FictionArchive/FictionArchive.Service.TranslationService.Tests/NanoGpt/NanoGptTranslationEngineTests.cs

using System.Net;
using System.Text;
using System.Text.Json;
using FictionArchive.Service.TranslationService.Models.Enums;
using FictionArchive.Service.TranslationService.Services.TranslationEngines.NanoGpt;
using FictionArchive.Service.TranslationService.Tests.TestSupport;
using FluentAssertions;
using Microsoft.Extensions.Logging.Abstractions;
using Polly;
using Xunit;
using Language = FictionArchive.Common.Enums.Language;

namespace FictionArchive.Service.TranslationService.Tests.NanoGpt;

public class NanoGptTranslationEngineTests
{
    private static (NanoGptTranslationEngine engine, TestHttpMessageHandler handler)
        BuildEngine(Func<HttpRequestMessage, HttpResponseMessage> responder, string modelKey = "nanogpt-test", string modelId = "test-model")
    {
        var handler = new TestHttpMessageHandler(responder);
        var httpClient = new HttpClient(handler) { BaseAddress = new Uri("https://nano-gpt.test/api/") };
        var apiClient = new NanoGptApiClient(httpClient, NullLogger<NanoGptApiClient>.Instance);
        var options = new NanoGptModelOptions { Key = modelKey, DisplayName = "Test", Model = modelId };
        var pipeline = new ResiliencePipelineBuilder().Build(); // no-retry pipeline for deterministic tests
        var engine = new NanoGptTranslationEngine(apiClient, options, pipeline, NullLogger<NanoGptTranslationEngine>.Instance);
        return (engine, handler);
    }

    private static HttpResponseMessage JsonResponse(HttpStatusCode status, string json)
    {
        return new HttpResponseMessage(status)
        {
            Content = new StringContent(json, Encoding.UTF8, "application/json")
        };
    }

    private const string SuccessJson = """
        {
          "choices": [{"message": {"role":"assistant","content":"안녕, 세상."}}],
          "usage": {"prompt_tokens": 10, "completion_tokens": 5, "total_tokens": 15}
        }
        """;

    [Fact]
    public void Descriptor_ReflectsConfiguredOptions()
    {
        var (engine, _) = BuildEngine(_ => JsonResponse(HttpStatusCode.OK, SuccessJson),
            modelKey: "nanogpt-gpt-4o");

        engine.Descriptor.Key.Should().Be("nanogpt-gpt-4o");
        engine.Descriptor.DisplayName.Should().Be("Test");
    }

    [Fact]
    public async Task GetTranslation_OnSuccess_ReturnsTranslatedTextAndUsage()
    {
        var (engine, _) = BuildEngine(_ => JsonResponse(HttpStatusCode.OK, SuccessJson));

        var result = await engine.GetTranslation("Hello, world.", Language.En, Language.Kr);

        result.Status.Should().Be(TranslationRequestStatus.Success);
        result.TranslatedText.Should().Be("안녕, 세상.");
        result.OriginalText.Should().Be("Hello, world.");
        result.From.Should().Be(Language.En);
        result.To.Should().Be(Language.Kr);
        result.TranslationEngineKey.Should().Be("nanogpt-test");
        result.BilledCharacterCount.Should().Be(15u);
    }

    [Fact]
    public async Task GetTranslation_PostsToChatCompletionsEndpoint()
    {
        var (engine, handler) = BuildEngine(_ => JsonResponse(HttpStatusCode.OK, SuccessJson));

        await engine.GetTranslation("Hello", Language.En, Language.Kr);

        handler.Requests.Should().HaveCount(1);
        var request = handler.Requests[0];
        request.Method.Should().Be(HttpMethod.Post);
        request.RequestUri!.AbsoluteUri.Should().Be("https://nano-gpt.test/api/v1/chat/completions");
    }

    [Fact]
    public async Task GetTranslation_SendsConfiguredModelAndTemperature()
    {
        var (engine, handler) = BuildEngine(
            _ => JsonResponse(HttpStatusCode.OK, SuccessJson),
            modelId: "chatgpt-4o-latest");

        await engine.GetTranslation("Hello", Language.En, Language.Kr);

        var body = JsonDocument.Parse(handler.RequestBodies[0]).RootElement;
        body.GetProperty("model").GetString().Should().Be("chatgpt-4o-latest");
        body.GetProperty("temperature").GetDouble().Should().Be(0.2);
    }

    [Fact]
    public async Task GetTranslation_WrapsSourceTextInDelimitersAndIncludesSystemPrompt()
    {
        var (engine, handler) = BuildEngine(_ => JsonResponse(HttpStatusCode.OK, SuccessJson));

        await engine.GetTranslation("Hello", Language.En, Language.Kr);

        var body = JsonDocument.Parse(handler.RequestBodies[0]).RootElement;
        var messages = body.GetProperty("messages");
        messages.GetArrayLength().Should().Be(2);

        var system = messages[0];
        system.GetProperty("role").GetString().Should().Be("system");
        var systemContent = system.GetProperty("content").GetString()!;
        systemContent.Should().Contain("English").And.Contain("Korean");
        systemContent.Should().Contain("<source_text>").And.Contain("Output only the translated text");

        var user = messages[1];
        user.GetProperty("role").GetString().Should().Be("user");
        user.GetProperty("content").GetString().Should().Be("<source_text>\nHello\n</source_text>");
    }

    [Theory]
    [InlineData(Language.En, "English")]
    [InlineData(Language.Kr, "Korean")]
    [InlineData(Language.Ch, "Chinese")]
    [InlineData(Language.Ja, "Japanese")]
    public async Task GetTranslation_RendersExpectedLanguageNamesInSystemPrompt(Language to, string expectedName)
    {
        var (engine, handler) = BuildEngine(_ => JsonResponse(HttpStatusCode.OK, SuccessJson));

        await engine.GetTranslation("Hello", Language.En, to);

        var body = JsonDocument.Parse(handler.RequestBodies[0]).RootElement;
        var systemContent = body.GetProperty("messages")[0].GetProperty("content").GetString()!;
        systemContent.Should().Contain(expectedName);
    }

    [Fact]
    public async Task GetTranslation_On4xx_ReturnsFailedResultWithoutThrowing()
    {
        var (engine, _) = BuildEngine(_ => JsonResponse(HttpStatusCode.BadRequest,
            """{"error":"bad model"}"""));

        var result = await engine.GetTranslation("Hello", Language.En, Language.Kr);

        result.Status.Should().Be(TranslationRequestStatus.Failed);
        result.TranslatedText.Should().BeNull();
        result.TranslationEngineKey.Should().Be("nanogpt-test");
        result.OriginalText.Should().Be("Hello");
    }

    [Fact]
    public async Task GetTranslation_On5xx_ReturnsFailedResultWithoutThrowing()
    {
        var (engine, _) = BuildEngine(_ => JsonResponse(HttpStatusCode.InternalServerError, "{}"));

        var result = await engine.GetTranslation("Hello", Language.En, Language.Kr);

        result.Status.Should().Be(TranslationRequestStatus.Failed);
        result.TranslatedText.Should().BeNull();
    }

    [Fact]
    public async Task GetTranslation_OnMalformedJson_ReturnsFailedResult()
    {
        var (engine, _) = BuildEngine(_ => JsonResponse(HttpStatusCode.OK, "not json"));

        var result = await engine.GetTranslation("Hello", Language.En, Language.Kr);

        result.Status.Should().Be(TranslationRequestStatus.Failed);
        result.TranslatedText.Should().BeNull();
    }

    [Fact]
    public async Task GetTranslation_OnEmptyChoices_ReturnsFailedResult()
    {
        const string emptyChoicesJson = """
            {
              "choices": [],
              "usage": {"prompt_tokens": 0, "completion_tokens": 0, "total_tokens": 0}
            }
            """;
        var (engine, _) = BuildEngine(_ => JsonResponse(HttpStatusCode.OK, emptyChoicesJson));

        var result = await engine.GetTranslation("Hello", Language.En, Language.Kr);

        result.Status.Should().Be(TranslationRequestStatus.Failed);
        result.TranslatedText.Should().BeNull();
    }
}