Attempt at solving the C# home assignment

jobs/Backend/ExchangeRateUpdater.Tests/ExchangeRateUpdater.Tests.csproj

@@ -0,0 +1,23 @@
+<Project Sdk="Microsoft.NET.Sdk">
+
+  <PropertyGroup>
+    <TargetFramework>net6.0</TargetFramework>
+
+    <IsPackable>false</IsPackable>
+  </PropertyGroup>
+
+  <ItemGroup>
+    <PackageReference Include="JetBrains.Annotations" Version="2024.2.0" />
+    <PackageReference Include="Microsoft.NET.Test.Sdk" Version="17.10.0" />
+    <PackageReference Include="xunit" Version="2.9.0" />
+    <PackageReference Include="xunit.runner.visualstudio" Version="2.8.2">
+      <PrivateAssets>all</PrivateAssets>
+      <IncludeAssets>runtime; build; native; contentfiles; analyzers; buildtransitive</IncludeAssets>
+    </PackageReference>
+  </ItemGroup>
+
+  <ItemGroup>
+    <ProjectReference Include="..\Task\ExchangeRateUpdater.csproj" />
+  </ItemGroup>
+
+</Project>

jobs/Backend/ExchangeRateUpdater.Tests/Ext/CnbFxRowTest.cs

@@ -0,0 +1,32 @@
+using System;
+using ExchangeRateUpdater.Model;
+using JetBrains.Annotations;
+using Xunit;
+
+namespace ExchangeRateUpdater.Tests.Ext;
+
+[TestSubject(typeof(CnbFxRow))]
+public class CnbFxRowTest
+{
+
+    [Fact]
+    public void ThrowsIfNot5Parts()
+    {
+        var testStr = "Brazílie|real|1|BRL";
+        Assert.Throws<ArgumentException>(() => CnbFxRow.FromSeparatedString(testStr, '|'));
+    }
+
+
+    [Fact]
+    public void ParsesDelimitedString()
+    {
+        var testStr = "Brazílie|real|1|BRL|4,283";
+        var result = CnbFxRow.FromSeparatedString(testStr, '|');
+
+        Assert.Equal("Brazílie", result.Country);
+        Assert.Equal("real", result.CurrencyStr);
+        Assert.Equal(1, result.Amount);
+        Assert.Equal(new Currency("BRL"), result.Currency);
+        Assert.Equal(4.283, (double) result.Rate);
+    }
+}

jobs/Backend/ExchangeRateUpdater.Tests/Ext/CnbTxtParserTest.cs

@@ -0,0 +1,23 @@
+using ExchangeRateUpdater.Model;
+using JetBrains.Annotations;
+using Xunit;
+
+namespace ExchangeRateUpdater.Tests.Ext;
+
+[TestSubject(typeof(CnbTxtParser))]
+public class CnbTxtParserTest
+{
+
+    [Fact]
+    public void CanParseResponse()
+    {
+        var response = @"15.07.2024 #135
+                       země|měna|množství|kód|kurz
+                       Austrálie|dolar|1|AUD|15,800
+                       Brazílie|real|1|BRL|4,283
+                       ";
+
+        var result = CnbTxtParser.ParseResponse(response);
+        Assert.Equal(2, result.Count);
+    }
+}

jobs/Backend/ExchangeRateUpdater.Tests/Program.cs

@@ -0,0 +1,10 @@
+namespace ExchangeRateUpdater.Tests;
+
+// I don't know why I need this class, but I cannot run tests without it
+public class Program
+{
+    public static void Main()
+    {
+
+    }
+}

jobs/Backend/Task/CurrencyRateClient.cs

@@ -0,0 +1,18 @@
+using System;
+using System.Collections.Generic;
+using System.Threading.Tasks;
+using ExchangeRateUpdater.Model;
+
+namespace ExchangeRateUpdater;
+
+public interface ICurrencyRateClient
+{
+
+    Task<List<ExchangeRate>> GetLatestExchangeRates()
+    {
+        return GetExchangeRates(DateOnly.FromDateTime(DateTime.Now));
+    }
+
+    Task<List<ExchangeRate>> GetExchangeRates(DateOnly forDate);
+
+}

jobs/Backend/Task/DbMock.cs

@@ -0,0 +1,30 @@
+#nullable enable
+
+using System;
+using System.Collections.Generic;
+using ExchangeRateUpdater.Model;
+
+namespace ExchangeRateUpdater;
+
+/**
+ * Simulates a real data store of some kind, perhaps an SQL instance
+ */
+
+public class DbMock
+{
+    private static readonly Dictionary<DateOnly, List<ExchangeRate>> Cache = new();
+
+    /**
+     * Would be a database call to see if we have already persisted rates for the given date
+     */
+    public static List<ExchangeRate>? FindExchangeRates(DateOnly forDate)
+    {
+        return Cache.GetValueOrDefault(forDate);
+    }
+
+    public static void Insert(DateOnly forDate, List<ExchangeRate> newRates)
+    {
+        Cache[forDate] = newRates;
+    }
+}
+

jobs/Backend/Task/ExchangeRateProvider.cs

@@ -1,10 +1,19 @@
-using System.Collections.Generic;
+using System;
+using System.Collections.Generic;
 using System.Linq;
+using ExchangeRateUpdater.Model;
 
 namespace ExchangeRateUpdater
 {
+
     public class ExchangeRateProvider
     {
+        private readonly ICurrencyRateClient _currencyRateClient;
+        public ExchangeRateProvider(ICurrencyRateClient currencyRateClient)
+        {
+            _currencyRateClient = currencyRateClient;
+        }
+
         /// <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 +22,20 @@ public class ExchangeRateProvider
         /// </summary>
         public IEnumerable<ExchangeRate> GetExchangeRates(IEnumerable<Currency> currencies)
         {
-            return Enumerable.Empty<ExchangeRate>();
+            var fromDateTime = DateOnly.FromDateTime(DateTime.Now);
+            var existingRates = DbMock.FindExchangeRates(fromDateTime);
+            var allRates = existingRates ?? FetchAndStoreNewRates(fromDateTime);
+            // Filter for the queried ones only
+            return allRates.FindAll(rate => currencies.Contains(rate.SourceCurrency));
+        }
+
+        private List<ExchangeRate> FetchAndStoreNewRates(DateOnly forDate)
+        {
+            Console.WriteLine($"Fetching new exchange rates for {forDate}");
+            var newRates = _currencyRateClient.GetExchangeRates(forDate).Result;
+            Console.WriteLine($"Received {newRates.Count} new rates from {_currencyRateClient.GetType().Name}");
+            DbMock.Insert(forDate, newRates);
+            return newRates;
         }
     }
 }

jobs/Backend/Task/ExchangeRateUpdater.sln

@@ -1,22 +1,28 @@
-
-Microsoft Visual Studio Solution File, Format Version 12.00
-# Visual Studio 14
-VisualStudioVersion = 14.0.25123.0
-MinimumVisualStudioVersion = 10.0.40219.1
-Project("{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}") = "ExchangeRateUpdater", "ExchangeRateUpdater.csproj", "{7B2695D6-D24C-4460-A58E-A10F08550CE0}"
-EndProject
-Global
-	GlobalSection(SolutionConfigurationPlatforms) = preSolution
-		Debug|Any CPU = Debug|Any CPU
-		Release|Any CPU = Release|Any CPU
-	EndGlobalSection
-	GlobalSection(ProjectConfigurationPlatforms) = postSolution
-		{7B2695D6-D24C-4460-A58E-A10F08550CE0}.Debug|Any CPU.ActiveCfg = Debug|Any CPU
-		{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
-	EndGlobalSection
-	GlobalSection(SolutionProperties) = preSolution
-		HideSolutionNode = FALSE
-	EndGlobalSection
-EndGlobal
+
+Microsoft Visual Studio Solution File, Format Version 12.00
+# Visual Studio Version 17
+VisualStudioVersion = 17.0.31903.59
+MinimumVisualStudioVersion = 10.0.40219.1
+Project("{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}") = "ExchangeRateUpdater", "ExchangeRateUpdater.csproj", "{FD216A8A-1723-4627-BB9C-D0FFB198D515}"
+EndProject
+Project("{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}") = "ExchangeRateUpdater.Tests", "..\ExchangeRateUpdater.Tests\ExchangeRateUpdater.Tests.csproj", "{0BFF5B5E-DACE-4BED-8B72-7E7D19A8643D}"
+EndProject
+Global
+	GlobalSection(SolutionConfigurationPlatforms) = preSolution
+		Debug|Any CPU = Debug|Any CPU
+		Release|Any CPU = Release|Any CPU
+	EndGlobalSection
+	GlobalSection(SolutionProperties) = preSolution
+		HideSolutionNode = FALSE
+	EndGlobalSection
+	GlobalSection(ProjectConfigurationPlatforms) = postSolution
+		{FD216A8A-1723-4627-BB9C-D0FFB198D515}.Debug|Any CPU.ActiveCfg = Debug|Any CPU
+		{FD216A8A-1723-4627-BB9C-D0FFB198D515}.Debug|Any CPU.Build.0 = Debug|Any CPU
+		{FD216A8A-1723-4627-BB9C-D0FFB198D515}.Release|Any CPU.ActiveCfg = Release|Any CPU
+		{FD216A8A-1723-4627-BB9C-D0FFB198D515}.Release|Any CPU.Build.0 = Release|Any CPU
+		{0BFF5B5E-DACE-4BED-8B72-7E7D19A8643D}.Debug|Any CPU.ActiveCfg = Debug|Any CPU
+		{0BFF5B5E-DACE-4BED-8B72-7E7D19A8643D}.Debug|Any CPU.Build.0 = Debug|Any CPU
+		{0BFF5B5E-DACE-4BED-8B72-7E7D19A8643D}.Release|Any CPU.ActiveCfg = Release|Any CPU
+		{0BFF5B5E-DACE-4BED-8B72-7E7D19A8643D}.Release|Any CPU.Build.0 = Release|Any CPU
+	EndGlobalSection
+EndGlobal

jobs/Backend/Task/Ext/CnbClient.cs

@@ -0,0 +1,32 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Net.Http;
+using System.Threading.Tasks;
+using ExchangeRateUpdater.Model;
+
+namespace ExchangeRateUpdater.Ext;
+
+public class CnbClient : ICurrencyRateClient
+{
+    private const string TxtUrl = "https://www.cnb.cz/cs/financni-trhy/devizovy-trh/kurzy-devizoveho-trhu/kurzy-devizoveho-trhu/denni_kurz.txt";
+    private const string DateFmt = "dd.MM.YYYY";
+    private static readonly Currency TargetRate = new("CZK");
+
+    public async Task<List<ExchangeRate>> GetExchangeRates(DateOnly forDate)
+    {
+        using var client = new HttpClient();
+        var fullUrl = TxtUrl + "?date=" + forDate.ToString(DateFmt);
+        var response = await client.GetStringAsync(fullUrl);
+        var parsedResponse = CnbTxtParser.ParseResponse(response);
+
+        return parsedResponse.Select(ToExchangeRate).ToList();
+    }
+
+    private static ExchangeRate ToExchangeRate(CnbFxRow row)
+    {
+        // Some exchange rates are reported in 100s, e.g. JPY, so adjust rate accordingly.
+        var adjustedRate = row.Rate / row.Amount;
+        return new ExchangeRate(row.Currency, TargetRate, adjustedRate);
+    }
+}

jobs/Backend/Task/Ext/CnbFxRow.cs

@@ -0,0 +1,28 @@
+using System;
+using System.Globalization;
+
+namespace ExchangeRateUpdater.Model;
+
+public record CnbFxRow(string Country, string CurrencyStr, long Amount, Currency Currency, decimal Rate)
+{
+    public static CnbFxRow FromSeparatedString(string theString, char delimiter)
+    {
+        var split = theString.Split(delimiter);
+        if (split.Length != 5)
+        {
+            throw new ArgumentException("Expected 5 parameters, got: " + split.Length);
+        }
+
+        Console.WriteLine(split[4]);
+        // Decimal notation is a `,` comma, so parse it with a comma-supported culture/fmt, using FR here
+        var parsedDecimal = decimal.Parse(split[4], NumberStyles.Number, new CultureInfo("fr-FR"));
+
+        return new CnbFxRow(
+            Country: split[0],
+            CurrencyStr: split[1],
+            Amount: long.Parse(split[2]),
+            Currency: new Currency(split[3]),
+            Rate: parsedDecimal
+        );
+    }
+}

jobs/Backend/Task/Ext/CnbTxtParser.cs

@@ -0,0 +1,20 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+
+namespace ExchangeRateUpdater.Model;
+
+public class CnbTxtParser
+{
+    private static readonly char DELIMITER = '|';
+
+    public static List<CnbFxRow> ParseResponse(string rawResponse)
+    {
+        return rawResponse
+            .Split("\n")
+            .Skip(2) // Header rows skipped
+            .SkipLast(1) // Skip empty row at the end
+            .Select(line => CnbFxRow.FromSeparatedString(line, DELIMITER))
+            .ToList();
+    }
+}

jobs/Backend/Task/Currency.cs → jobs/Backend/Task/Model/Currency.cs

@@ -1,9 +1,16 @@
-namespace ExchangeRateUpdater
+using System;
+using System.Text.RegularExpressions;
+
+namespace ExchangeRateUpdater.Model
 {
-    public class Currency
+    public record Currency
     {
+
+        private static readonly Regex ISO_4217 = new("^[A-Z]{3}$");
+
         public Currency(string code)
         {
+            if (!ISO_4217.IsMatch(code)) throw new ArgumentException("Invalid currency code: " + code);
             Code = code;
         }
 

jobs/Backend/Task/ExchangeRate.cs → jobs/Backend/Task/Model/ExchangeRate.cs

@@ -1,4 +1,4 @@
-namespace ExchangeRateUpdater
+namespace ExchangeRateUpdater.Model
 {
     public class ExchangeRate
     {

jobs/Backend/Task/Program.cs

@@ -1,6 +1,8 @@
 using System;
 using System.Collections.Generic;
 using System.Linq;
+using ExchangeRateUpdater.Ext;
+using ExchangeRateUpdater.Model;
 
 namespace ExchangeRateUpdater
 {
@@ -23,13 +25,19 @@ public static void Main(string[] args)
         {
             try
             {
-                var provider = new ExchangeRateProvider();
+                var provider = new ExchangeRateProvider(new CnbClient());
                 var rates = provider.GetExchangeRates(currencies);
 
                 Console.WriteLine($"Successfully retrieved {rates.Count()} exchange rates:");
                 foreach (var rate in rates)
                 {
-                    Console.WriteLine(rate.ToString());
+                    Console.WriteLine(rate);
+                }
+
+                foreach (var rate in rates)
+                {
+                    var exampleAmount = Math.Round(1000 * rate.Value, 2, MidpointRounding.AwayFromZero);
+                    Console.WriteLine($"Converting 1000 {rate.SourceCurrency} to {rate.TargetCurrency} using ({rate}): {exampleAmount} {rate.TargetCurrency}");
                 }
             }
             catch (Exception e)