Updated: added (working) unit tests and an explanation asto how I'd speed up the password check

Server/.idea/.idea.BackEnd/.idea/vcs.xml

@@ -0,0 +1,6 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<project version="4">
+  <component name="VcsDirectoryMappings">
+    <mapping directory="$PROJECT_DIR$/.." vcs="Git" />
+  </component>
+</project>

Server/BackEnd.sln

@@ -1,8 +1,9 @@
 
 Microsoft Visual Studio Solution File, Format Version 12.00
+# 
 Project("{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}") = "BackEnd", "BackEnd\BackEnd.csproj", "{936595E0-B2A6-42B9-84F8-AA8D52466E5E}"
 EndProject
-Project("{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}") = "Test.BackEnd", "Test.BackEnd\Test.BackEnd.csproj", "{911B1058-1BDB-46E7-8365-CB6CEC864C54}"
+Project("{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}") = "Backend.Tests", "Backend.Tests\Backend.Tests.csproj", "{2C7E25E6-5C68-4F23-96C1-67F953719CA9}"
 EndProject
 Global
 	GlobalSection(SolutionConfigurationPlatforms) = preSolution
@@ -14,9 +15,9 @@ Global
 		{936595E0-B2A6-42B9-84F8-AA8D52466E5E}.Debug|Any CPU.Build.0 = Debug|Any CPU
 		{936595E0-B2A6-42B9-84F8-AA8D52466E5E}.Release|Any CPU.ActiveCfg = Release|Any CPU
 		{936595E0-B2A6-42B9-84F8-AA8D52466E5E}.Release|Any CPU.Build.0 = Release|Any CPU
-		{911B1058-1BDB-46E7-8365-CB6CEC864C54}.Debug|Any CPU.ActiveCfg = Debug|Any CPU
+		{2C7E25E6-5C68-4F23-96C1-67F953719CA9}.Debug|Any CPU.ActiveCfg = Debug|Any CPU
-		{911B1058-1BDB-46E7-8365-CB6CEC864C54}.Debug|Any CPU.Build.0 = Debug|Any CPU
+		{2C7E25E6-5C68-4F23-96C1-67F953719CA9}.Debug|Any CPU.Build.0 = Debug|Any CPU
-		{911B1058-1BDB-46E7-8365-CB6CEC864C54}.Release|Any CPU.ActiveCfg = Release|Any CPU
+		{2C7E25E6-5C68-4F23-96C1-67F953719CA9}.Release|Any CPU.ActiveCfg = Release|Any CPU
-		{911B1058-1BDB-46E7-8365-CB6CEC864C54}.Release|Any CPU.Build.0 = Release|Any CPU
+		{2C7E25E6-5C68-4F23-96C1-67F953719CA9}.Release|Any CPU.Build.0 = Release|Any CPU
 	EndGlobalSection
 EndGlobal

Server/BackEnd/Controllers/PasswordController.cs

@@ -14,8 +14,7 @@ public class PasswordController : ControllerBase
     public PasswordController(ILogger<PasswordController> logger, IPasswordService passwordService)
     {
         _logger = logger;
-        _passwordService = passwordService;
+        _passwordService = new PasswordService("Data/common-passwords.txt");
-        _passwordService.LoadCommonPasswords("Data/common-passwords.txt");
     }
 
     [HttpPost("change")]

Server/BackEnd/Services/PasswordService.cs

@@ -4,8 +4,81 @@ namespace back_end.Services;
 
 public class PasswordService : IPasswordService
 {
+    
+    
+     private class PasswordTreeNode {
+        
+        public Dictionary<char, PasswordTreeNode> children { get; set; }
+        public bool endOfWord { get; set; }
+
+        public PasswordTreeNode()
+        {
+            children = new Dictionary<char, PasswordTreeNode>();
+            endOfWord = false;
+        }
+    }
+
+    private class PasswordTree
+    {
+        private readonly PasswordTreeNode _root;
+
+        public PasswordTree()
+        {
+            _root = new PasswordTreeNode();
+        }
+
+        public void Insert(string password)
+        {
+            var currentNode = _root;
+            foreach (var c in password)
+            {
+                if (!currentNode.children.ContainsKey(c))
+                {
+                    currentNode.children.Add(c, new PasswordTreeNode());
+                }
+                currentNode = currentNode.children[c];
+            }
+            currentNode.endOfWord = true;
+        }
+
+        public bool Search(string password)
+        {
+            var currentNode = _root;
+            foreach (var c in password)
+            {
+                // This is where the logic gets a bit confusing
+                //  if we want to just check whether the password the user gives 'passwords' EXACTLY
+                //  then this traversal is easy, we just do the following
+                if (!currentNode.children.ContainsKey(c)) return false;
+                currentNode = currentNode.children[c];
+            }
+            return currentNode.endOfWord;   
+            /* However, I had the idea that instead of just checking the tree like this, I'd want it to loop through
+             *  the password given:
+             *
+             * For example
+             *  123!passwords should loop through the first four characters (123!)
+             *      and just continue looping through the above once it gets to p and return true for the common check
+             *  I think this would just be done by popping off the first character, so we get down to just 'passwords'
+             *      for the example which would work just fine, but for something like '123!someotherpasswords' it would
+             *      not
+             *  For this to work, I'd have to really slice up the stirng in multiple (if not all) of the possible spaces
+             *      and then run this check in parallel since now I'd be giving the program a LOT more work
+             *  I believe that that would be overengineering this however, as someotherpasswords is (marginally) more
+             *      secure than the common password 'passwords', but still fun to think about!
+             */
+        }
+        
+    }
+    
+    
     private readonly List<string> _commonPasswords = new List<string>();
 
+    public PasswordService(string commonPasswordFilepath)
+    {
+        LoadCommonPasswords(commonPasswordFilepath);
+    }
+    
     // load common passwords on start up to save us from having to re-load the file over and over 
     // if the list would change, then this is a bad idea
     public void LoadCommonPasswords(string filepath)
@@ -25,7 +98,28 @@ public class PasswordService : IPasswordService
     {
         // RegEx feels like cheating it's so good
         Regex regex = new Regex("^(?=.*?[a-zA-Z])(?=.*?[0-9])(?=.*?[!£$^*#])[a-zA-Z0-9!£$^*#]{7,14}$");
-        return !regex.IsMatch(password);
+        return !(regex.IsMatch(password) 
+               && this.IsPasswordLengthValid(password) 
+               && this.IsPasswordContainingMinimumCharacters(password) 
+               && this.IsPasswordContainingOnlyLegalCharacters(password));
+    }
+    
+    public bool IsPasswordLengthValid(string password) {
+        // check if all the characters between the start and end of the password add to 7-14 characters
+        Regex regex = new Regex("^.{7,14}$");
+        return regex.IsMatch(password);
+    }
+
+    public bool IsPasswordContainingMinimumCharacters(string password) {
+        // look ahead for the special characters and digits and match them at least once
+        Regex regex = new Regex("^(?=.*?[!£$^*#])(?=.*?[0-9]).*$");
+        return regex.IsMatch(password);
+    }
+    
+    public bool IsPasswordContainingOnlyLegalCharacters(string password) {
+        // check for any characters not in the allowed list
+        Regex regex = new Regex("^[a-zA-Z0-9!£$^*#]*$");
+        return regex.IsMatch(password);
     }
     
     public bool IsPasswordCommon(string? password)

Server/Backend.Tests/Backend.Tests.csproj

@@ -0,0 +1,35 @@
+<Project Sdk="Microsoft.NET.Sdk">
+
+  <PropertyGroup>
+    <TargetFramework>net7.0</TargetFramework>
+    <ImplicitUsings>enable</ImplicitUsings>
+    <Nullable>enable</Nullable>
+
+    <IsPackable>false</IsPackable>
+    <IsTestProject>true</IsTestProject>
+  </PropertyGroup>
+
+  <ItemGroup>
+    <PackageReference Include="Microsoft.NET.Test.Sdk" Version="17.7.1" />
+    <PackageReference Include="xunit" Version="2.4.2" />
+    <PackageReference Include="xunit.runner.visualstudio" Version="2.4.5">
+      <IncludeAssets>runtime; build; native; contentfiles; analyzers; buildtransitive</IncludeAssets>
+      <PrivateAssets>all</PrivateAssets>
+    </PackageReference>
+    <PackageReference Include="coverlet.collector" Version="3.2.0">
+      <IncludeAssets>runtime; build; native; contentfiles; analyzers; buildtransitive</IncludeAssets>
+      <PrivateAssets>all</PrivateAssets>
+    </PackageReference>
+  </ItemGroup>
+
+  <ItemGroup>
+    <ProjectReference Include="..\BackEnd\BackEnd.csproj" />
+  </ItemGroup>
+
+  <ItemGroup>
+    <None Update="common-passwords.txt">
+      <CopyToOutputDirectory>Always</CopyToOutputDirectory>
+    </None>
+  </ItemGroup>
+
+</Project>

Server/Backend.Tests/GlobalUsings.cs

@@ -0,0 +1 @@
+global using Xunit;

Server/Backend.Tests/PasswordTests.cs

@@ -0,0 +1,76 @@
+using back_end.Services;
+
+namespace Backend.Tests;
+
+public class PasswordTests
+{
+    private readonly PasswordService _passwordService;
+
+    public PasswordTests()
+    {
+        _passwordService = new PasswordService(Path.Combine(AppDomain.CurrentDomain.BaseDirectory,"common-passwords.txt"));
+    }
+    
+    [Theory]
+    [InlineData("abcdefg",true)]
+    [InlineData("abcdef",false)]
+    [InlineData("abcdefghijklmn",true)]
+    [InlineData("abcdefghijklmno",false)]
+    public void PasswordLengthValid(string password, bool valid)
+    {
+        var check = _passwordService.IsPasswordLengthValid(password);
+        Assert.Equal(valid, check);
+    }
+
+    [Theory]
+    [InlineData("password", false)]
+    [InlineData("password1", false)]
+    [InlineData("password!", false)]
+    [InlineData("password1!", true)]
+    public void PasswordContainsMinimumCharacters(string password, bool valid)
+    {
+        var check = _passwordService.IsPasswordContainingMinimumCharacters(password);
+        Assert.Equal(valid, check);
+    }
+    
+    [Theory]
+    [InlineData("password",true)]
+    [InlineData("password(",false)]
+    [InlineData("password\u00a9",false)]
+    public void PasswordContainsOnlyLegalCharacters(string password, bool valid)
+    {
+        var check = _passwordService.IsPasswordContainingOnlyLegalCharacters(password);
+        Assert.Equal(valid, check);
+    }
+    
+    // not sure why, however, 'password' on it's own is not considered a 'common' password
+    // As explained in the comments for the PasswordTree, for the '123!passwords' test, I feel like it should count
+    //  as a common password but doesnt in this implementation!
+    [Theory]
+    [InlineData("123!passwords",false)] 
+    [InlineData("password123!",true)]
+    [InlineData("123!haslo",false)]
+    [InlineData("haslo123!",false)]
+    [InlineData("passwords",true)]
+    [InlineData("haslo",false)]
+    public void PasswordIsCommon(string password, bool valid)
+    {
+        var check = _passwordService.IsPasswordCommon(password);
+        Assert.Equal(valid, check);
+    }
+    
+    [Theory]
+    [InlineData("123!passwords",true)]
+    [InlineData("password123!",true)] // this is a valid password, regardless of it being common
+    [InlineData("123!haslo",true)]
+    [InlineData("haslo123!",true)]
+    [InlineData("passwords",false)]
+    [InlineData("haslo",false)]
+    [InlineData("password\u00a9",false)]
+    public void PasswordIsValid(string password, bool valid)
+    {
+        // The logic here is hard to read, the test checks whether I am passing in a VALID password
+        var check = !(_passwordService.IsPasswordInvalid(password));
+        Assert.Equal(valid, check);
+    }
+}

Server/PasswordTests/GlobalUsings.cs

@@ -0,0 +1 @@
+global using Xunit;

Server/PasswordTests/PasswordTests.csproj

@@ -0,0 +1,25 @@
+<Project Sdk="Microsoft.NET.Sdk">
+
+  <PropertyGroup>
+    <TargetFramework>net7.0</TargetFramework>
+    <ImplicitUsings>enable</ImplicitUsings>
+    <Nullable>enable</Nullable>
+
+    <IsPackable>false</IsPackable>
+    <IsTestProject>true</IsTestProject>
+  </PropertyGroup>
+
+  <ItemGroup>
+    <PackageReference Include="Microsoft.NET.Test.Sdk" Version="17.7.1" />
+    <PackageReference Include="xunit" Version="2.4.2" />
+    <PackageReference Include="xunit.runner.visualstudio" Version="2.4.5">
+      <IncludeAssets>runtime; build; native; contentfiles; analyzers; buildtransitive</IncludeAssets>
+      <PrivateAssets>all</PrivateAssets>
+    </PackageReference>
+    <PackageReference Include="coverlet.collector" Version="3.2.0">
+      <IncludeAssets>runtime; build; native; contentfiles; analyzers; buildtransitive</IncludeAssets>
+      <PrivateAssets>all</PrivateAssets>
+    </PackageReference>
+  </ItemGroup>
+
+</Project>

Server/PasswordTests/UnitTest1.cs

@@ -0,0 +1,10 @@
+namespace PasswordTests;
+
+public class UnitTest1
+{
+    [Fact]
+    public void Test1()
+    {
+
+    }
+}