[CmdletBinding(DefaultParameterSetName = "Url")]
param(
    [Parameter(Mandatory = $true, ParameterSetName = "Url")]
    [string]$Url,

    [Parameter(Mandatory = $true, ParameterSetName = "Path")]
    [string]$Path
)

$ErrorActionPreference = "Stop"

function Fail($Message) {
    Write-Error $Message
    exit 1
}

$feedSource = $null
$feedContent = $null

if ($PSCmdlet.ParameterSetName -eq "Url") {
    $parsedUrl = $null
    if (-not [System.Uri]::TryCreate($Url, [System.UriKind]::Absolute, [ref]$parsedUrl)) {
        Fail "Desktop update feed URL is not absolute: $Url"
    }

    if ($parsedUrl.Scheme -ne "https") {
        Fail "Desktop update feed URL must use HTTPS: $Url"
    }

    try {
        $response = Invoke-WebRequest -Uri $Url -UseBasicParsing -TimeoutSec 30
    } catch {
        Fail "Desktop update feed request failed for ${Url}: $($_.Exception.Message)"
    }

    if ($response.StatusCode -ne 200) {
        Fail "Desktop update feed must return HTTP 200. Actual status: $($response.StatusCode)"
    }

    $feedSource = $Url
    $feedContent = $response.Content
} else {
    if (-not (Test-Path -LiteralPath $Path)) {
        Fail "Desktop update feed file was not found: $Path"
    }

    $feedSource = (Resolve-Path -LiteralPath $Path).Path
    $feedContent = Get-Content -LiteralPath $feedSource -Raw
}

try {
    [xml]$xml = $feedContent
} catch {
    Fail "Desktop update feed is not valid XML: $($_.Exception.Message)"
}

if ($xml.DocumentElement.Name -ne "QSfera") {
    Fail "Desktop update feed root element must be <QSfera>."
}

$requiredElements = @("version", "versionstring", "web", "downloadurl")
foreach ($name in $requiredElements) {
    if ($null -eq $xml.QSfera.$name) {
        Fail "Desktop update feed is missing <$name>."
    }
}

$version = [string]$xml.QSfera.version
$versionString = [string]$xml.QSfera.versionstring
$web = [string]$xml.QSfera.web
$downloadUrl = [string]$xml.QSfera.downloadurl

if (-not [string]::IsNullOrWhiteSpace($web)) {
    $webUri = $null
    if (-not [System.Uri]::TryCreate($web, [System.UriKind]::Absolute, [ref]$webUri) -or $webUri.Scheme -ne "https") {
        Fail "Desktop update feed <web> must be an absolute HTTPS URL when present."
    }
}

if ([string]::IsNullOrWhiteSpace($version)) {
    if (-not [string]::IsNullOrWhiteSpace($downloadUrl)) {
        Fail "Desktop no-update feed must leave <downloadurl> empty when <version> is empty."
    }
} else {
    $parsedVersion = $null
    if (-not [System.Version]::TryParse($version, [ref]$parsedVersion)) {
        Fail "Desktop update feed <version> must be parseable as a version number."
    }

    if ([string]::IsNullOrWhiteSpace($versionString)) {
        Fail "Desktop update feed must set <versionstring> when <version> is present."
    }

    $downloadUri = $null
    if (-not [System.Uri]::TryCreate($downloadUrl, [System.UriKind]::Absolute, [ref]$downloadUri) -or $downloadUri.Scheme -ne "https") {
        Fail "Desktop update feed <downloadurl> must be an absolute HTTPS URL when an update is advertised."
    }

    if (-not $downloadUri.AbsolutePath.EndsWith(".msi", [System.StringComparison]::OrdinalIgnoreCase)) {
        Fail "Desktop Windows updater expects an MSI download URL."
    }
}

Write-Host "QSfera Desktop update feed validation passed: $feedSource"