Convert ZIP Files to 7z, but Only When It Actually Saves Space

<#
.SYNOPSIS
    Repacks ZIP archives as 7z and tells you how much space it actually saved.
.DESCRIPTION
    Extracts each ZIP to a private temp folder, builds a .7z from the contents, tests the new
    archive with 7-Zip, and only then moves it into place. If the 7z isn't smaller than the ZIP,
    it's thrown away and the ZIP is kept (unless you say otherwise). The original ZIP is only
    deleted when you ask for it with -RemoveSource. Supports -WhatIf.
.PARAMETER Path
    ZIP files, or folders to search for ZIP files. Accepts pipeline input.
.PARAMETER Recurse
    Search subfolders when Path is a folder.
.PARAMETER Destination
    Folder for the new .7z files. Defaults to the same folder as each ZIP.
.PARAMETER CompressionLevel
    7-Zip -mx level: 1 (fastest) to 9 (smallest). Default 9.
.PARAMETER KeepLarger
    Keep the .7z even when it's the same size or bigger than the ZIP.
.PARAMETER RemoveSource
    Delete each ZIP after its .7z has been built and tested.
.PARAMETER Force
    Overwrite a .7z that already exists at the destination.
.PARAMETER SevenZipPath
    Full path to 7z.exe. If omitted, the script checks PATH, then the usual Program Files folders.
.EXAMPLE
    .\Convert-ZipTo7z.ps1 -Path D:\Downloads -WhatIf
.EXAMPLE
    .\Convert-ZipTo7z.ps1 -Path D:\Archive -Recurse -RemoveSource
#>
[CmdletBinding(SupportsShouldProcess)]
param(
    [Parameter(ValueFromPipeline, ValueFromPipelineByPropertyName)]
    [Alias('FullName')]
    [string[]]$Path = '.',
    [switch]$Recurse,
    [string]$Destination,
    [ValidateSet(1, 3, 5, 7, 9)][int]$CompressionLevel = 9,
    [switch]$KeepLarger,
    [switch]$RemoveSource,
    [switch]$Force,
    [string]$SevenZipPath
)

begin {
    if (-not $SevenZipPath) {
        $SevenZipPath = @(
            Get-Command -Name 7z.exe, 7z -CommandType Application -ErrorAction SilentlyContinue | Select-Object -First 1 -ExpandProperty Source
            if ($env:ProgramFiles) { Join-Path $env:ProgramFiles '7-Zip\7z.exe' }
            if (${env:ProgramFiles(x86)}) { Join-Path ${env:ProgramFiles(x86)} '7-Zip\7z.exe' }
        ) | Where-Object { $_ -and (Test-Path -LiteralPath $_) } | Select-Object -First 1
    }
    if (-not $SevenZipPath -or -not (Test-Path -LiteralPath $SevenZipPath)) {
        throw '7-Zip not found. Install it from 7-zip.org or pass -SevenZipPath.'
    }
    Write-Verbose "Using 7-Zip at $SevenZipPath"

    function Invoke-SevenZip([string[]]$Arguments) {
        $out = & $SevenZipPath @Arguments 2>&1
        if ($LASTEXITCODE -ge 2) {
            $errText = $out | Where-Object { $_ -is [System.Management.Automation.ErrorRecord] } | ForEach-Object { "$_".Trim() } | Where-Object { $_ -match '\w' -and $_ -notmatch 'RemoteException|^ERRORS:$' -and $_ -ne $Arguments[-1] }
            throw "7-Zip exit code ${LASTEXITCODE}: $(($errText | Select-Object -Unique) -join '; ')"
        }
        if ($LASTEXITCODE -eq 1) { Write-Warning "7-Zip reported warnings for $($Arguments[-1])" }
    }
}

process {
    $zips = foreach ($item in $Path) {
        if (Test-Path -LiteralPath $item -PathType Container) {
            Get-ChildItem -LiteralPath $item -Filter *.zip -File -Recurse:$Recurse
        }
        else { Get-Item -LiteralPath $item }
    }

    foreach ($zip in $zips) {
        $outDir = if ($Destination) { $Destination } else { $zip.DirectoryName }
        $target = Join-Path $outDir ($zip.BaseName + '.7z')
        $result = [ordered]@{ Name = $zip.Name; ZipKB = [math]::Round($zip.Length / 1KB); SevenZipKB = $null; SavedPercent = $null; Status = $null; Target = $target }

        if ((Test-Path -LiteralPath $target) -and -not $Force) {
            $result.Status = 'SkippedExists'
            [pscustomobject]$result
            continue
        }
        if (-not $PSCmdlet.ShouldProcess($zip.FullName, "Repack as $target")) { continue }

        $work = Join-Path ([IO.Path]::GetTempPath()) ('zip2-7z-' + [guid]::NewGuid().ToString('N'))
        try {
            $files = Join-Path $work 'files'
            $null = New-Item -ItemType Directory -Path $files -Force
            Invoke-SevenZip @('x', '-y', '-bd', "-o$files", $zip.FullName)
            if (-not (Get-ChildItem -LiteralPath $files -Force)) { throw 'ZIP is empty.' }

            $temp7z = Join-Path $work 'out.7z'
            Invoke-SevenZip @('a', '-t7z', "-mx=$CompressionLevel", '-y', '-bd', $temp7z, (Join-Path $files '*'))
            Invoke-SevenZip @('t', '-bd', $temp7z)

            $newSize = (Get-Item -LiteralPath $temp7z).Length
            $result.SevenZipKB = [math]::Round($newSize / 1KB)
            $result.SavedPercent = if ($zip.Length) { [math]::Round((1 - $newSize / $zip.Length) * 100, 1) } else { 0 }

            if ($newSize -ge $zip.Length -and -not $KeepLarger) {
                $result.Status = 'NoGainKeptZip'
            }
            else {
                if (-not (Test-Path -LiteralPath $outDir)) { $null = New-Item -ItemType Directory -Path $outDir }
                Move-Item -LiteralPath $temp7z -Destination $target -Force -WhatIf:$false
                $result.Status = 'Converted'
                if ($RemoveSource -and $PSCmdlet.ShouldProcess($zip.FullName, 'Delete original ZIP')) {
                    Remove-Item -LiteralPath $zip.FullName -Force
                    $result.Status = 'ConvertedRemovedZip'
                }
            }
        }
        catch {
            $result.Status = "Failed: $($_.Exception.Message)"
            Write-Warning "$($zip.Name): $($_.Exception.Message)"
        }
        finally {
            Remove-Item -LiteralPath $work -Recurse -Force -ErrorAction SilentlyContinue -WhatIf:$false
        }
        [pscustomobject]$result
    }
}