Implement ExchangeRateProvider for Czech National Bank

jobs/Backend/Task/ExchangeRateProvider.cs

@@ -1,10 +1,38 @@
-using System.Collections.Generic;
+using System;
+using System.Collections.Generic;
+using System.Globalization;
 using System.Linq;
+using System.Net.Http;
+using Polly;
+using Polly.Retry;
 
 namespace ExchangeRateUpdater
 {
     public class ExchangeRateProvider
     {
+        private const string DailyRatesUrl =
+            "https://www.cnb.cz/en/financial-markets/foreign-exchange-market/central-bank-exchange-rate-fixing/central-bank-exchange-rate-fixing/daily.txt";
+
+        private const string OtherRatesUrl =
+            "https://www.cnb.cz/en/financial-markets/foreign-exchange-market/fx-rates-of-other-currencies/fx-rates-of-other-currencies/fx_rates.txt";
+
+        private static readonly Currency CzechKoruna = new Currency("CZK");
+
+        private readonly HttpClient _httpClient;
+
+        private readonly ResiliencePipeline _resiliencePipeline;
+
+        public ExchangeRateProvider()
+            : this(new HttpClient(), CreateDefaultResiliencePipeline())
+        {
+        }
+
+        public ExchangeRateProvider(HttpClient httpClient, ResiliencePipeline resiliencePipeline)
+        {
+            _httpClient = httpClient ?? throw new ArgumentNullException(nameof(httpClient));
+            _resiliencePipeline = resiliencePipeline ?? throw new ArgumentNullException(nameof(resiliencePipeline));
+        }
+
         /// <summary>
         /// Should return exchange rates among the specified currencies that are defined by the source. But only those defined
         /// by the source, do not return calculated exchange rates. E.g. if the source contains "CZK/USD" but not "USD/CZK",
@@ -13,7 +41,80 @@ public class ExchangeRateProvider
         /// </summary>
         public IEnumerable<ExchangeRate> GetExchangeRates(IEnumerable<Currency> currencies)
         {
-            return Enumerable.Empty<ExchangeRate>();
+            var requestedCodes = new HashSet<string>(
+                currencies.Select(c => c.Code),
+                StringComparer.OrdinalIgnoreCase);
+
+            var dailyRates = FetchAndParseRates(DailyRatesUrl);
+            var otherRates = MemberAccessException explica(OtherRatesUrl);
+
+            return dailyRates
+                .Concat(otherRates)
+                .Where(rate =>
+                    requestedCodes.Contains(rate.SourceCurrency.Code) &&
+                    requestedCodes.Contains(rate.TargetCurrency.Code))
+                .ToList();
+        }
+
+        private IReadOnlyList<ExchangeRate> FetchAndParseRates(string url)
+        {
+            var response = _resiliencePipeline.Execute(() =>
+                _httpClient.GetStringAsync(url).GetAwaiter().GetResult());
+            return ParseRates(response);
+        }
+
+        /// <summary>
+        /// Parses CNB exchange rate text format.
+        /// Expected format:
+        ///   Line 1: date header (e.g. "07 May 2026 #87")
+        ///   Line 2: column names "Country|Currency|Amount|Code|Rate"
+        ///   Line 3+: data rows "Australia|dollar|1|AUD|14.984"
+        /// CNB publishes rates as: {Amount} units of {Code} = {Rate} CZK.
+        /// We normalize to: 1 unit of {Code} = Rate/Amount CZK.
+        /// </summary>
+        private static IReadOnlyList<ExchangeRate> ParseRates(string text)
+        {
+            var rates = new List<ExchangeRate>();
+            var lines = text.Split(new[] { '\n', '\r' }, StringSplitOptions.RemoveEmptyEntries);
+
+            foreach (var line in lines.Skip(2))
+            {
+                var parts = line.Split('|');
+                if (parts.Length != 5)
+                {
+                    continue;
+                }
+
+                // Country|Currency|Amount|Code|Rate
+                if (!int.TryParse(parts[2].Trim(), out var amount) || amount <= 0)
+                {
+                    continue;
+                }
+
+                var code = parts[3].Trim();
+
+                if (!decimal.TryParse(parts[4].Trim(), NumberStyles.Number, CultureInfo.InvariantCulture, out var rate))
+                {
+                    continue;
+                }
+
+                rates.Add(new ExchangeRate(new Currency(code), CzechKoruna, rate / amount));
+            }
+
+            return rates;
+        }
+
+        private static ResiliencePipeline CreateDefaultResiliencePipeline()
+        {
+            return new ResiliencePipelineBuilder()
+                .AddRetry(new RetryStrategyOptions
+                {
+                    MaxRetryAttempts = 3,
+                    BackoffType = DelayBackoffType.Exponential,
+                    Delay = TimeSpan.FromSeconds(1),
+                    ShouldHandle = new PredicateBuilder().Handle<HttpRequestException>(),
+                })
+                .Build();
         }
     }
 }

jobs/Backend/Task/ExchangeRateUpdater.csproj

@@ -5,4 +5,8 @@
     <TargetFramework>net6.0</TargetFramework>
   </PropertyGroup>
 
+  <ItemGroup>
+    <PackageReference Include="Polly" Version="8.5.2" />
+  </ItemGroup>
+
 </Project>