Add `usage` to `OpenAI` payload struct

[eastriverlee]

Hi, I enjoyed working with your library. but I wanted to know how many tokens I've used, so I made a very small change:

public struct OpenAI<T: Payload>: Codable {
    public let object: String
    public let model: String?
    public let choices: [T]
    public let usage: UsageResult
}

public struct UsageResult: Payload {
    public let prompt_tokens: Int
    public let completion_tokens: Int
    public let total_tokens: Int
}

This way, we can track tokens usage for sendEdits, sendCompletion, and sendChats without changing anything else.

[bogdanf]

I'd love to see this merged, but with better, swifter names for the members of UsageResult, like promptTokens instead of prompt_tokens.

[eastriverlee]

To get decoded by JsonDecoder, this is a must, since these are parameter names of the OpenAI's API result. If one wants to have more 'swifty' name, computed property extensions are possible, like:

public extension UsageResult {
    public var promptTokens: Int { prompt_tokens }
    ...
}

but I thought keeping changes to an absolute minimum was more important.

Maybe @adamrushy can merge with extra changes like the above.

[arjun-dureja]

@eastriverlee You can just create an enum that conforms to CodingKeys to set the correct parameter names: https://developer.apple.com/documentation/foundation/archives_and_serialization/encoding_and_decoding_custom_types#2904057

[eastriverlee]

That's good to know! Honestly I did not know that. In that case, I guess there is no reason to not change names.

I renamed properties and tested a new commit:

public struct UsageResult: Payload {
    public let promptTokens: Int
    public let completionTokens: Int
    public let totalTokens: Int

    enum CodingKeys: String, CodingKey {
        case promptTokens = "prompt_tokens"
        case completionTokens = "completion_tokens"
        case totalTokens = "total_tokens"
    }
}

[bogdanf]

@eastriverlee That's exactly what I had in mind.

[bogdanf]

One more thing, UsageResults should not conform to Payload but to Codable. Payload is intended to model one of the choices in the OpenAI struct, not anything else.

[eastriverlee]

Make sense. I wondered why Payload exist and just went it 🤭 without thinking too much.

[ConfusedVorlon]

nice - I was about to write this :)

[adamrushy]

Amazing extra! Great job @eastriverlee just awaiting GH Actions then will merge 🚀