Utility for converting files with indices in their names to be the quotient/remainder of a divisor. This is useful if there's a large number of files that actually represent groups of 3/4/5/etc, and it'd be less confusing for them to be indexed by what group they're in.

filename_muxy.ps1

param
(
    [int]$divisor = 3,
    [int]$indexingOrigin = 1, # Set to "0" if you want zero-based indexing, and likewise to "1" for one-based.
    [bool]$padNumbers = $true
)

[regex]$indexRegex = "\d+"

Get-ChildItem -File -Recurse | ForEach-Object `
{
    $fileItem = $_

    Write-Host -NoNewLine "$($fileItem.Name), "

    $regexMatch = $indexRegex.Match($fileItem.BaseName)

    if ($regexMatch.Success)
    {
        $oldIndex = $regexMatch.Value
        $newMainIndex = [int]([Math]::Floor(($oldIndex - $indexingOrigin) / $divisor) + $indexingOrigin)
        
        # If desired, pad the output to the same number of digits as the input.
        if ($padNumbers)
        {
            $newMainIndex = ([string]$newMainIndex).PadLeft($oldIndex.Length, '0')
        }
        
        $directoryName = Split-Path -Path $fileItem.PSPath

        # Generate a new filename with an additional sub-index that avoids collisions with any existing file.
        $newFilename = ""
        for ($newSubIndex = $indexingOrigin; <# internal break #>; $newSubIndex++)
        {
            # Replace the first instance of the old index.
            $newFilename = $indexRegex.Replace($fileItem.Name, "$($newMainIndex)_$($newSubIndex)", 1)

            if (-Not (Test-Path (Join-Path $directoryName $newFilename) -PathType Leaf))
            {
                # The filename does't collide, so we'll accept it.
                break
            }
        }
        
        Write-Host "[$($oldIndex)], `"$($fileItem.Name)`" -> `"$($newFilename)`""

        Rename-Item -Path $_.PSPath -NewName $newFilename
    }
    else
    {
        Write-Host "ignored"
    }
}

Write-Host -NoNewLine "Press any key to exit...";
$null = $Host.UI.RawUI.ReadKey("NoEcho,IncludeKeyDown");