feat: Implement CNB Exchange Rate Provider - Backend job Task

jobs/Backend/Task/Clients/CnpApiClient.cs

@@ -0,0 +1,53 @@
+using ExchangeRateUpdater.Configuration;
+using ExchangeRateUpdater.Exceptions;
+using Microsoft.Extensions.Options;
+using System.Net.Http;
+using System.Threading.Tasks;
+using System.Threading;
+using System;
+using System.Runtime.ConstrainedExecution;
+
+namespace ExchangeRateUpdater.Clients
+{
+    /// <summary>
+    /// Czech National Bank specific implementation of exchange rate API client.
+    /// Fetches data from CNB's daily exchange rate fixing endpoint.
+    /// </summary>
+    public class CnbApiClient : IExchangeRateApiClient
+    {
+        private readonly HttpClient _httpClient;
+        private readonly ExchangeRateApiSettings _settings;
+
+        public CnbApiClient(HttpClient httpClient, IOptions<ExchangeRateApiSettings> settings)
+        {
+            _httpClient = httpClient ?? throw new ArgumentNullException(nameof(httpClient));
+            _settings = settings?.Value ?? throw new ArgumentNullException(nameof(settings));
+
+            _httpClient.Timeout = TimeSpan.FromSeconds(_settings.TimeoutSeconds);
+        }
+
+        public async Task<string> GetDailyRatesAsync(CancellationToken cancellationToken = default)
+        {
+            try
+            {
+                var response = await _httpClient.GetAsync(_settings.DailyRatesUrl, cancellationToken);
+                response.EnsureSuccessStatusCode();
+
+                return await response.Content.ReadAsStringAsync(cancellationToken);
+            }
+            //Consistent error handling regardless of which client implementation is used
+            catch (HttpRequestException ex)
+            {
+                throw new ExchangeRateException(
+                    "Failed to retrieve exchange rates from CNB API.",
+                    ex);
+            }
+            catch (TaskCanceledException ex)
+            {
+                throw new ExchangeRateException(
+                    "Request to CNB API timed out.",
+                    ex);
+            }
+        }
+    }
+}

jobs/Backend/Task/Clients/IExchangeRateApiClient.cs

@@ -0,0 +1,17 @@
+using System.Threading.Tasks;
+using System.Threading;
+
+namespace ExchangeRateUpdater.Clients
+{
+    /// <summary>
+    /// Generic contract for fetching exchange rate data from any source.
+    /// Implementation can target different central banks or financial data providers.
+    /// </summary>
+    public interface IExchangeRateApiClient
+    {
+        /// <summary>
+        /// Fetches raw exchange rate data from the source.
+        /// </summary>
+        Task<string> GetDailyRatesAsync(CancellationToken cancellationToken = default);
+    }
+}

jobs/Backend/Task/Configuration/ExchangeRateApiSettings.cs

@@ -0,0 +1,12 @@
+namespace ExchangeRateUpdater.Configuration
+{
+    /// <summary>
+    /// Configuration settings for exchange rate API access - Generic structure usable for any exchange rate source.
+    /// Compile time binding via IOptions pattern better than raw string keys
+    /// </summary>
+    public class ExchangeRateApiSettings
+    {
+        public string DailyRatesUrl { get; set; } = string.Empty;
+        public int TimeoutSeconds { get; set; } = 30;
+    }
+}

jobs/Backend/Task/Exceptions/ExchangeRateException.cs

@@ -0,0 +1,20 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Runtime.InteropServices;
+using System.Text;
+using System.Threading.Tasks;
+
+namespace ExchangeRateUpdater.Exceptions
+{
+    /// <summary>
+    /// Custom exception for exchange rate operations.
+    /// caller can distinguish domain errors from infrastructure errors
+    /// </summary>
+    public class ExchangeRateException : Exception
+    {
+        public ExchangeRateException(string message) : base(message) { }
+        public ExchangeRateException(string message, Exception innerException)
+            : base(message, innerException) { }
+    }
+}

jobs/Backend/Task/ExchangeRateUpdater.csproj

@@ -5,4 +5,26 @@
     <TargetFramework>net6.0</TargetFramework>
   </PropertyGroup>
 
+  <ItemGroup>
+    <PackageReference Include="Microsoft.Extensions.Configuration" Version="9.0.10" />
+    <PackageReference Include="Microsoft.Extensions.Configuration.Binder" Version="9.0.10" />
+    <PackageReference Include="Microsoft.Extensions.Configuration.EnvironmentVariables" Version="9.0.10" />
+    <PackageReference Include="Microsoft.Extensions.Configuration.Json" Version="9.0.10" />
+    <PackageReference Include="Microsoft.Extensions.DependencyInjection" Version="9.0.10" />
+    <PackageReference Include="Microsoft.Extensions.Http" Version="9.0.10" />
+    <PackageReference Include="Microsoft.Extensions.Options.ConfigurationExtensions" Version="9.0.10" />
+  </ItemGroup>
+
+  <ItemGroup>
+    <None Update="appsettings.Production.json">
+      <CopyToOutputDirectory>Always</CopyToOutputDirectory>
+    </None>
+    <None Update="appsettings.Development.json">
+      <CopyToOutputDirectory>Always</CopyToOutputDirectory>
+    </None>
+    <None Update="appsettings.json">
+      <CopyToOutputDirectory>Always</CopyToOutputDirectory>
+    </None>
+  </ItemGroup>
+
 </Project>

jobs/Backend/Task/ExchangeRateUpdater.sln

@@ -1,9 +1,11 @@
 
 Microsoft Visual Studio Solution File, Format Version 12.00
-# Visual Studio 14
-VisualStudioVersion = 14.0.25123.0
+# Visual Studio Version 17
+VisualStudioVersion = 17.10.35122.118
 MinimumVisualStudioVersion = 10.0.40219.1
-Project("{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}") = "ExchangeRateUpdater", "ExchangeRateUpdater.csproj", "{7B2695D6-D24C-4460-A58E-A10F08550CE0}"
+Project("{9A19103F-16F7-4668-BE54-9A1E7A4F7556}") = "ExchangeRateUpdater", "ExchangeRateUpdater.csproj", "{7B2695D6-D24C-4460-A58E-A10F08550CE0}"
+EndProject
+Project("{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}") = "ExchangeRateUpdater.XUnitTests", "..\ExchangeRateUpdater.XUnitTests\ExchangeRateUpdater.XUnitTests.csproj", "{ABAEC4CE-7FFB-4DAE-AFED-CBCD6D699BF4}"
 EndProject
 Global
 	GlobalSection(SolutionConfigurationPlatforms) = preSolution
@@ -15,8 +17,15 @@ Global
 		{7B2695D6-D24C-4460-A58E-A10F08550CE0}.Debug|Any CPU.Build.0 = Debug|Any CPU
 		{7B2695D6-D24C-4460-A58E-A10F08550CE0}.Release|Any CPU.ActiveCfg = Release|Any CPU
 		{7B2695D6-D24C-4460-A58E-A10F08550CE0}.Release|Any CPU.Build.0 = Release|Any CPU
+		{ABAEC4CE-7FFB-4DAE-AFED-CBCD6D699BF4}.Debug|Any CPU.ActiveCfg = Debug|Any CPU
+		{ABAEC4CE-7FFB-4DAE-AFED-CBCD6D699BF4}.Debug|Any CPU.Build.0 = Debug|Any CPU
+		{ABAEC4CE-7FFB-4DAE-AFED-CBCD6D699BF4}.Release|Any CPU.ActiveCfg = Release|Any CPU
+		{ABAEC4CE-7FFB-4DAE-AFED-CBCD6D699BF4}.Release|Any CPU.Build.0 = Release|Any CPU
 	EndGlobalSection
 	GlobalSection(SolutionProperties) = preSolution
 		HideSolutionNode = FALSE
 	EndGlobalSection
+	GlobalSection(ExtensibilityGlobals) = postSolution
+		SolutionGuid = {7A601B6F-E9B2-4165-8DB1-8AB0DD43684F}
+	EndGlobalSection
 EndGlobal

jobs/Backend/Task/Models/Currency.cs

@@ -0,0 +1,37 @@
+using System;
+
+namespace ExchangeRateUpdater.Models
+{
+    /// <summary>
+    /// Immutable value object representing a currency.
+    /// </summary>
+    public class Currency
+    {
+        //Validate in constructor. Don't allow invalid states.
+        public Currency(string code)
+        {
+            if (string.IsNullOrWhiteSpace(code))
+                throw new ArgumentException("Currency code cannot be null or empty.", nameof(code));
+
+            //Normalize to uppercase (usd : USD) for consistency
+            Code = code.ToUpperInvariant();
+        }
+
+        /// <summary>
+        /// Three-letter ISO 4217 code of the currency.
+        /// </summary>
+        public string Code { get; }
+
+        public override string ToString()
+        {
+            return Code;
+        }
+
+        // Equality based on Code to use it in HashSet/Dictionary
+        public override bool Equals(object? obj) =>
+            obj is Currency other && Code == other.Code;
+
+        // Required when overriding Equals: Ensures Dictionary/HashSet work correctly
+        public override int GetHashCode() => Code.GetHashCode();
+    }
+}

jobs/Backend/Task/Models/ExchangeRate.cs

@@ -0,0 +1,31 @@
+using System;
+using System.Runtime.ConstrainedExecution;
+
+namespace ExchangeRateUpdater.Models
+{
+    /// <summary>
+    /// Immutable value object representing an exchange rate between two currencies.
+    /// </summary>
+    public class ExchangeRate
+    {
+        //validations in constructor
+        public ExchangeRate(Currency sourceCurrency, Currency targetCurrency, decimal value)
+        {
+            SourceCurrency = sourceCurrency ?? throw new ArgumentNullException(nameof(sourceCurrency));
+            TargetCurrency = targetCurrency ?? throw new ArgumentNullException(nameof(targetCurrency));
+
+            if (value <= 0)
+                throw new ArgumentException("Exchange rate value must be positive.", nameof(value));
+
+            Value = value;
+        }
+
+        public Currency SourceCurrency { get; }
+        public Currency TargetCurrency { get; }
+        public decimal Value { get; }
+
+        //consistent output(4 decimal places)
+        public override string ToString() =>
+            $"{SourceCurrency}/{TargetCurrency}={Value:F4}";
+    }
+}

jobs/Backend/Task/Parsers/CnbDataParser.cs

@@ -0,0 +1,86 @@
+using ExchangeRateUpdater.Exceptions;
+using ExchangeRateUpdater.Models;
+using System.Collections.Generic;
+using System.Linq;
+using System;
+
+namespace ExchangeRateUpdater.Parsers
+{
+    /// <summary>
+    /// Parses CNB daily exchange rates text format.
+    /// Format: Country|Currency|Amount|Code|Rate
+    /// Example: USA|dollar|1|USD|24.123
+    /// </summary>
+    public class CnbDataParser : IExchangeRateDataParser
+    {
+        private const int ExpectedColumnCount = 5;
+        private const int AmountIndex = 2;
+        private const int CodeIndex = 3;
+        private const int RateIndex = 4;
+        private static readonly Currency CzkCurrency = new("CZK");
+
+        public IEnumerable<ExchangeRate> Parse(string rawData, IEnumerable<Currency> targetCurrencies)
+        {
+            if (string.IsNullOrWhiteSpace(rawData))
+                throw new ExchangeRateException("CNB data is empty.");
+
+            // enhances performance and ensures that the filtering process is accurate and efficient.
+            // HashSet for O(1) lookup performance instead of O(n) with List.Contains--> Checking if currency is requested happens for every parsed line
+            var currencyCodesSet = new HashSet<string>(
+                targetCurrencies.Select(c => c.Code),
+                StringComparer.OrdinalIgnoreCase);
+
+            //Parse all lines and then filter
+            return ParseLines(rawData)
+                .Where(rate => currencyCodesSet.Contains(rate.SourceCurrency.Code))
+                .ToList();
+        }
+
+        private IEnumerable<ExchangeRate> ParseLines(string rawData)
+        {
+            var lines = rawData.Split(new[] { '\r', '\n' }, StringSplitOptions.RemoveEmptyEntries);
+
+            // first 2 lines: date and column headers
+            return lines.Skip(2)
+                .Select(ParseLine)
+                .Where(rate => rate != null)
+                .Cast<ExchangeRate>();
+        }
+
+        private ExchangeRate? ParseLine(string line)
+        {
+            try
+            {
+                var columns = line.Split('|');
+
+                if (columns.Length != ExpectedColumnCount)
+                    return null;
+
+                var currencyCode = columns[CodeIndex].Trim();
+                var amount = ParseDecimal(columns[AmountIndex]);
+                var rate = ParseDecimal(columns[RateIndex]);
+
+                // CNB rates are: {amount} {currency} = {rate} CZK
+                // We want: 1 {currency} = {rate/amount} CZK. We normalize it to 1 unit.
+                var normalizedRate = rate / amount;
+
+                return new ExchangeRate(
+                    sourceCurrency: new Currency(currencyCode),
+                    targetCurrency: CzkCurrency,
+                    value: normalizedRate);
+            }
+            catch
+            {
+                // Skip malformed lines
+                return null;
+            }
+        }
+
+        private decimal ParseDecimal(string value)
+        {
+            // CNB uses comma as decimal separator
+            var normalized = value.Trim().Replace(',', '.');
+            return decimal.Parse(normalized, System.Globalization.CultureInfo.InvariantCulture);
+        }
+    }
+}