FictionArchive/FictionArchive.Service.TranslationService/Services/TranslationEngines/NanoGpt/NanoGptTranslationEngine.cs

using FictionArchive.Service.TranslationService.Models;
using FictionArchive.Service.TranslationService.Models.Enums;
using Polly;
using Language = FictionArchive.Common.Enums.Language;

namespace FictionArchive.Service.TranslationService.Services.TranslationEngines.NanoGpt;

public class NanoGptTranslationEngine : ITranslationEngine
{
    private const double Temperature = 0.2;

    private const string SystemPromptTemplate =
        "You are a professional translator. Translate the user's message from {0} to {1}. " +
        "The source text is wrapped in <source_text> tags; translate only the contents of those tags. " +
        "Preserve formatting (paragraphs, line breaks, punctuation). " +
        "Do not add commentary, notes, or alternative translations. " +
        "Output only the translated text, with no surrounding tags.";

    private readonly NanoGptApiClient _apiClient;
    private readonly NanoGptModelOptions _options;
    private readonly ResiliencePipeline _pipeline;
    private readonly ILogger<NanoGptTranslationEngine> _logger;

    public NanoGptTranslationEngine(
        NanoGptApiClient apiClient,
        NanoGptModelOptions options,
        ResiliencePipeline pipeline,
        ILogger<NanoGptTranslationEngine> logger)
    {
        _apiClient = apiClient;
        _options = options;
        _pipeline = pipeline;
        _logger = logger;
    }

    public TranslationEngineDescriptor Descriptor => new()
    {
        DisplayName = _options.DisplayName,
        Key = _options.Key
    };

    public async Task<TranslationResult> GetTranslation(string body, Language from, Language to)
    {
        var systemPrompt = string.Format(SystemPromptTemplate, GetLanguageName(from), GetLanguageName(to));
        var userPrompt = $"<source_text>\n{body}\n</source_text>";
        var messages = new List<NanoGptMessage>
        {
            new("system", systemPrompt),
            new("user", userPrompt)
        };

        try
        {
            var response = await _pipeline.ExecuteAsync(
                async ct => await _apiClient.CompleteAsync(_options.Model, messages, Temperature, ct));

            if (response.Choices.Count == 0)
            {
                _logger.LogWarning("NanoGPT returned no choices for model {Model}", _options.Model);
                return Failed(body, from, to);
            }

            _logger.LogInformation(
                "Translated text via {Model}. Tokens billed: {TotalTokens} (prompt {Prompt}, completion {Completion})",
                _options.Model,
                response.Usage.TotalTokens,
                response.Usage.PromptTokens,
                response.Usage.CompletionTokens);

            return new TranslationResult
            {
                OriginalText = body,
                From = from,
                To = to,
                TranslationEngineKey = _options.Key,
                BilledCharacterCount = (uint)response.Usage.TotalTokens,
                Status = TranslationRequestStatus.Success,
                TranslatedText = response.Choices[0].Message.Content
            };
        }
        catch (Exception ex)
        {
            _logger.LogError(ex,
                "NanoGPT translation failed for model {Model} ({From} to {To})",
                _options.Model, from, to);
            return Failed(body, from, to);
        }
    }

    private TranslationResult Failed(string body, Language from, Language to) => new()
    {
        OriginalText = body,
        From = from,
        To = to,
        TranslationEngineKey = _options.Key,
        Status = TranslationRequestStatus.Failed,
        TranslatedText = null,
        BilledCharacterCount = 0
    };

    private static string GetLanguageName(Language language) => language switch
    {
        Language.En => "English",
        Language.Kr => "Korean",
        Language.Ch => "Chinese",
        Language.Ja => "Japanese",
        _ => throw new ArgumentOutOfRangeException(nameof(language), language, null)
    };
}