extension PureXML.Schema.XSDParser {
    /// The unprefixed attributes each XSD component admits (XSD 1.0 schema for
    /// schemas). The element/attribute entries are the permissive union across
    /// global and local use (so a context-only attribute like `form` or
    /// `minOccurs` is never wrongly flagged); `id` is admitted everywhere. An
    /// attribute from a foreign namespace (prefixed) is always allowed and is not
    /// listed here.
    private static let allowedAttributes: [String: Set<String>] = {
        let facetAttrs: Set = ["value", "fixed", "id"]
        var table: [String: Set<String>] = [
            "schema": ["targetNamespace", "version", "finalDefault", "blockDefault", "attributeFormDefault", "elementFormDefault", "id"],
            "element": ["name", "ref", "type", "minOccurs", "maxOccurs", "default", "fixed", "nillable", "abstract", "substitutionGroup", "final", "block", "form", "id"],
            "attribute": ["name", "ref", "type", "use", "default", "fixed", "form", "id"],
            "complexType": ["name", "abstract", "final", "block", "mixed", "id"],
            "simpleType": ["name", "final", "id"],
            "complexContent": ["mixed", "id"],
            "simpleContent": ["id"],
            "restriction": ["base", "id"],
            "extension": ["base", "id"],
            "group": ["name", "ref", "minOccurs", "maxOccurs", "id"],
            "attributeGroup": ["name", "ref", "id"],
            "sequence": ["minOccurs", "maxOccurs", "id"],
            "choice": ["minOccurs", "maxOccurs", "id"],
            "all": ["minOccurs", "maxOccurs", "id"],
            "any": ["namespace", "processContents", "minOccurs", "maxOccurs", "id"],
            "anyAttribute": ["namespace", "processContents", "id"],
            "unique": ["name", "id"],
            "key": ["name", "id"],
            "keyref": ["name", "refer", "id"],
            "selector": ["xpath", "id"],
            "field": ["xpath", "id"],
            "list": ["itemType", "id"],
            "union": ["memberTypes", "id"],
            "import": ["namespace", "schemaLocation", "id"],
            "include": ["schemaLocation", "id"],
            "redefine": ["schemaLocation", "id"],
            "notation": ["name", "public", "system", "id"],
            "annotation": ["id"],
        ]
        for facet in ["minExclusive", "minInclusive", "maxExclusive", "maxInclusive", "totalDigits", "fractionDigits", "length", "minLength", "maxLength", "whiteSpace"] {
            table[facet] = facetAttrs
        }
        for facet in ["enumeration", "pattern"] {
            table[facet] = ["value", "id"]
        }
        return table
    }()

    /// Findings for the unprefixed attributes on `node`: any not admitted by its
    /// component, the `ref` exclusions (`ref` excludes `name` and `type`), and the
    /// `type`-excludes-inline-type exclusion. A prefixed (foreign-namespace)
    /// attribute and a namespace declaration are not the component's own and are
    /// not checked.
    static func attributeApplicabilityErrors(_ node: XSDTree, local: String) -> [String] {
        guard let allowed = allowedAttributes[local] else { return [] }
        var errors: [String] = []
        var present: Set<String> = []
        for attribute in node.attributes where attribute.name.prefix == nil && attribute.name.localName != "xmlns" {
            let name = attribute.name.localName
            present.insert(name)
            if !allowed.contains(name) {
                errors.append("attribute '\(name)' is not allowed on '\(local)'")
            }
        }
        if present.contains("ref"), present.contains("name") {
            errors.append("'\(local)' may not have both 'ref' and 'name'")
        }
        if present.contains("ref"), present.contains("type") {
            errors.append("'\(local)' may not have both 'ref' and 'type'")
        }
        // src-element.3 / src-attribute.3-4: a `type` attribute and an inline
        // anonymous type definition are mutually exclusive. An attribute may carry
        // only an inline `simpleType`; an element may carry a `simpleType` or
        // `complexType`.
        if present.contains("type"), local == "element" || local == "attribute", hasInlineType(node, local) {
            errors.append("'\(local)' may not have both a 'type' attribute and an inline type definition")
        }
        return errors
    }

    private static func hasInlineType(_ node: XSDTree, _ local: String) -> Bool {
        PureXML.Schema.XSDNode.elementChildren(node).contains { child in
            // Only an inline type in the XSD namespace counts; a foreign-namespace
            // child that happens to be named `complexType`/`simpleType` is not the
            // component's own type definition (foreign content is not checked here).
            guard child.name?.namespaceURI == xsdNamespace else { return false }
            let kind = PureXML.Schema.XSDNode.localName(child)
            return kind == "simpleType" || (local == "element" && kind == "complexType")
        }
    }
}