Param(
    [Parameter(Mandatory=$true)]
    [string]$SiteRoot,

    [Parameter(Mandatory=$true)]
    [string]$BaseUrl,

    [string[]]$Extensions = @('.html', '.htm', '.php', '.asp', '.aspx'),

    [string]$OutputFile = "sitemap.xml",

    [ValidateSet('always','hourly','daily','weekly','monthly','yearly','never')]
    [string]$ChangeFreq = 'weekly',

    [ValidateRange(0.0,1.0)]
    [double]$Priority = 0.5
)

# Normalize inputs
$SiteRoot = (Resolve-Path -Path $SiteRoot).ProviderPath
if (-not $BaseUrl.EndsWith('/')) {
    $BaseUrl = "$BaseUrl/"
}
$OutputPath = Join-Path -Path $SiteRoot -ChildPath $OutputFile

# Collect files
$files = Get-ChildItem -Path $SiteRoot -Recurse -File |
    Where-Object { $Extensions -contains $_.Extension.ToLower() }

# Setup XML writer
$settings = New-Object System.Xml.XmlWriterSettings
$settings.Indent = $true
$settings.Encoding = [System.Text.Encoding]::UTF8

$writer = [System.Xml.XmlWriter]::Create($OutputPath, $settings)
$writer.WriteStartDocument()
$writer.WriteStartElement("urlset")
$writer.WriteAttributeString("xmlns", "http://www.sitemaps.org/schemas/sitemap/0.9")

foreach ($file in $files) {
    # Build relative URL
    $relativePath = $file.FullName.Substring($SiteRoot.Length).TrimStart('\','/')
    $relativePath = $relativePath -replace '\\','/'
    $loc = $BaseUrl + $relativePath
    # Escape characters per RFC 3986
    $locEscaped = [System.Uri]::EscapeUriString($loc)

    $lastMod = $file.LastWriteTimeUtc.ToString("yyyy-MM-ddTHH:mm:sszzz")

    $writer.WriteStartElement("url")

    $writer.WriteElementString("loc", $locEscaped)
    $writer.WriteElementString("lastmod", $lastMod)
    $writer.WriteElementString("changefreq", $ChangeFreq)
    $writer.WriteElementString("priority", $Priority.ToString("0.0"))

    $writer.WriteEndElement() # url
}

$writer.WriteEndElement() # urlset
$writer.WriteEndDocument()
$writer.Flush()
$writer.Close()

Write-Host "Sitemap generated at: $OutputPath"