#######################################################################
## Determine and compare MD5 checksums with PowerShell               ##
## ----------------------------------------------------------------- ##
## Author:  Helmut Kaczmarek <email@helmutkaczmarek.de>              ##
## URL:     https://wiki.helmutkaczmarek.de/code:powershell:checksum ##
#######################################################################

# Settings
$dataFolder = "D:\Folder"
$scriptFolder = "D:\Folder\Checksum"
$checksumFolderPath = Join-Path -Path $scriptFolder -ChildPath "Lists"
$logFolderPath = Join-Path -Path $scriptFolder -ChildPath "Logs"
$allowedExtensions = "*.jpg", "*.jpeg", "*.dng", "*.cr2"
$timestamp = Get-Date -Format "yyyyMMdd_HHmmss"

# Create folder for checksum files (will be subfolder of $scriptFolder)
if (-not (Test-Path -Path $checksumFolderPath -PathType Container)) {
    New-Item -Path $checksumFolderPath -ItemType Directory
}

# Create folder for log files (will be subfolder of $scriptFolder)
if (-not (Test-Path -Path $logFolderPath -PathType Container)) {
    New-Item -Path $logFolderPath -ItemType Directory
}

# Create the checksum file
$checksumList = Join-Path -Path $checksumFolderPath -ChildPath ("${timestamp}_checksum.txt")

Add-Content -Path $checksumList -Value "MD5 checksums on: $(Get-Date -Format 'yyyy-MM-dd HH:mm:ss')"
Get-ChildItem -Recurse -File -Path $dataFolder -Include $allowedExtensions | ForEach-Object {
    $file = $_.FullName
    $md5 = Get-FileHash -Algorithm MD5 -Path $file
    $logEntry = "$($md5.Hash)  $file"
    Add-Content -Path $checksumList -Value $logEntry
    Write-Host "Processing $file"
}

# Keep the 10 most recent checksum files
$allChecksumFiles = Get-ChildItem -Path $checksumFolderPath -Filter "*_checksum.txt" | Sort-Object -Property LastWriteTime -Descending
$oldChecksumFiles = $allChecksumFiles | Select-Object -Skip 10

foreach ($oldFile in $oldChecksumFiles) {
    Remove-Item -Path $oldFile.FullName -Force
}

# Determine the last two checksum files
$allChecksumFiles = Get-ChildItem -Path $checksumFolderPath -Filter "*_checksum.txt" | Sort-Object -Property LastWriteTime -Descending

if ($allChecksumFiles.Count -lt 2) {
    Write-Host "INFO: Checksums could not be compared because there is currently only one checksum file."
    $logFile = Join-Path -Path $logFolderPath -ChildPath ("${timestamp}_log.txt")
    Add-Content -Path $logFile -Value "$(Get-Date -Format 'yyyy-MM-dd HH:mm:ss'): INFO: Checksums could not be compared because there is currently only one checksum file."
    Write-Host "The following log file was created:"
    Write-Host $logFile
}
else {
    $latestChecksumFile = $allChecksumFiles[0]
    $previousChecksumFile = $allChecksumFiles[1]

    # Read the checksums and filenames from the last two checksum files
    $latestChecksums = Get-Content -Path $latestChecksumFile.FullName
    $previousChecksums = Get-Content -Path $previousChecksumFile.FullName

    # Compare the checksums and create the log file
    $differentFiles = @()

    for ($i = 0; $i -lt $latestChecksums.Count; $i++) {
        $latestChecksum = $latestChecksums[$i] -split ' ', 2
        $previousChecksum = $previousChecksums[$i] -split ' ', 2

        $latestHash = $latestChecksum[0]
        $latestFile = $latestChecksum[1]

        $previousHash = $previousChecksum[0]
        $previousFile = $previousChecksum[1]

        if ($latestFile -eq $previousFile -and $latestHash -ne $previousHash) {
            $differentFiles += $latestFile
        }
    }

    # Create the log file
    $logFile = Join-Path -Path $logFolderPath -ChildPath ("${timestamp}_log.txt")

    Add-Content -Path $logFile -Value "Directory monitoring on: $(Get-Date -Format 'yyyy-MM-dd HH:mm:ss')"
    if ($differentFiles.Count -gt 0) {
        Write-Host "ATTENTION: Different MD5 checksums found!"
        Add-Content -Path $logFile -Value "ATTENTION: The following files have different checksums:"
        foreach ($file in $differentFiles) {
            Add-Content -Path $logFile -Value "  $file"
        }
    } else {
        Write-Host "INFO: No different MD5 checksums found."
        Add-Content -Path $logFile -Value "INFO: No different MD5 checksums found."
    }

    # Keep the 10 most recent log files
    $allLogFiles = Get-ChildItem -Path $logFolderPath -Filter "*_log.txt" | Sort-Object -Property LastWriteTime -Descending
    $oldLogFiles = $allLogFiles | Select-Object -Skip 10

    foreach ($oldFile in $oldLogFiles) {
        Remove-Item -Path $oldFile.FullName -Force
    }

    # Display a message with the path to the created log file in the command prompt
    Write-Host "The following log file was created:"
    Write-Host $logFile
}