<#
.SYNOPSIS
    Moves (or copies) a list of folders to another drive with robocopy, keeping their structure.
.DESCRIPTION
    For each source folder, works out a matching path under -DestinationRoot and hands the job
    to robocopy, which is far better than Move-Item at big trees, long paths, retries and
    logging. Each source gets its own log file and its own result object, and one failure
    doesn't stop the rest. Supports -WhatIf.
.PARAMETER Source
    One or more folders to move, e.g. C:\Media, D:\Projects.
.PARAMETER DestinationRoot
    Where they go, e.g. E:\ or E:\Migrated.
.PARAMETER IncludeDriveLetter
    Put each source under a folder named for its drive (E:\C\Media, E:\D\Media). Use this when two
    sources share a name on different drives; the script refuses to merge them otherwise.
.PARAMETER Copy
    Copy instead of move. Sources are left alone.
.PARAMETER LogFolder
    Where robocopy logs go. Default: the current folder.
.PARAMETER Threads
    robocopy /MT value. Default 8. Use 1 for USB drives and old spinning disks.
.EXAMPLE
    .\Move-FolderToDrive.ps1 -Source C:\Media, D:\Projects -DestinationRoot E:\ -WhatIf
.EXAMPLE
    .\Move-FolderToDrive.ps1 -Source C:\Media, D:\Media -DestinationRoot E:\Migrated -IncludeDriveLetter -Copy
#>
[CmdletBinding(SupportsShouldProcess)]
param(
    [Parameter(Mandatory)][ValidateScript({ Test-Path -LiteralPath $_ -PathType Container })][string[]]$Source,
    [Parameter(Mandatory)][string]$DestinationRoot,
    [switch]$IncludeDriveLetter,
    [switch]$Copy,
    [string]$LogFolder = (Get-Location).Path,
    [ValidateRange(1, 128)][int]$Threads = 8
)

$exitMeaning = @{
    0 = 'Nothing to do'; 1 = 'Files copied'; 2 = 'Extra files at destination'; 3 = 'Files copied, extras at destination'
    4 = 'Mismatches found'; 5 = 'Files copied, mismatches found'; 6 = 'Extras and mismatches'; 7 = 'Files copied, extras and mismatches'
}

# Work out every destination first, so collisions are caught before anything moves.
$plan = foreach ($src in $Source) {
    $full = (Resolve-Path -LiteralPath $src).ProviderPath.TrimEnd('\')
    if ($full -notmatch '^([A-Za-z]):\\(.+)$') { throw "$full isn't a folder on a local drive. Point the script at folders like D:\Media, not drive roots or UNC paths." }
    $child = if ($IncludeDriveLetter) { '{0}\{1}' -f $Matches[1].ToUpper(), $Matches[2] } else { $Matches[2] }
    # Plain string join: Join-Path insists the destination drive already exists, which breaks -WhatIf planning.
    [pscustomobject]@{ Source = $full; Destination = '{0}\{1}' -f $DestinationRoot.TrimEnd('\'), $child }
}

$dupes = $plan | Group-Object Destination | Where-Object Count -gt 1
if ($dupes) {
    throw "These sources would land in the same place: $(($dupes.Group.Source) -join ', '). Use -IncludeDriveLetter."
}

$verb = if ($Copy) { 'Copy' } else { 'Move' }
foreach ($item in $plan) {
    if (-not $PSCmdlet.ShouldProcess($item.Source, "$verb to $($item.Destination)")) { continue }

    $log  = Join-Path $LogFolder ('robocopy_{0}_{1:yyyyMMdd-HHmmss}.log' -f ($item.Source -replace '[:\\ ]+', '_').Trim('_'), (Get-Date))
    $rcArgs = @($item.Source, $item.Destination, '/E', '/COPY:DAT', '/DCOPY:DAT', '/XJ', '/R:2', '/W:5', "/MT:$Threads", '/NP', "/LOG:$log")
    if (-not $Copy) { $rcArgs += '/MOVE' }

    Write-Verbose "robocopy $($rcArgs -join ' ')"
    try {
        robocopy @rcArgs | Out-Null
        $code = $LASTEXITCODE
        [pscustomobject]@{
            Source      = $item.Source
            Destination = $item.Destination
            ExitCode    = $code
            Result      = if ($code -ge 8) { 'FAILED - see log' } else { $exitMeaning[$code] }
            Log         = $log
        }
        if ($code -ge 8) { Write-Warning "$($item.Source): robocopy exit code $code. Check $log" }
    }
    catch {
        Write-Warning "$($item.Source): $($_.Exception.Message)"
    }
}