DeutschLernen/GermanApp/Infrastructure/Services/MistralConnector.cs

using System.Net;
using System.Text;
using System.Text.Json;
using GermanApp.Application.Models;
using GermanApp.Domain.Interfaces;
using GermanApp.Infrastructure.Configuration;
using Microsoft.Extensions.Caching.Memory;
using Microsoft.Extensions.Logging;

namespace GermanApp.Infrastructure.Services;

/// <summary>
/// HTTP client implementation for Mistral API.
/// Implements IMistralConnector from Domain layer.
/// </summary>
public class MistralConnector : IMistralConnector
{
    private readonly HttpClient _httpClient;
    private readonly MistralConfig _config;
    private readonly ILogger<MistralConnector> _logger;
    private readonly MistralRateLimiter _rateLimiter;
    private readonly IMemoryCache _cache;
    private readonly MistralCircuitBreaker _circuitBreaker;
    private readonly JsonSerializerOptions _jsonOptions;

    public MistralConnector(HttpClient httpClient, MistralConfig config,
        ILogger<MistralConnector> logger, IMemoryCache cache)
    {
        _httpClient = httpClient;
        _config = config;
        _logger = logger;
        _cache = cache;
        _config.Validate();

        _httpClient.BaseAddress = new Uri(_config.BaseUrl);
        _httpClient.Timeout = TimeSpan.FromSeconds(_config.TimeoutSeconds);
        _httpClient.DefaultRequestHeaders.Add("Authorization", $"Bearer {_config.ApiKey}");
        _httpClient.DefaultRequestHeaders.Accept.Add(new System.Net.Http.Headers.MediaTypeWithQualityHeaderValue("application/json"));

        _jsonOptions = new JsonSerializerOptions
        {
            PropertyNameCaseInsensitive = true,
            PropertyNamingPolicy = JsonNamingPolicy.CamelCase
        };

        _rateLimiter = new MistralRateLimiter(_config.RateLimitPerMinute);
        _circuitBreaker = new MistralCircuitBreaker(
            _config.CircuitBreakerFailureThreshold,
            TimeSpan.FromMinutes(_config.CircuitBreakerResetMinutes));
    }

    public async Task<MistralResponse> CompleteAsync(MistralRequest request,
        CancellationToken cancellationToken = default)
    {
        return await ExecuteRequestAsync<MistralRequest, MistralResponse>(
            "completions", request, cancellationToken);
    }

    public async Task<MistralResponse> ChatAsync(MistralChatRequest request,
        CancellationToken cancellationToken = default)
    {
        return await ExecuteRequestAsync<MistralChatRequest, MistralResponse>(
            "chat/completions", request, cancellationToken);
    }

    public async Task<IReadOnlyList<MistralModel>> ListModelsAsync(
        CancellationToken cancellationToken = default)
    {
        CheckCircuitAndRateLimit("models");
        try
        {
            var response = await ExecuteWithRetryAsync(async () =>
            {
                var httpResponse = await _httpClient.GetAsync("models", cancellationToken);
                return await HandleResponseAsync<MistralListModelsResponse>(httpResponse);
            }, "ListModels");
            _circuitBreaker.RecordSuccess();
            return response.Data;
        }
        catch (Exception ex)
        {
            HandleRequestError(ex, "ListModels");
            throw;
        }
    }

    public async Task<MistralModel> GetModelAsync(string modelId,
        CancellationToken cancellationToken = default)
    {
        if (string.IsNullOrWhiteSpace(modelId))
            throw new ArgumentException("Model ID cannot be null or empty", nameof(modelId));
        CheckCircuitAndRateLimit("models");
        try
        {
            var response = await ExecuteWithRetryAsync(async () =>
            {
                var httpResponse = await _httpClient.GetAsync($"models/{modelId}", cancellationToken);
                return await HandleResponseAsync<MistralModel>(httpResponse);
            }, "GetModel");
            _circuitBreaker.RecordSuccess();
            return response;
        }
        catch (Exception ex)
        {
            HandleRequestError(ex, "GetModel");
            throw;
        }
    }

    private async Task<TResponse> ExecuteRequestAsync<TRequest, TResponse>(
        string endpoint, TRequest request, CancellationToken cancellationToken)
        where TResponse : class
    {
        CheckCircuitAndRateLimit(endpoint);
        
        // Generate cache key for caching
        var cacheKey = GenerateCacheKey(request);
        if (_config.EnableCaching && _cache.TryGetValue(cacheKey, out TResponse cachedResponse))
        {
            _logger.LogDebug("Cache hit for {Endpoint}", endpoint);
            return cachedResponse!;
        }

        try
        {
            var response = await ExecuteWithRetryAsync(async () =>
            {
                var json = JsonSerializer.Serialize(request, _jsonOptions);
                var content = new StringContent(json, Encoding.UTF8, "application/json");
                var httpResponse = await _httpClient.PostAsync(endpoint, content, cancellationToken);
                return await HandleResponseAsync<TResponse>(httpResponse);
            }, endpoint);

            _circuitBreaker.RecordSuccess();
            
            // Cache the response if caching is enabled
            if (_config.EnableCaching && response != null)
            {
                _cache.Set(cacheKey, response, TimeSpan.FromMinutes(_config.CacheTTLMinutes));
            }
            
            return response;
        }
        catch (Exception ex)
        {
            HandleRequestError(ex, endpoint);
            throw;
        }
    }

    private void CheckCircuitAndRateLimit(string endpoint)
    {
        if (!_circuitBreaker.IsClosed)
            throw new AiServiceException("Mistral API circuit breaker is open",
                AiErrorCode.ServiceUnavailable);
        if (!_rateLimiter.TryAcquire(endpoint))
            throw new AiServiceException("Rate limit exceeded",
                AiErrorCode.RateLimited);
    }

    private async Task<T> ExecuteWithRetryAsync<T>(Func<Task<T>> action, string operationName)
    {
        int retryCount = 0;
        while (true)
        {
            try
            {
                using var cts = new CancellationTokenSource(TimeSpan.FromSeconds(_config.TimeoutSeconds));
                return await action();
            }
            catch (Exception ex) when (retryCount < _config.MaxRetries && IsRetryable(ex))
            {
                retryCount++;
                var delay = TimeSpan.FromSeconds(Math.Pow(2, retryCount));
                _logger.LogWarning(ex, "{Operation} failed (attempt {RetryCount}), retrying in {Delay}s...",
                    operationName, retryCount, delay.TotalSeconds);
                await Task.Delay(delay);
            }
        }
    }

    private bool IsRetryable(Exception ex) => ex switch
    {
        HttpRequestException or TimeoutException or TaskCanceledException => true,
        _ => false
    };

    private async Task<T> HandleResponseAsync<T>(HttpResponseMessage httpResponse)
    {
        if (!httpResponse.IsSuccessStatusCode)
        {
            var errorContent = await httpResponse.Content.ReadAsStringAsync();
            var errorResponse = JsonSerializer.Deserialize<MistralErrorResponse>(errorContent, _jsonOptions);
            
            var errorCode = errorResponse?.ErrorCode ?? MapStatusCodeToErrorCode(httpResponse.StatusCode);
            var message = errorResponse?.Message ?? $"HTTP {(int)httpResponse.StatusCode}: {httpResponse.ReasonPhrase}";
            
            _logger.LogError("Mistral API error: {StatusCode} - {Message}", 
                (int)httpResponse.StatusCode, message);
            
            httpResponse.EnsureSuccessStatusCode(); // This will throw
        }

        var content = await httpResponse.Content.ReadAsStringAsync();
        return JsonSerializer.Deserialize<T>(content, _jsonOptions) 
            ?? throw new AiServiceException("Invalid response from Mistral API",
                AiErrorCode.InvalidResponse);
    }

    private string GenerateCacheKey<T>(T request)
    {
        var json = JsonSerializer.Serialize(request, _jsonOptions);
        using var sha256 = System.Security.Cryptography.SHA256.Create();
        var hashBytes = sha256.ComputeHash(Encoding.UTF8.GetBytes(json));
        return Convert.ToBase64String(hashBytes);
    }

    private AiErrorCode MapStatusCodeToErrorCode(HttpStatusCode statusCode)
    {
        return statusCode switch
        {
            HttpStatusCode.Unauthorized => AiErrorCode.AuthenticationError,
            HttpStatusCode.TooManyRequests => AiErrorCode.RateLimited,
            HttpStatusCode.RequestTimeout => AiErrorCode.Timeout,
            HttpStatusCode.BadRequest => AiErrorCode.InvalidRequest,
            HttpStatusCode.InternalServerError or 
            HttpStatusCode.ServiceUnavailable or
            HttpStatusCode.BadGateway or
            HttpStatusCode.GatewayTimeout => AiErrorCode.Temporary,
            _ => AiErrorCode.Unknown
        };
    }

    private void HandleRequestError(Exception ex, string operationName)
    {
        _circuitBreaker.RecordFailure();
        _logger.LogError(ex, "Mistral API request failed: {Operation}", operationName);
        if (ex is not AiServiceException)
        {
            throw new AiServiceException($"Mistral {operationName} failed: {ex.Message}", 
                ex, AiErrorCode.Temporary);
        }
    }
}