DeutschLernen/GermanApp/Application/Services/MistralService.cs

using GermanApp.Application.Models;
using GermanApp.Domain.Interfaces;
using GermanApp.Infrastructure.Configuration;
using Microsoft.Extensions.Options;

namespace GermanApp.Application.Services;

/// <summary>
/// Application service for Mistral AI text generation.
/// This is part of the Application layer.
/// </summary>
public class MistralService : IMistralService
{
    private readonly IMistralConnector _connector;
    private readonly MistralConfig _config;

    public MistralService(
        IMistralConnector connector,
        IOptions<MistralConfig> config)
    {
        _connector = connector;
        _config = config.Value ?? new MistralConfig();
    }

    /// <summary>
    /// Generates text from a prompt using Mistral API.
    /// </summary>
    public virtual async Task<string> GenerateTextAsync(
        string prompt,
        string? model = null,
        float temperature = 0.7f,
        int? maxTokens = null,
        CancellationToken cancellationToken = default)
    {
        model ??= _config.DefaultModel;
        maxTokens ??= 500;

        var request = new MistralRequest
        {
            Model = model,
            Prompt = prompt,
            Temperature = temperature,
            MaxTokens = maxTokens.Value
        };

        var response = await _connector.CompleteAsync(request, cancellationToken);
        
        if (response.Choices == null || response.Choices.Count == 0)
            throw new InvalidOperationException("No choices returned from Mistral API");

        return response.Choices[0].Text ?? string.Empty;
    }

    /// <summary>
    /// Generates text from chat messages using Mistral API.
    /// </summary>
    public virtual async Task<string> GenerateChatAsync(
        IReadOnlyList<(string role, string content)> messages,
        string? model = null,
        float temperature = 0.7f,
        int? maxTokens = null,
        CancellationToken cancellationToken = default)
    {
        model ??= _config.DefaultModel;
        maxTokens ??= 500;

        var chatMessages = messages
            .Select(m => new MistralMessage { Role = m.role, Content = m.content })
            .ToList();

        var request = new MistralChatRequest
        {
            Model = model,
            Messages = chatMessages,
            Temperature = temperature,
            MaxTokens = maxTokens.Value
        };

        var response = await _connector.ChatAsync(request, cancellationToken);
        
        if (response.Choices == null || response.Choices.Count == 0)
            throw new InvalidOperationException("No choices returned from Mistral API");

        return response.Choices[0].Message?.Content ?? string.Empty;
    }

    /// <summary>
    /// Generates a story based on lesson context and vocabulary.
    /// </summary>
    public virtual async Task<string> GenerateStoryAsync(
        string level,
        string topic,
        IReadOnlyList<string> vocabularyWords,
        int length = 200,
        CancellationToken cancellationToken = default)
    {
        var prompt = BuildStoryPrompt(level, topic, vocabularyWords, length);
        
        return await GenerateTextAsync(
            prompt,
            model: _config.DefaultModel,
            temperature: 0.8f,
            maxTokens: 1000,
            cancellationToken);
    }

    /// <summary>
    /// Provides feedback on user writing.
    /// </summary>
    public virtual async Task<string> GenerateWritingFeedbackAsync(
        string userText,
        string level,
        string? prompt = null,
        CancellationToken cancellationToken = default)
    {
        var messages = new List<(string role, string content)>
        {
            ("system", BuildFeedbackSystemPrompt(level)),
            ("user", prompt ?? "Please provide feedback on the following German text:"),
            ("user", userText)
        };

        return await GenerateChatAsync(
            messages,
            model: _config.DefaultModel,
            temperature: 0.3f, // Lower temperature for more deterministic feedback
            maxTokens: 800,
            cancellationToken);
    }

    /// <summary>
    /// Tests the Mistral API connection.
    /// </summary>
    public virtual async Task<bool> TestConnectionAsync(CancellationToken cancellationToken = default)
    {
        try
        {
            // Send a simple test prompt
            var testPrompt = "Say 'test successful'";
            var result = await GenerateTextAsync(
                testPrompt,
                maxTokens: 10,
                cancellationToken: cancellationToken);
            
            return result.Contains("test successful", System.StringComparison.OrdinalIgnoreCase);
        }
        catch
        {
            return false;
        }
    }

    /// <summary>
    /// Builds a prompt for story generation.
    /// </summary>
    private string BuildStoryPrompt(string level, string topic, IReadOnlyList<string> vocabularyWords, int length)
    {
        var vocabularyList = string.Join(", ", vocabularyWords);
        
        return $"You are a helpful German language teacher. Create an engaging story for a {level} level learner.\n\nRequirements:\n- Topic: {topic}\n- Length: approximately {length} words\n- Use these German vocabulary words: {vocabularyList}\n- Write in German language\n- Appropriate for A1-A2 learners (simple sentences, common vocabulary)\n- Include dialogue\n- End with a question for the reader\n\nWrite only the story text, no additional explanation or formatting."
        .Replace("\n\n", "\n");
    }

    /// <summary>
    /// Builds a system prompt for writing feedback.
    /// </summary>
    private string BuildFeedbackSystemPrompt(string level)
    {
        return $"You are a helpful German language tutor. Provide constructive feedback on the user's German writing.\n\nGuidelines:\n- Respond in English\n- First, identify and correct any grammar mistakes\n- Then, provide suggestions for improvement\n- Finally, give encouragement\n- Be specific and helpful\n- Keep feedback concise (3-5 sentences)\n- Level: {level}"
        .Replace("\n\n", "\n");
    }
}