Compact.exe already shows you the algorithm by default
PS D:\compressed> compact *.txt
Listing D:\compressed\
New files added to this directory will not be compressed.
195634 : 86016 = 2,3 to 1 C LZNT1.txt
195634 : 45056 = 4,3 to 1 l LZX.txt
195634 : 195634 = 1,0 to 1 No_compression.txt
195634 : 69632 = 2,8 to 1 X XPRESS16K.txt
195634 : 81920 = 2,4 to 1 x XPRESS4K.txt
195634 : 73728 = 2,7 to 1 X XPRESS8K.txt
↑ compression algorithm
Of 6 files within 1 directories
5 are compressed and 1 are not compressed.
1.173.804 total bytes of data are stored in 551.986 bytes.
The compression ratio is 2,1 to 1.
See the second-to-last column, C means the default LZNT1 algorithm, l is LZX, x is XPRESS4K and X is XPRESS8K or XPRESS16K. In fact compact list files just like dir and you can use compact /a to show system/hidden files or compact /s to show files recursively
Unfortunately there isn't a way to differentiate XPRESS8K and XPRESS16K with compact.exe though, but you can do that by calling the Win32 API directly. The new algorithms for CompactOs are handled via Windows Overlay Filter (WOF) and you can use its WofIsExternalFile API to retrieve compression info. I've written a PowerShell script named compression.ps1 do do that. Here's its output for the same files above
PS D:\compressed> .\compression.ps1 (ls *.txt)
FileName Algorithm
-------- ---------
D:\compressed\LZNT1.txt LZNT1
D:\compressed\LZX.txt LZX
D:\compressed\No_compression.txt NO_COMPRESSION
D:\compressed\XPRESS16K.txt XPRESS16K
D:\compressed\XPRESS4K.txt XPRESS4K
D:\compressed\XPRESS8K.txt XPRESS8K
Here's its source code, just save as *.ps1 and run as above
param(
[Parameter()]
[string[]]$fileList
)
Add-Type -Name 'Wof' -Namespace 'Wofutil' -MemberDefinition @'
public struct ExternalFileInfo
{
public uint algorithm;
public uint flags;
}
[DllImport("Wofutil.dll")]public static extern int WofIsExternalFile(
[MarshalAs(UnmanagedType.LPWStr)] string Filepath,
out int IsExternalFile,
out uint Provider,
out ExternalFileInfo Info,
ref uint BufferLength
);
'@
enum Algorithms {
NO_COMPRESSION = -2
LZNT1 = -1
XPRESS4K = 0
LZX = 1
XPRESS8K = 2
XPRESS16K = 3
}
class CompressionInfo {
[string]$FileName
[Algorithms]$Algorithm
}
foreach ($fileName in $fileList) {
$f = Get-ChildItem -LiteralPath $fileName
if (($f.Attributes -band [IO.FileAttributes]::Compressed) -ne 0) {
[CompressionInfo]@{
FileName = $f
Algorithm = 'LZNT1'
}
} else {
[int]$isExternalFile = 0
[uint32]$provider = 0
$fileInfo = New-Object Wofutil.Wof+ExternalFileInfo
[uint32]$length = 8
[void][Wofutil.Wof]::WofIsExternalFile($f, [ref]$isExternalFile,
[ref]$provider, [ref]$fileInfo, [ref]$length)
if ($isExternalFile -ne 0) {
[CompressionInfo]@{
FileName = $f
Algorithm = $fileInfo.algorithm
}
} else {
[CompressionInfo]@{
FileName = $f
Algorithm = 'NO_COMPRESSION'
}
}
}
}
Note that NTFS compression and CompactOS are completely different features. When MS introduced the new CompactOS feature they merged it into the compact command because of their relation to compression. But CompactOS is purely to compress read-only system files to save the OS footprint. You can clearly see that the option is called /exe because it's meant to compress executable files. That's why the new algorithms aren't designed for editing and writing to the files will decompress them
PS D:\compressed> echo x >.\LZX.txt # Write more data to file
PS D:\compressed> compact .\LZX.txt # Now the file is uncompressed
Listing D:\compressed\
New files added to this directory will not be compressed.
8 : 8 = 1,0 to 1 LZX.txt
Of 1 files within 1 directories
0 are compressed and 1 are not compressed.
8 total bytes of data are stored in 8 bytes.
The compression ratio is 1,0 to 1.
OTOH the NTFS compression feature uses the old LZNT1 algorithm and is designed to work with any files. It supports on-the-fly editing where you can write to the files arbitrarily and they're still compressed. You can also mark new files in the folder to be compressed automatically with LZNT1 but not with the new algorithms
Note that the /exe option isn't for checking the algorithm but to specify an algorithm when compressing. compact /C /exe:<algorithm> your_files will compress the specified files with the specified EXE algorithm, whereas compact /C your_files will compress the files using NTFS compression
