param(
    [Parameter(Mandatory = $true)][int]$TemplateId,
    [string]$ConfigPath = (Join-Path (Split-Path -Parent $PSScriptRoot) 'config.ini'),
    [string]$OutputRoot
)

$ErrorActionPreference = 'Stop'
. (Join-Path $PSScriptRoot 'template-common.ps1')
if (-not $OutputRoot) { $OutputRoot = Join-Path (Split-Path -Parent $ConfigPath) 'temp\previews' }
if ($TemplateId -lt 1) { throw 'TemplateId must be positive' }
$context = Get-MailContext -ConfigPath $ConfigPath
$connection = Open-MailDatabase $context
try {
    $command = $connection.CreateCommand()
    $command.CommandText = 'SELECT [HtmlContent],[ContentHash],[ValidationStatus] FROM [TemplateVersion] WHERE [Id]=?'
    $null = $command.Parameters.Add('id', [System.Data.OleDb.OleDbType]::Integer)
    $command.Parameters[0].Value = $TemplateId
    $reader = $command.ExecuteReader()
    try {
        if (-not $reader.Read()) { throw "Template ID $TemplateId was not found" }
        $html = [string]$reader.GetValue(0)
        $hash = [string]$reader.GetValue(1)
        if ([string]$reader.GetValue(2) -ne 'validated' -or
            (Get-HashHex ([System.Text.Encoding]::UTF8.GetBytes($html))) -ne $hash) {
            throw 'Template validation or content hash failed'
        }
    }
    finally { $reader.Close() }
    $document = [xml]$html
    Assert-TemplateNode $document.DocumentElement
    $previewPath = Join-Path $OutputRoot ([guid]::NewGuid().ToString('N'))
    $null = New-Item -ItemType Directory -Path $previewPath -Force
    $command = $connection.CreateCommand()
    $command.CommandText = 'SELECT [OriginalRelativePath],[ContentType],[ContentHash],[SnapshotBytes],[ContentId] FROM [TemplateImage] WHERE [TemplateVersionId]=? ORDER BY [Id]'
    $null = $command.Parameters.Add('id', [System.Data.OleDb.OleDbType]::Integer)
    $command.Parameters[0].Value = $TemplateId
    $reader = $command.ExecuteReader()
    try {
        $index = 0
        while ($reader.Read()) {
            $index++
            $bytes = [byte[]]$reader.GetValue(3)
            if ((Get-HashHex $bytes) -ne [string]$reader.GetValue(2)) { throw 'Stored image hash failed' }
            $type = [string]$reader.GetValue(1)
            $ext = switch ($type) { 'image/png' { 'png' } 'image/jpeg' { 'jpg' } 'image/gif' { 'gif' } default { throw 'Unsupported stored image type' } }
            $name = "image-$index.$ext"
            [System.IO.File]::WriteAllBytes((Join-Path $previewPath $name), $bytes)
            $cid = 'cid:' + [string]$reader.GetValue(4)
            $found = $false
            foreach ($node in $document.GetElementsByTagName('img')) {
                if ($node.GetAttribute('src') -eq $cid) { $node.SetAttribute('src', $name); $found = $true }
            }
            if (-not $found) { throw 'Stored image is not referenced by the template' }
        }
    }
    finally { $reader.Close() }
    foreach ($node in $document.GetElementsByTagName('img')) {
        if ($node.GetAttribute('src').StartsWith('cid:')) { throw 'Template image snapshot is missing' }
    }
    $file = Join-Path $previewPath 'preview.html'
    [System.IO.File]::WriteAllText($file, $document.OuterXml, (New-Object System.Text.UTF8Encoding($false)))
    Write-Output $file
}
finally { $connection.Dispose() }