param(
    # The folder containing the images
    [Parameter()][string]$imgPath = $env:imgPath,
    [Parameter()][string]$bgiPath = $env:bgiPath
)
if ([string]::IsNullOrEmpty($imgPath) -or [string]::IsNullOrEmpty($bgiPath)) {
    throw "You need to set imgPath and bgiPath environment variables or pass them in."
}

#region function definitions
function Set-WallPaper($value) {
    Set-ItemProperty -Path 'HKCU:\Control Panel\Desktop\' -Name WallPaper -Value $value
    Set-ItemProperty -Path 'HKCU:\Control Panel\Desktop\' -Name WallpaperStyle -Value 10
    Set-ItemProperty -Path 'HKCU:\Control Panel\Desktop\' -Name TileWallpaper -Value 0
    RUNDLL32.EXE USER32.DLL,UpdatePerUserSystemParameters 1, True
}
#endregion

# get image list
$imgList = Get-ChildItem $imgPath |
            Where-Object {('.jpg','.bmp','.png').Contains($_.Extension)} |
            Sort-Object { [regex]::Replace($_, '\d+', { $args[0].Value.PadLeft(20) }) } # natual sort
if ($imgList.Length -eq 0) { throw ('No image found under ' + $imgPath) }

# select a wallpaper based on date
[System.IO.FileSystemInfo]$img = $null
$imgIdx = 0
$now = Get-Date
if ($imgList.Length -gt 12) {
    # more than 12 images found (distributed over a year)
    $imgIdx = ($now.DayOfYear - 1) % $imgList.Length
}
else {
    # less than 12 images (one per month)
    $imgIdx = ($now.Month - 1) % $imgList.Length
}
$img = $imgList[$imgIdx]


# apply configuration
Set-WallPaper $img.FullName
Start-Process -FilePath 'bginfo.exe' -ArgumentList 'config.bgi /timer:00' -WorkingDirectory $bgiPath
Write-Host ('Wallpaper set to ' + $img.FullName)