<#
.SYNOPSIS
    Zips up folders that haven't been touched in N days, and optionally removes the originals.
.DESCRIPTION
    Looks at each subfolder of -Path and finds the newest file anywhere inside it. If nothing
    in the folder has changed for -OlderThanDays, the folder is zipped to -Destination as
    <FolderName>_<yyyy-MM-dd>.zip (the date of the last change). The ZIP is checked against
    the folder's file count before -RemoveSource is allowed to delete anything. Empty folders
    are skipped.
    Supports -WhatIf.
.PARAMETER Path
    The parent folder. Each folder directly under it is considered separately.
.PARAMETER Destination
    Where the ZIP files go. Created if it doesn't exist.
.PARAMETER OlderThanDays
    Archive a folder only if its newest file is older than this. Default 180.
.PARAMETER RemoveSource
    Delete each folder after its ZIP has been written and verified.
.PARAMETER Force
    Overwrite a ZIP with the same name that's already in the destination.
.EXAMPLE
    .\Compress-OldFolder.ps1 -Path D:\Projects -Destination E:\Archive\Projects -WhatIf
.EXAMPLE
    .\Compress-OldFolder.ps1 -Path D:\Projects -Destination E:\Archive\Projects -OlderThanDays 365 -RemoveSource
#>
[CmdletBinding(SupportsShouldProcess)]
param(
    [Parameter(Mandatory)][ValidateScript({ Test-Path -LiteralPath $_ -PathType Container })][string]$Path,
    [Parameter(Mandatory)][string]$Destination,
    [ValidateRange(1, 36500)][int]$OlderThanDays = 180,
    [switch]$RemoveSource,
    [switch]$Force
)

Add-Type -AssemblyName System.IO.Compression, System.IO.Compression.FileSystem
$cutoff = (Get-Date).AddDays(-$OlderThanDays)
$Destination = $ExecutionContext.SessionState.Path.GetUnresolvedProviderPathFromPSPath($Destination)

foreach ($folder in Get-ChildItem -LiteralPath $Path -Directory) {
    $files  = @(Get-ChildItem -LiteralPath $folder.FullName -File -Recurse -Force -ErrorAction SilentlyContinue)
    if (-not $files) {
        Write-Verbose "$($folder.Name): no files inside, skipped."
        continue
    }
    $newest = ($files | Sort-Object LastWriteTime -Descending | Select-Object -First 1).LastWriteTime

    if ($newest -ge $cutoff) {
        Write-Verbose "$($folder.Name): last change $($newest.ToString('yyyy-MM-dd')), too recent."
        continue
    }

    $zipPath = Join-Path $Destination ('{0}_{1:yyyy-MM-dd}.zip' -f $folder.Name, $newest)
    $result  = [ordered]@{ Folder = $folder.Name; LastChange = $newest.Date; Files = $files.Count; SizeMB = [math]::Round(($files | Measure-Object Length -Sum).Sum / 1MB, 1); ZipMB = $null; Status = $null; Zip = $zipPath }

    if ((Test-Path -LiteralPath $zipPath) -and -not $Force) {
        $result.Status = 'SkippedExists'
        [pscustomobject]$result
        continue
    }
    if (-not $PSCmdlet.ShouldProcess($folder.FullName, "Archive to $zipPath")) { continue }

    $partial = "$zipPath.partial"
    try {
        if (-not (Test-Path -LiteralPath $Destination)) { $null = New-Item -ItemType Directory -Path $Destination }
        if (Test-Path -LiteralPath $partial) { Remove-Item -LiteralPath $partial -Force }

        # CreateFromDirectory includes hidden files and handles big archives, unlike Compress-Archive in 5.1.
        [System.IO.Compression.ZipFile]::CreateFromDirectory($folder.FullName, $partial, [System.IO.Compression.CompressionLevel]::Optimal, $true)

        $zip = [System.IO.Compression.ZipFile]::OpenRead($partial)
        try { $zipped = @($zip.Entries | Where-Object { $_.Name }).Count }
        finally { $zip.Dispose() }
        if ($zipped -ne $files.Count) { throw "ZIP holds $zipped files but the folder has $($files.Count)." }

        Move-Item -LiteralPath $partial -Destination $zipPath -Force -WhatIf:$false
        $result.ZipMB = [math]::Round((Get-Item -LiteralPath $zipPath).Length / 1MB, 1)
        $result.Status = 'Archived'

        if ($RemoveSource -and $PSCmdlet.ShouldProcess($folder.FullName, 'Delete folder')) {
            Remove-Item -LiteralPath $folder.FullName -Recurse -Force
            $result.Status = 'ArchivedRemoved'
        }
    }
    catch {
        Remove-Item -LiteralPath $partial -Force -ErrorAction SilentlyContinue -WhatIf:$false
        $result.Status = "Failed: $($_.Exception.Message)"
        Write-Warning "$($folder.Name): $($_.Exception.Message)"
    }
    [pscustomobject]$result
}