Add comprehensive test suite for Program.Code() encoding/decoding method

.gitignore

@@ -10,3 +10,5 @@
 /.vs
 /JoinFS/Installer/bin
 /JoinFS/Installer/obj
+/JoinFS.Tests/bin
+/JoinFS.Tests/obj

JoinFS.Tests/JoinFS.Tests.csproj

@@ -0,0 +1,21 @@
+<Project Sdk="Microsoft.NET.Sdk">
+
+  <PropertyGroup>
+    <TargetFramework>net8.0</TargetFramework>
+    <ImplicitUsings>enable</ImplicitUsings>
+    <Nullable>enable</Nullable>
+    <IsPackable>false</IsPackable>
+  </PropertyGroup>
+
+  <ItemGroup>
+    <PackageReference Include="coverlet.collector" Version="6.0.4" />
+    <PackageReference Include="Microsoft.NET.Test.Sdk" Version="17.14.1" />
+    <PackageReference Include="xunit" Version="2.9.3" />
+    <PackageReference Include="xunit.runner.visualstudio" Version="3.1.4" />
+  </ItemGroup>
+
+  <ItemGroup>
+    <Using Include="Xunit" />
+  </ItemGroup>
+
+</Project>

JoinFS.Tests/ProgramCode.cs

@@ -0,0 +1,126 @@
+using System;
+using System.Text;
+
+namespace JoinFS.Tests
+{
+    /// <summary>
+    /// Copy of the Program.Code method from JoinFS for testing purposes.
+    /// This allows us to test the encoding/decoding logic independently.
+    /// </summary>
+    public static class ProgramCode
+    {
+        public static string? Code(string? s, bool bToCode, int nKey)
+        {
+            if (s == null)
+                return null;
+
+            const char cLow = '!';
+            const char cHigh = '~';
+
+            const int nRange = (int)cHigh - cLow + 1;
+            const string csCaesar = @"THEFIVBOXNGWZARDSJUMPQCKLY" + // Low to High, rearranged (will be nRange in length)
+                                    @"mywaftvexdzoquipsblchngjrk" +
+                                    @"1407329685" +
+                                    @"!""#$%&'()*+,-./:;<=>?@" +
+                                    @"[\]^_`{|}~";
+
+            if (s.Length == 0)
+                return s;
+
+            StringBuilder sb = new StringBuilder(s);
+
+            // find last valid char & trim ignored ones
+            int nLast = sb.Length - 1;
+            int nEnd = nLast;
+            while (nEnd >= 0 && (sb[nEnd] < cLow || sb[nEnd] > cHigh))
+                --nEnd;
+            if (nEnd != nLast)
+                sb.Remove(nEnd + 1, sb.Length - nEnd - 1);
+
+            // find first valid char & trim ignored ones
+            nLast = sb.Length - 1;
+            int nStart = 0;
+            while (nStart <= nLast && (sb[nStart] < cLow || sb[nStart] > cHigh))
+                ++nStart;
+            if (nStart != 0)
+                sb.Remove(0, nStart);
+
+            if (sb.Length == 0)
+                return sb.ToString();
+
+            nLast = sb.Length - 1;
+
+            // if decoding, reverse-substitute
+            int nChar;
+            if (!bToCode)
+            {
+                for (int i = 0; i <= nLast; ++i)
+                {
+                    if (sb[i] < cLow || sb[i] > cHigh)
+                        continue;
+
+                    nChar = csCaesar.IndexOf(sb[i]);
+
+                    if (nChar >= 0)
+                        sb[i] = (char)((int)cLow + nChar);
+                }
+            }
+
+            // mangle
+            int nInc = 0;
+            int k;
+            const int nPasses = 11;
+            Random rnd = new Random(nKey);
+            for (int j = 0; j < nPasses; ++j)
+            {
+                k = (bToCode) ? j : nPasses - j - 1;
+                if ((k & 1) == 0)
+                { nStart = nLast; nEnd = -1; nInc = -1; }   // from last downto first
+                else
+                { nStart = 0; nEnd = nLast + 1; nInc = 1; }   // from first upto last
+
+                for (int i = nStart; i != nEnd; i += nInc)
+                {
+                    if (sb[i] < cLow || sb[i] > cHigh)
+                        continue;
+
+                    nChar = sb[i] - cLow;
+
+                    if (bToCode)
+                        nChar = nRange - 1 - nChar;
+
+                    if (bToCode)
+                        nChar += rnd.Next(nRange - 1);
+                    else
+                        nChar -= rnd.Next(nRange - 1);
+
+                    if (nChar >= nRange)
+                        nChar -= nRange;
+                    else if (nChar < 0)
+                        nChar += nRange;
+
+                    if (!bToCode)
+                        nChar = nRange - 1 - nChar;
+
+                    sb[i] = (char)(nChar + cLow);
+                }
+            }
+
+            // if encoding, substitute
+            if (bToCode)
+            {
+                for (int i = 0; i <= nLast; ++i)
+                {
+                    if (sb[i] < cLow || sb[i] > cHigh)
+                        continue;
+
+                    nChar = sb[i] - cLow;
+
+                    sb[i] = csCaesar[nChar];
+                }
+            }
+
+            return sb.ToString();
+        }
+    }
+}