 0
            ? Of(promptTokens.Value, completionTokens is > 0 ? completionTokens.Value : 0)
            : UNKNOWN;
}