From 000f9f982481d3fc460755f0fd484bcc614f9d1f Mon Sep 17 00:00:00 2001
From: Divyam Bhagchandani <divyambhagchandani@gmail.com>
Date: Mon, 10 Nov 2025 00:52:55 +0100
Subject: [PATCH] Implement CNB exchange rate provider

---
 jobs/Backend/Task/ExchangeRateProvider.cs | 148 +++++++++++++++++++++-
 1 file changed, 141 insertions(+), 7 deletions(-)

diff --git a/jobs/Backend/Task/ExchangeRateProvider.cs b/jobs/Backend/Task/ExchangeRateProvider.cs
index 6f82a97fbe..1eb16adee1 100644
--- a/jobs/Backend/Task/ExchangeRateProvider.cs
+++ b/jobs/Backend/Task/ExchangeRateProvider.cs
@@ -1,19 +1,153 @@
-using System.Collections.Generic;
+using System;
+using System.Collections.Generic;
 using System.Linq;
+using System.Net.Http;
+using System.Xml.Linq;
+using System.Xml;
+using System.Globalization;
+
+
+
+
 
 namespace ExchangeRateUpdater
 {
     public class ExchangeRateProvider
     {
+        // Czech National Bank XML endpoint for daily exchange rates
+        private static readonly string BankApiUrl = 
+            "https://www.cnb.cz/cs/financni_trhy/devizovy_trh/kurzy_devizoveho_trhu/denni_kurz.xml";
+        
+        
+        private const int TimeoutSeconds = 10;
+        private const string TargetCurrency = "CZK";
+
+        private static readonly HttpClient _httpClient;
+        
+        static ExchangeRateProvider()
+        {
+            _httpClient = new HttpClient();
+            _httpClient.Timeout = TimeSpan.FromSeconds(TimeoutSeconds);
+        }
+
+
+
         /// <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",
-        /// do not return exchange rate "USD/CZK" with value calculated as 1 / "CZK/USD". If the source does not provide
-        /// some of the currencies, ignore them.
+        /// This method returns exchange rates among the specified currencies that are defined by the source.
+        /// Provides exchange rates from Czech National Bank's public API.
         /// </summary>
         public IEnumerable<ExchangeRate> GetExchangeRates(IEnumerable<Currency> currencies)
         {
-            return Enumerable.Empty<ExchangeRate>();
+            if (currencies == null)
+            {
+                throw new ArgumentNullException(nameof(currencies), "Currency collection cannot be null");
+            }
+            var currencyList = currencies.ToList();
+            if (!currencyList.Any())
+            {
+                return Enumerable.Empty<ExchangeRate>(); 
+            }
+            
+            var exchangeRatesDict = FetchandParseBankData();
+            return GetRequestedCurrenciesExchangeRate(exchangeRatesDict, currencies);
+        }
+        
+
+        private List<ExchangeRate> GetRequestedCurrenciesExchangeRate(Dictionary<string, decimal> allExchangeRates, IEnumerable<Currency> requestedCurrencies)
+        {
+            var requestedCurrenciesExchangeRate = new List<ExchangeRate>();
+            // Used to keep track of the processed currencies and prevent duplicates
+            var seenCurrencies = new HashSet<string>(); 
+
+            foreach (var currency in requestedCurrencies)
+            {
+                if (currency == null)
+                {
+                    continue;
+                }
+
+                string sourceCurrency = currency.Code;
+
+                if (!seenCurrencies.Add(sourceCurrency))
+                {
+                    continue;
+                }
+                
+                if (allExchangeRates.ContainsKey(sourceCurrency))
+                {
+                    requestedCurrenciesExchangeRate.Add(new ExchangeRate(
+                        new Currency(sourceCurrency),
+                        new Currency(TargetCurrency),
+                        allExchangeRates[sourceCurrency]
+                    ));
+                }
+            }
+            return requestedCurrenciesExchangeRate;
+
+        }
+
+        private Dictionary<string, decimal> FetchandParseBankData()
+        {
+            try{
+
+            LogInfo("Fetching exchange rates provided currencies");
+
+            var response = _httpClient.GetAsync(BankApiUrl).Result;
+            response.EnsureSuccessStatusCode();
+            var xmlBankData = response.Content.ReadAsStringAsync().Result;
+
+            var parsedXmlData = XDocument.Parse(xmlBankData);
+            
+            Dictionary<string, decimal> exchangeRates = parsedXmlData.Descendants("radek")
+            .Where(x => x.Attribute("kod") != null && 
+                        x.Attribute("kurz") != null && 
+                        x.Attribute("mnozstvi") != null)
+            .ToDictionary(
+                x => x.Attribute("kod").Value,
+                x => 
+                {
+                    // // Use Czech culture for decimal parsing
+                    decimal kurz = decimal.Parse(x.Attribute("kurz").Value, CultureInfo.GetCultureInfo("cs-CZ"));
+                    decimal mnozstvi = decimal.Parse(x.Attribute("mnozstvi").Value);
+                    if (mnozstvi == 0)
+                    {
+                        throw new InvalidOperationException($"Invalid mnozstvi (0) for currency {x.Attribute("kod").Value}");
+                    }
+                    return kurz / mnozstvi;
+                }
+            );
+            return exchangeRates;
+            }
+             catch (XmlException ex)
+            {
+                LogError("Failed to parse XML from the Bank", ex);
+                throw;
+            }
+            catch (FormatException ex)
+            {
+                LogError("Invalid number format in Bank data", ex);
+                throw;
+            }
+            catch (Exception ex)
+            {
+                LogError("Failed to fetch and parse CNB data", ex);
+                throw;
+            }
+        }
+
+    private void LogError(string message, Exception ex = null)
+    {
+        Console.Error.WriteLine($"[ERROR] {message}");
+        if (ex != null)
+        {
+            Console.Error.WriteLine($"Exception: {ex.Message}");
         }
     }
-}
+
+    private void LogInfo(string message)
+    {
+        Console.WriteLine($"[INFO] {message}");
+    }
+
+    }
+}
\ No newline at end of file