fix(engines): find LibreOffice where its Windows installer recorded it

src/Filee.Engines/Infrastructure/EngineEnvironment.cs

@@ -5,7 +5,10 @@
 // the PC (on the PATH or where its installer puts it), so nobody has to download a second FFmpeg they already have.
 
 using System.Collections.Concurrent;
+using System.Runtime.Versioning;
+using System.Security;
 using Filee.Core.Settings;
+using Microsoft.Win32;
 
 namespace Filee.Engines.Infrastructure;
 
@@ -219,6 +222,8 @@ private static IEnumerable<string> SystemFolders(string id)
             if (id == "calibre")
                 folders.AddRange(["/Applications/calibre.app", Path.Combine(Environment.GetFolderPath(Environment.SpecialFolder.UserProfile), "Applications", "calibre.app"), "/opt/calibre"]);
         }
+        else if (id == "libreoffice")
+            folders.AddRange(RegisteredFolders(LibreOfficeInstallPathKey));
         foreach (var root in (OperatingSystem.IsWindows()
                      ? new[] { Environment.SpecialFolder.ProgramFiles, Environment.SpecialFolder.ProgramFilesX86 }
                      : [])
@@ -256,6 +261,37 @@ private static IEnumerable<string> SystemFolders(string id)
             .Where(f => !IsWithin(f, DownloadRoot, comparison) && !IsWithin(f, AppContext.BaseDirectory, comparison));
     }
 
+    /// <summary>Where LibreOffice's Windows installer records its program folder (the key's default value).</summary>
+    internal const string LibreOfficeInstallPathKey = @"SOFTWARE\LibreOffice\UNO\InstallPath";
+
+    /// <summary>
+    /// The folders recorded as the default value of <paramref name="keyPath"/>, for this user and for all users, in
+    /// the 64-bit and 32-bit views. LibreOffice installed somewhere else than Program Files (another drive, a folder
+    /// of its own) is found this way without being on the PATH.
+    /// </summary>
+    [SupportedOSPlatform("windows")]
+    internal static List<string> RegisteredFolders(string keyPath)
+    {
+        var found = new List<string>();
+        foreach (var hive in new[] { RegistryHive.CurrentUser, RegistryHive.LocalMachine })
+        {
+            foreach (var view in new[] { RegistryView.Registry64, RegistryView.Registry32 })
+            {
+                try
+                {
+                    using var root = RegistryKey.OpenBaseKey(hive, view);
+                    using var key = root.OpenSubKey(keyPath);
+                    if (key?.GetValue(null) is string folder && folder.Trim().Trim('"') is { Length: > 0 } trimmed)
+                        found.Add(Environment.ExpandEnvironmentVariables(trimmed));
+                }
+                catch (Exception ex) when (ex is IOException or UnauthorizedAccessException or SecurityException)
+                {
+                }
+            }
+        }
+        return found;
+    }
+
     private static bool IsWithin(string path, string root, StringComparison comparison) =>
         string.Equals(Path.TrimEndingDirectorySeparator(path), Path.TrimEndingDirectorySeparator(root), comparison)
         || path.StartsWith(Path.TrimEndingDirectorySeparator(root) + Path.DirectorySeparatorChar, comparison);

tests/Filee.Engines.Tests/OwnCopyTests.cs

@@ -89,6 +89,30 @@ public void A_tool_on_the_path_is_found_when_filee_has_no_copy()
         Assert.Equal(Path.Combine(_folder, EngineEnvironment.ProgramName("ffmpeg.exe")), EngineEnvironment.SystemProgram("ffmpeg", "ffmpeg.exe"));
     }
 
+    [Fact]
+    [System.Runtime.Versioning.SupportedOSPlatform("windows")] // skips elsewhere
+    public void LibreOffice_is_found_in_the_folder_its_installer_recorded()
+    {
+        Assert.SkipUnless(OperatingSystem.IsWindows(), "The Windows registry.");
+        const string key = @"Software\FileeTests\LibreOffice\UNO\InstallPath";
+        File.WriteAllBytes(Path.Combine(_folder, "soffice.exe"), [0x4D, 0x5A]);
+        try
+        {
+            using (var written = Microsoft.Win32.Registry.CurrentUser.CreateSubKey(key))
+                written.SetValue(null, $"\"{_folder}\"");
+
+            var folders = EngineEnvironment.RegisteredFolders(key);
+
+            Assert.Contains(_folder, folders);
+            Assert.Equal(_folder, EngineEnvironment.FolderWithPrograms("libreoffice", _folder));
+            Assert.Empty(EngineEnvironment.RegisteredFolders(@"Software\FileeTests\Missing"));
+        }
+        finally
+        {
+            Microsoft.Win32.Registry.CurrentUser.DeleteSubKeyTree(@"Software\FileeTests", throwOnMissingSubKey: false);
+        }
+    }
+
     [Theory]
     [InlineData("ffmpeg", "ffmpeg version 7.1.1-full_build-www.gyan.dev Copyright (c) 2000-2025", "7.1")]
     [InlineData("ffmpeg", "ffmpeg version n8.0 Copyright (c) 2000-2025", "8.0")]