AssetManager.API/AssetManager.Infrastructure/Services/ExchangeRateService.cs

using Microsoft.Extensions.Caching.Memory;
using Microsoft.Extensions.Logging;
using System.Net.Http.Json;

namespace AssetManager.Infrastructure.Services;

/// <summary>
/// 真实汇率服务，接入 exchangerate-api.com 免费接口
/// </summary>
public class ExchangeRateService : IExchangeRateService
{
    private readonly HttpClient _httpClient;
    private readonly IMemoryCache _cache;
    private readonly ILogger<ExchangeRateService> _logger;
    
    // 缓存键前缀
    private const string CacheKeyPrefix = "ExchangeRate_";
    // 缓存过期时间：1小时
    private const int CacheExpirationHours = 1;
    // 基础汇率币种（用USD作为中间货币，减少API调用次数）
    private const string BaseCurrency = "USD";

    public ExchangeRateService(
        HttpClient httpClient, 
        IMemoryCache cache,
        ILogger<ExchangeRateService> logger)
    {
        _httpClient = httpClient;
        _cache = cache;
        _logger = logger;
    }

    public async Task<decimal> GetExchangeRateAsync(string fromCurrency, string toCurrency)
    {
        _logger.LogInformation("获取汇率: {FromCurrency} -> {ToCurrency}", fromCurrency, toCurrency);

        // 同币种直接返回1
        if (fromCurrency.Equals(toCurrency, StringComparison.OrdinalIgnoreCase))
        {
            return 1.00m;
        }

        // 先尝试从缓存获取
        string cacheKey = $"{CacheKeyPrefix}{fromCurrency}_{toCurrency}";
        if (_cache.TryGetValue(cacheKey, out decimal cachedRate))
        {
            _logger.LogDebug("缓存命中: {Key} = {Rate}", cacheKey, cachedRate);
            return cachedRate;
        }

        try
        {
            // 使用免费的开放汇率接口，无需API key
            var response = await _httpClient.GetFromJsonAsync<OpenExchangeRateResponse>($"https://open.er-api.com/v6/latest/{BaseCurrency}");
            
            if (response == null || response.Rates == null || response.Result != "success")
            {
                _logger.LogError("汇率API调用失败: {Message}", response?.ErrorType ?? "未知错误");
                throw new Exception("汇率服务暂时不可用");
            }

            // 转换为USD -> 目标币种的汇率字典
            var usdRates = response.Rates;

            // 计算 from -> to 的汇率：(1 from = x USD) * (1 USD = y to) = x*y to
            decimal fromToUsd = 1 / usdRates[fromCurrency.ToUpper()];
            decimal usdToTo = usdRates[toCurrency.ToUpper()];
            decimal rate = fromToUsd * usdToTo;

            // 存入缓存
            var cacheOptions = new MemoryCacheEntryOptions
            {
                AbsoluteExpirationRelativeToNow = TimeSpan.FromHours(CacheExpirationHours)
            };
            _cache.Set(cacheKey, rate, cacheOptions);

            // 同时缓存反向汇率
            string reverseCacheKey = $"{CacheKeyPrefix}{toCurrency}_{fromCurrency}";
            _cache.Set(reverseCacheKey, 1 / rate, cacheOptions);

            _logger.LogInformation("汇率获取成功: {FromCurrency} -> {ToCurrency} = {Rate}", fromCurrency, toCurrency, rate);
            return rate;
        }
        catch (Exception ex)
        {
            _logger.LogError(ex, "获取汇率失败: {FromCurrency} -> {ToCurrency}", fromCurrency, toCurrency);
            // 失败时降级到Mock汇率
            return await GetMockFallbackRate(fromCurrency, toCurrency);
        }
    }

    public async Task<decimal> ConvertAmountAsync(decimal amount, string fromCurrency, string toCurrency)
    {
        _logger.LogInformation("转换金额: {Amount} {FromCurrency} -> {ToCurrency}", amount, fromCurrency, toCurrency);

        if (fromCurrency == toCurrency)
        {
            return amount;
        }

        var rate = await GetExchangeRateAsync(fromCurrency, toCurrency);
        return amount * rate;
    }

    /// <summary>
    /// 失败降级到Mock汇率
    /// </summary>
    private Task<decimal> GetMockFallbackRate(string fromCurrency, string toCurrency)
    {
        _logger.LogWarning("使用Mock汇率降级: {FromCurrency} -> {ToCurrency}", fromCurrency, toCurrency);
        
        var mockRates = new Dictionary<string, decimal>
        {
            { "CNY-USD", 0.14m },
            { "USD-CNY", 7.10m },
            { "CNY-HKD", 1.09m },
            { "HKD-CNY", 0.92m },
            { "USD-HKD", 7.75m },
            { "HKD-USD", 0.13m }
        };

        string key = $"{fromCurrency.ToUpper()}-{toCurrency.ToUpper()}";
        return Task.FromResult(mockRates.TryGetValue(key, out var rate) ? rate : 1.00m);
    }
}

/// <summary>
/// 汇率API响应模型（exchangerate-api.com）
/// </summary>
public class ExchangeRateResponse
{
    public string Result { get; set; } = string.Empty;
    public string ErrorType { get; set; } = string.Empty;
    public Dictionary<string, decimal> ConversionRates { get; set; } = new();
}

/// <summary>
/// 开放汇率API响应模型（er-api.com）
/// </summary>
public class OpenExchangeRateResponse
{
    public string Result { get; set; } = string.Empty;
    public string ErrorType { get; set; } = string.Empty;
    public Dictionary<string, decimal> Rates { get; set; } = new();
}