extension PureXML.Schema.XSDParser {
    private struct ResolutionContext {
        let check: ReferenceCheckContext

        init(
            types: Set<String>,
            pools: [String: Set<String>],
            bindings: [String: String],
            targetNamespace: String?,
            foreignPools: [String?: [String: Set<String>]],
            chameleonNamespace: Bool,
        ) {
            check = ReferenceCheckContext(
                types: types,
                pools: pools,
                bindings: bindings,
                targetNamespace: targetNamespace,
                foreignPools: foreignPools,
                chameleonNamespace: chameleonNamespace,
            )
        }
    }

    /// Schema-validity findings for unresolvable references: every QName a schema
    /// names (a `type`/`base`/`itemType`/`memberTypes` type, an `element`/
    /// `attribute`/`group`/`attributeGroup` `ref`, an element `substitutionGroup`)
    /// must resolve to a declared component or a built-in. The reference is matched
    /// by local name, as the rest of the compiler resolves it.
    ///
    /// Skipped when the document pulls in external definitions through
    /// `import`/`include`/`redefine`: the default compile does not load them, so
    /// the pools would be incomplete and a reference into them must not be flagged.
    static func referenceFindings(
        _ schema: XSDTree,
        in context: PureXML.Schema.XSDContext,
        elements: [String: PureXML.Schema.ElementType],
        containers: [XSDTree],
    ) -> [PureXML.Schema.SchemaLocatedFinding] {
        collectReferenceFindings(schema, in: context, elements: elements, containers: containers)
    }

    private static func collectReferenceFindings(
        _ schema: XSDTree,
        in context: PureXML.Schema.XSDContext,
        elements: [String: PureXML.Schema.ElementType],
        containers: [XSDTree],
    ) -> [PureXML.Schema.SchemaLocatedFinding] {
        let xsdFindings = xsdNamespaceReferenceFindings(schema)
        let simpleContentFindings = simpleContentBaseFindings(schema, in: context)
        if skipsCrossDocumentRules(schema, compositionLoaded: context.compositionLoaded) {
            return xsdFindings + simpleContentFindings
                + undeclaredNamespaceReferenceFindings(schema, containers: containers)
        }
        let types = referenceBuiltins.union(context.simpleTypes.keys).union(context.complexTypeNodes.keys)
        let pools: [String: Set<String>] = [
            "element": Set(elements.keys),
            "attribute": Set(context.globalAttributes.keys),
            "group": Set(context.groups.keys),
            "attributeGroup": Set(context.attributeGroups.keys),
        ]
        let targetNamespace = context.targetNamespace
        let foreignPools = context.compositionLoaded
            ? foreignComponentPools(containers, mainTargetNamespace: targetNamespace)
            : [:]
        var findings: [PureXML.Schema.SchemaLocatedFinding] = []
        let referenceSources: [(XSDTree, String?)]
        if context.compositionLoaded {
            let namespaceMap = resolveContainerNamespaces(containers, mainTargetNamespace: targetNamespace)
            referenceSources = containers.compactMap { container -> (XSDTree, String?)? in
                guard PureXML.Schema.XSDNode.localName(container) == "schema" else { return nil }
                let index = containers.firstIndex(where: { $0 === container }) ?? containers.count - 1
                let effectiveNamespace = namespaceMap[index] ?? targetNamespace
                return (container, effectiveNamespace)
            }
        } else {
            referenceSources = [(schema, targetNamespace)]
        }
        for (source, effectiveNamespace) in referenceSources {
            let sourceBindings = PureXML.Schema.XSDNode.namespaceBindings(of: source)
            let chameleonNamespace = PureXML.Schema.XSDNode.attribute(source, "targetNamespace") == nil
            let resolutionContext = ResolutionContext(
                types: types,
                pools: pools,
                bindings: sourceBindings,
                targetNamespace: effectiveNamespace,
                foreignPools: foreignPools,
                chameleonNamespace: chameleonNamespace,
            )
            collectReferenceFindings(source, in: resolutionContext, inheritedBindings: sourceBindings, into: &findings)
        }
        var result = xsdFindings
        result += simpleContentFindings
        result += findings
        result += redefineComponentExistenceFindings(containers)
        result += importNamespaceFindings(context, containers)
        return result
    }

    /// XSD 1.0 `src-redefine.6`/`7.2.1`/`7.2.2`: each component a `redefine` names
    /// must have a same-kind component of that name in the redefined schema, and a
    /// `redefine` may redefine a given component at most once. Runs only past the
    /// cross-document skip guard above, so the redefined schema is loaded; a
    /// redefinition whose name matches nothing in any non-`redefine` container (the
    /// loaded targets and the root) names a component that does not exist, and a
    /// name redefined twice in one `redefine` is a duplicate redefinition.
    private static func redefineComponentExistenceFindings(_ containers: [XSDTree]) -> [PureXML.Schema.SchemaLocatedFinding] {
        let kinds = ["complexType", "simpleType", "group", "attributeGroup"]
        var defined: [String: Set<String>] = [:]
        for container in containers where PureXML.Schema.XSDNode.localName(container) != "redefine" {
            for kind in kinds {
                for node in PureXML.Schema.XSDNode.children(container, named: kind) {
                    if let name = PureXML.Schema.XSDNode.attribute(node, "name") {
                        defined[kind, default: []].insert(name)
                    }
                }
            }
        }
        var findings: [PureXML.Schema.SchemaLocatedFinding] = []
        for container in containers where PureXML.Schema.XSDNode.localName(container) == "redefine" {
            var redefinedHere: [String: Set<String>] = [:]
            for kind in kinds {
                for node in PureXML.Schema.XSDNode.children(container, named: kind) {
                    guard let name = PureXML.Schema.XSDNode.attribute(node, "name") else { continue }
                    if !redefinedHere[kind, default: []].insert(name).inserted {
                        findings.append(PureXML.Schema.SchemaLocatedFinding(
                            reason: "a redefine redefines \(kind) '\(name)' more than once",
                            node: node,
                        ))
                    } else if defined[kind]?.contains(name) != true {
                        findings.append(PureXML.Schema.SchemaLocatedFinding(
                            reason: "a redefine of \(kind) '\(name)' has no '\(name)' to redefine in the redefined schema",
                            node: node,
                        ))
                    }
                }
            }
        }
        return findings
    }

    /// Whether cross-document schema-validity rules should stand down: an external
    /// reference is declared but no external document was loaded through a
    /// `schemaLoader`, so the merged component set is incomplete.
    static func skipsCrossDocumentRules(_ schema: XSDTree, compositionLoaded: Bool) -> Bool {
        hasExternalReference(schema) && !compositionLoaded
    }

    /// Whether the document declares an `import`, `include`, or `redefine`, so its
    /// component pools may be completed by an external document the default compile
    /// does not load.
    static func hasExternalReference(_ schema: XSDTree) -> Bool {
        var found = false
        func walk(_ node: XSDTree) {
            if isExternalDefinition(node) {
                found = true
                return
            }
            for child in PureXML.Schema.XSDNode.elementChildren(node) where !found {
                walk(child)
            }
        }
        walk(schema)
        return found
    }

    /// Whether `node` is an `xs:import`/`xs:include`/`xs:redefine` (an XSD-namespace
    /// element bringing in external definitions).
    private static func isExternalDefinition(_ node: XSDTree) -> Bool {
        guard node.name?.namespaceURI == xsdNamespace, let local = PureXML.Schema.XSDNode.localName(node) else {
            return false
        }
        return local == "import" || local == "include" || local == "redefine"
    }

    private static func collectReferenceFindings(
        _ node: XSDTree,
        in resolutionContext: ResolutionContext,
        inheritedBindings: [String: String],
        into findings: inout [PureXML.Schema.SchemaLocatedFinding],
    ) {
        let local = PureXML.Schema.XSDNode.localName(node)
        if local == "appinfo" || local == "documentation" {
            return
        }
        let bindings = mergedNamespaceBindings(on: node, inherited: inheritedBindings)
        let nodeContext = ResolutionContext(
            types: resolutionContext.check.types,
            pools: resolutionContext.check.pools,
            bindings: bindings,
            targetNamespace: resolutionContext.check.targetNamespace,
            foreignPools: resolutionContext.check.foreignPools,
            chameleonNamespace: resolutionContext.check.chameleonNamespace,
        )
        if node.name?.namespaceURI == xsdNamespace, let local {
            findings += referenceFindings(at: node, local: local, in: nodeContext)
        }
        for child in PureXML.Schema.XSDNode.elementChildren(node) {
            collectReferenceFindings(child, in: resolutionContext, inheritedBindings: bindings, into: &findings)
        }
    }

    private static func referenceFindings(
        at node: XSDTree,
        local: String,
        in resolutionContext: ResolutionContext,
    ) -> [PureXML.Schema.SchemaLocatedFinding] {
        var findings: [PureXML.Schema.SchemaLocatedFinding] = []
        let context = resolutionContext.check
        for attribute in ["type", "base", "itemType"] {
            guard let qname = PureXML.Schema.XSDNode.attribute(node, attribute) else { continue }
            if isUndeclaredReferenceType(qname, in: context) {
                findings.append(PureXML.Schema.SchemaLocatedFinding(reason: "\(attribute) references undeclared type '\(qname)'", node: node))
            }
        }
        if let members = PureXML.Schema.XSDNode.attribute(node, "memberTypes") {
            for token in members.split(whereSeparator: \.isWhitespace) {
                let qname = String(token)
                if isUndeclaredReferenceType(qname, in: context) {
                    findings.append(PureXML.Schema.SchemaLocatedFinding(reason: "memberTypes references undeclared type '\(qname)'", node: node))
                }
            }
        }
        if let head = PureXML.Schema.XSDNode.attribute(node, "substitutionGroup") {
            if isUndeclaredReferenceRef(head, poolName: "element", in: context) {
                findings.append(PureXML.Schema.SchemaLocatedFinding(reason: "substitutionGroup references undeclared element '\(head)'", node: node))
            }
        }
        if let reference = PureXML.Schema.XSDNode.attribute(node, "ref"), context.pools[local] != nil {
            if isUndeclaredReferenceRef(reference, poolName: local, in: context) {
                findings.append(PureXML.Schema.SchemaLocatedFinding(reason: "\(local) ref references undeclared '\(reference)'", node: node))
            }
        }
        return findings
    }
}