param(
    [Parameter(Mandatory=$true)]
    [string]$Url,

    [Parameter(Mandatory=$true)]
    [string]$Destination
)

$ErrorActionPreference = 'Stop'

Write-Host "============================================"
Write-Host "  Resumable File Download"
Write-Host "============================================"
Write-Host ""
Write-Host "Destination: $Destination"
Write-Host ""

try {
    # Check for partial file to resume
    $startBytes = 0
    if (Test-Path $Destination) {
        $startBytes = (Get-Item $Destination).Length
        if ($startBytes -gt 0) {
            Write-Host "Partial file found: $([math]::Round($startBytes / 1MB, 1)) MB already downloaded"
            Write-Host "Resuming..."
            Write-Host ""
        }
    }

    # Ensure destination directory exists
    $destDir = Split-Path $Destination -Parent
    if ($destDir -and !(Test-Path $destDir)) {
        New-Item -ItemType Directory -Path $destDir -Force | Out-Null
    }

    # Build headers
    $headers = @{}
    if ($startBytes -gt 0) {
        $headers["Range"] = "bytes=$startBytes-"
    }

    # Use Invoke-WebRequest with domain credentials for SharePoint auth
    $params = @{
        Uri                = $Url
        OutFile            = $Destination
        UseDefaultCredentials = $true
        UserAgent          = "Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36"
        Headers            = $headers
        UseBasicParsing    = $true
    }

    # If resuming, we need to handle appending manually
    if ($startBytes -gt 0) {
        # Download to a temp file, then append
        $tempFile = "$Destination.partial"
        $params.OutFile = $tempFile

        Write-Host "Downloading..."
        Invoke-WebRequest @params

        # Check if we got actual content
        $tempSize = (Get-Item $tempFile).Length
        if ($tempSize -eq 0) {
            Remove-Item $tempFile -Force
            Write-Host ""
            Write-Host "WARNING: Server returned 0 bytes. The URL may have expired."
            Write-Host "Get a fresh SharePoint link and try again."
            exit 1
        }

        # Append to existing file
        $existingBytes = [System.IO.File]::ReadAllBytes($Destination)
        $newBytes = [System.IO.File]::ReadAllBytes($tempFile)
        $combined = New-Object byte[] ($existingBytes.Length + $newBytes.Length)
        [System.Buffer]::BlockCopy($existingBytes, 0, $combined, 0, $existingBytes.Length)
        [System.Buffer]::BlockCopy($newBytes, 0, $combined, $existingBytes.Length, $newBytes.Length)
        [System.IO.File]::WriteAllBytes($Destination, $combined)
        Remove-Item $tempFile -Force

        $totalMB = [math]::Round(($existingBytes.Length + $newBytes.Length) / 1MB, 1)
        Write-Host ""
        Write-Host "Download complete: $totalMB MB saved to $Destination"
    }
    else {
        Write-Host "Downloading..."
        Invoke-WebRequest @params

        # Verify we got actual content
        $fileSize = (Get-Item $Destination).Length
        if ($fileSize -eq 0) {
            Remove-Item $Destination -Force
            Write-Host ""
            Write-Host "WARNING: Downloaded 0 bytes. Possible causes:"
            Write-Host "  - SharePoint URL expired or requires browser login"
            Write-Host "  - URL is a redirect/login page, not the actual file"
            Write-Host ""
            Write-Host "Try this: In Edge, start the download, then go to"
            Write-Host "edge://downloads and copy the source URL from there."
            exit 1
        }

        $totalMB = [math]::Round($fileSize / 1MB, 1)
        Write-Host ""
        Write-Host "Download complete: $totalMB MB saved to $Destination"
    }

} catch {
    Write-Host ""
    Write-Host "Error: $_"
    Write-Host ""
    if (Test-Path $Destination) {
        $partialMB = [math]::Round((Get-Item $Destination).Length / 1MB, 1)
        Write-Host "Partial file kept: $partialMB MB"
    }
    Write-Host "Re-run with the same arguments to resume."
    exit 1
}