extension PureXML.Schema.XSDParser {
    /// Keys of identity-constraint fields whose target element is, by declaration,
    /// not of simple type: the ur-type `anyType` from an absent type, or an inline
    /// complex type without simple content. XSD 1.0 §3.11.4 (cvc-identity-constraint.3)
    /// requires a field to identify a node of simple type, so such a field makes the
    /// instance invalid (XSTS idZ010). Resolved conservatively: a key is recorded only
    /// when the selector and field paths resolve unambiguously, by element-name steps,
    /// to a declared element that is provably non-simple. Any wildcard, descendant
    /// axis, predicate, named (unresolved) type, or `ref` leaves the key out, so a
    /// valid field, which always names a simple-typed element or attribute, is never
    /// flagged.
    static func identityNonSimpleFieldKeys(_ containers: [XSDTree], _ context: PureXML.Schema.XSDContext) -> Set<String> {
        var keys: Set<String> = []
        for container in containers {
            for element in descendants(container, named: "element") {
                let constraints = PureXML.Schema.XSDNode.elementChildren(element).compactMap(constraint)
                for constraint in constraints {
                    guard let selectorTarget = pathTargetElement(from: element, path: constraint.selector) else { continue }
                    for field in constraint.fields where !field.hasPrefix("@") {
                        guard let fieldTarget = pathTargetElement(from: selectorTarget, path: field),
                              elementIsDeclaredNonSimple(fieldTarget, context)
                        else { continue }
                        keys.insert(identityFieldKey(constraint: constraint, field: field))
                    }
                }
            }
        }
        return keys
    }

    /// The declared element node a selector or field XPath reaches from `start`,
    /// following only the restricted-XPath child-name steps (a leading or interior
    /// `.` is self). Returns nil for any step that is not a plain name test (a
    /// wildcard `*`, a `//` descendant step, an axis `::`, a predicate `[`, an
    /// attribute `@`), or that names no direct child element, so the caller flags
    /// only paths it has resolved with certainty.
    private static func pathTargetElement(from start: XSDTree, path: String) -> XSDTree? {
        var current: XSDTree? = start
        for rawStep in path.split(separator: "/", omittingEmptySubsequences: false) {
            let step = String(rawStep).trimmingXMLWhitespace()
            if step == "." {
                continue
            }
            guard isSimpleNameTest(step), let node = current,
                  let child = directChildElement(named: PureXML.Schema.XSDNode.stripPrefix(step), of: node)
            else { return nil }
            current = child
        }
        return current
    }

    /// Whether `step` is a plain element name test (`NCName` or `prefix:NCName`),
    /// not a wildcard, axis, predicate, or `//` (an empty step from a split).
    private static func isSimpleNameTest(_ step: String) -> Bool {
        guard !step.isEmpty, !step.contains(where: { "*[]()|@ \t".contains($0) }) else { return false }
        return step.count(where: { $0 == ":" }) <= 1
    }

    /// The direct child element declaration of `element`'s `complexType` model with
    /// the given local name, descending through model groups but not into another
    /// element's own type, and never matching a `ref` (which has no `name`).
    private static func directChildElement(named local: String, of element: XSDTree) -> XSDTree? {
        guard let complexType = PureXML.Schema.XSDNode.firstChild(element, named: "complexType") else { return nil }
        return modelChildElements(of: complexType).first { PureXML.Schema.XSDNode.attribute($0, "name") == local }
    }

    /// Every direct element declaration in a complex type's content model, reached
    /// through the model-group and derivation wrappers but stopping at each
    /// `element` (so a nested element's own children are not collected).
    private static func modelChildElements(of node: XSDTree) -> [XSDTree] {
        var result: [XSDTree] = []
        for child in PureXML.Schema.XSDNode.elementChildren(node) {
            switch PureXML.Schema.XSDNode.localName(child) {
            case "element": result.append(child)
            case "sequence", "choice", "all", "group", "complexContent", "restriction", "extension":
                result += modelChildElements(of: child)
            default: break
            }
        }
        return result
    }

    /// Whether `element`'s declared type is provably not a simple type: an explicit
    /// `xsd:anyType`, an inline `complexType` without `simpleContent`, or no type at
    /// all (the ur-type `anyType`). A builtin, `anySimpleType`, an inline
    /// `simpleType`, `simpleContent`, or any named non-builtin type is treated as
    /// possibly-simple and not flagged, so the check never rejects a valid field.
    private static func elementIsDeclaredNonSimple(_ element: XSDTree, _ context: PureXML.Schema.XSDContext) -> Bool {
        if let typeName = PureXML.Schema.XSDNode.attribute(element, "type") {
            let bindings = PureXML.Schema.XSDParser.namespaceBindingsInScope(of: element, defaultBindings: context.namespaceBindings)
            let local = PureXML.Schema.XSDNode.stripPrefix(typeName)
            guard PureXML.Schema.XSDNode.referenceNamespace(typeName, bindings) == PureXML.Schema.XSDParser.xsdNamespace else {
                return false
            }
            return local == "anyType"
        }
        if PureXML.Schema.XSDNode.firstChild(element, named: "simpleType") != nil {
            return false
        }
        if let complex = PureXML.Schema.XSDNode.firstChild(element, named: "complexType") {
            return PureXML.Schema.XSDNode.firstChild(complex, named: "simpleContent") == nil
        }
        return true
    }
}