Checksum the checksums!

Get-DirectoryChecksums.ps1

[CmdletBinding()]
param (
    [Parameter(Mandatory = $true)]
    [string]$LiteralPath,

    [Parameter(Mandatory = $false)]
    [string]$CompareCsvPath,

    [Parameter(Mandatory = $false)]
    [ValidateRange(1, 64)]
    [int]$ThrottleLimit = 8,

    [Parameter(Mandatory = $false)]
    [ValidateSet("SHA256", "SHA1", "MD5", "SHA384", "SHA512")]
    [string]$Algorithm = "SHA256",

    [Parameter(Mandatory = $false)]
    [string]$CSVDirectory
)

if (-not (Test-Path $LiteralPath)) { 
    throw "Path not found: $LiteralPath" 
}
if ($CompareCsvPath -and -not (Test-Path $CompareCsvPath)) { 
    throw "Compare CSV not found: $CompareCsvPath" 
}

$sep = [System.IO.Path]::DirectorySeparatorChar
$CmdletName = [System.IO.Path]::GetFileNameWithoutExtension($MyInvocation.MyCommand.Name)

if ([string]::IsNullOrEmpty($CSVDirectory)) {
    $CSVDirectory = Join-Path ([System.IO.Path]::GetTempPath()) $CmdletName
}

if (-not (Test-Path $CSVDirectory)) { 
    New-Item -Path $CSVDirectory -ItemType Directory | Out-Null 
}

$Algorithm = $Algorithm.ToLower();
$Timestamp = Get-Date -Format "yyyyMMdd_HHmmss"
$Guid = [guid]::NewGuid().ToString()
$fileLedgerCsvPath = Join-Path $CSVDirectory "checksums_${Algorithm}_${Timestamp}_${Guid}.csv"
$BasePath = (Resolve-Path $LiteralPath).Path.TrimEnd($sep)

function Write-LfCsv {
    [CmdletBinding()]
    param (
        [Parameter(Mandatory = $true)]
        [string]$Path,

        [Parameter(Mandatory = $false)]
        [switch]$NoBom
    )

    if (-not (Test-Path $Path)) {
        throw "CSV not found: $Path"
    }

    $content = Get-Content -Path $Path -Raw
    $content = $content -replace "`r`n", "`n"

    if ($NoBom) {
        $utf8NoBom = [System.Text.UTF8Encoding]::new($false)
        [System.IO.File]::WriteAllText($Path, $content, $utf8NoBom)
    }
    else {
        Set-Content -Path $Path -Value $content -NoNewline -Encoding UTF8
    }
}

function Get-ChecksumData {
    param (
        [Parameter(Mandatory)]
        [string]$BaseDirectory,

        [Parameter(Mandatory)]
        [int]$Throttle,

        [Parameter(Mandatory)]
        [string]$HashAlgorithm
    )

    $files = Get-ChildItem -Path $BaseDirectory -Recurse -File
    $total = $files.Count

    Write-Host "🔢 Total files to process: $total"

    $files | ForEach-Object -Parallel {
        $hash = Get-FileHash -LiteralPath $_.FullName -Algorithm $using:HashAlgorithm
        $sep = [System.IO.Path]::DirectorySeparatorChar
        $relPath = $_.FullName.Substring($using:BaseDirectory.Length).TrimStart($sep)

        [pscustomobject]@{
            RelativePath = $relPath
            FileName     = $_.Name
            Algorithm    = $using:HashAlgorithm
            Hash         = $hash.Hash.ToLower()
        }
    } -ThrottleLimit $Throttle | Sort-Object RelativePath
}

# Main Execution
Write-Host "📁 Scanning directory (PARALLEL=$ThrottleLimit, Algorithm=$Algorithm): $BasePath"

$directoryChecksumData = Get-ChecksumData -BaseDirectory $BasePath -Throttle $ThrottleLimit -HashAlgorithm $Algorithm
$directoryChecksumData | Export-Csv -Path $fileLedgerCsvPath -NoTypeInformation -Encoding UTF8

# Normalize to LF-only line endings and write without BOM
Write-LfCsv -Path $fileLedgerCsvPath -NoBom

Write-Host "✅ File Ledger CSV generated:"
Write-Host "   $fileLedgerCsvPath"

$newHash = (Get-FileHash -LiteralPath $fileLedgerCsvPath -Algorithm $Algorithm).Hash.ToLower()

# Verify against existing CSV if provided
if ($CompareCsvPath) {
    $oldHash = (Get-FileHash -LiteralPath $CompareCsvPath -Algorithm $Algorithm).Hash.ToLower()

    if ($oldHash -eq $newHash) {
        Write-Host "✅ CSV HASH MATCH — DATA IDENTICAL" -ForegroundColor Green
    }
    else {
        Write-Host "❌ CSV HASH MISMATCH" -ForegroundColor Red
        Write-Host "   Expected: $oldHash"
        Write-Host "   Actual:   $newHash"
    }
}

# Output the new hash to the pipeline
$newHash

Example Usages:

# Creates a CSV in $env:TEMP and outputs the checksum of that CSV to the pipeline.
.\Get-DirectoryChecksums.ps1 -Path "D:\bkp"

# Creates a CSV in $env:TEMP, verifies the checksum of the newly generated CSV matches the `-CompareCsvPath` file, and outputs the checksum of the new CSV to the pipeline.
.\Get-DirectoryChecksums.ps1 -Path "D:\bkp" -CompareCsvPath "$env:TEMP\Get-DirectoryChecksums\checksums_SHA256_20260108_225107_30768d14-6777-4705-96ff-4aebecb3a28b.csv"

# Creates a CSV in $env:TEMP using SHA512 for the checksums, and outputs the checksum of that CSV to the pipeline.
.\Get-DirectoryChecksums.ps1 -Path "D:\bkp" -Algorithm SHA512

# Creates a CSV in the "D:\temp" and outputs the checksum of that CSV to the pipeline.
.\Get-DirectoryChecksums.ps1 -Path "D:\bkp" -CSVDirectory "D:\temp"