Function validateMxCellStructure

frontend/lib/utils.ts:541–671

(xml: string)

 * @returns null if valid, error message string if invalid
 */
export function validateMxCellStructure(xml: string): string | null {
    const parser = new DOMParser()
    const doc = parser.parseFromString(xml, "text/xml")

    // Check for XML parsing errors (includes unescaped special characters)
    const parseError = doc.querySelector("parsererror")
    if (parseError) {
        return `Invalid XML: The XML contains syntax errors (likely unescaped special characters like <, >, & in attribute values). Please escape special characters: use &lt; for <, &gt; for >, &amp; for &, &quot; for ". Regenerate the diagram with properly escaped values.`
    }

    // Get all mxCell elements once for all validations
    const allCells = doc.querySelectorAll("mxCell")

    // Single pass: collect IDs, check for duplicates, nesting, orphans, and invalid parents
    const cellIds = new Set<string>()
    const duplicateIds: string[] = []
    const nestedCells: string[] = []
    const orphanCells: string[] = []
    const invalidParents: { id: string; parent: string }[] = []
    const edgesToValidate: {
        id: string
        source: string | null
        target: string | null
    }[] = []

    allCells.forEach((cell) => {
        const id = cell.getAttribute("id")
        const parent = cell.getAttribute("parent")
        const isEdge = cell.getAttribute("edge") === "1"

        // Check for duplicate IDs
        if (id) {
            if (cellIds.has(id)) {
                duplicateIds.push(id)
            } else {
                cellIds.add(id)
            }
        }

        // Check for nested mxCell (parent element is also mxCell)
        if (cell.parentElement?.tagName === "mxCell") {
            nestedCells.push(id || "unknown")
        }

        // Check parent attribute (skip root cell id="0")
        if (id !== "0") {
            if (!parent) {
                if (id) orphanCells.push(id)
            } else {
                // Store for later validation (after all IDs collected)
                invalidParents.push({ id: id || "unknown", parent })
            }
        }

        // Collect edges for connection validation
        if (isEdge) {
            edgesToValidate.push({
                id: id || "unknown",