extension PureXML.Schema.XSDParser {
    /// Schema-validity findings for component-name uniqueness. Within a schema,
    /// global type names (simpleType and complexType share one symbol space),
    /// global element names, global attribute names, named model-group names,
    /// named attribute-group names, and notation names must each be unique;
    /// identity-constraint names
    /// (unique/key/keyref) must be unique across the whole schema. A duplicate was
    /// accepted, with a later definition silently overwriting the earlier one.
    ///
    /// Only the document's own globals are examined (`xs:redefine` children and
    /// included documents are not direct globals here), so a redefinition is not
    /// mistaken for a clash.
    static func componentNameErrors(_ schema: XSDTree, _ containers: [XSDTree], _ context: PureXML.Schema.XSDContext) -> [String] {
        let namespaceMap = resolveContainerNamespaces(containers, mainTargetNamespace: context.targetNamespace)
        let uniqueIndices = findUniqueIndices(
            in: containers,
            containerLocations: context.containerLocations,
            namespaceMap: namespaceMap,
            mainTargetNamespace: context.targetNamespace,
        )

        var errors = checkDuplicateGlobals(
            in: containers,
            indices: uniqueIndices,
            namespaceMap: namespaceMap,
            mainTargetNamespace: context.targetNamespace,
        )
        errors += identityConstraintNameErrors(
            containers,
            uniqueIndices,
            namespaceMap: namespaceMap,
            mainTargetNamespace: context.targetNamespace,
        )
        errors += keyrefReferErrors(schema, containers, context)
        return errors
    }

    private static func findUniqueIndices(
        in containers: [XSDTree],
        containerLocations: [ObjectIdentifier: String?],
        namespaceMap: [Int: String?],
        mainTargetNamespace: String?,
    ) -> [Int] {
        var uniqueIndices: [Int] = []
        for index in containers.indices {
            let container = containers[index]
            let namespaceURI = namespaceMap[index] ?? mainTargetNamespace
            let location = containerLocations[ObjectIdentifier(container)] ?? nil
            let alreadyExists = uniqueIndices.contains { prevIndex in
                let prevNS = namespaceMap[prevIndex] ?? mainTargetNamespace
                let prevLocation = containerLocations[ObjectIdentifier(containers[prevIndex])] ?? nil
                guard prevNS == namespaceURI,
                      isStructurallyEqual(containers[prevIndex], container)
                else {
                    return false
                }
                if prevLocation == location { return true }
                // A circular import reloads the root schema under a schemaLocation while
                // the compile root is appended last; treat that as one document, not a clash.
                let rootIndex = containers.count - 1
                return index == rootIndex || prevIndex == rootIndex
            }
            if !alreadyExists {
                uniqueIndices.append(index)
            }
        }
        return uniqueIndices
    }

    private struct ComponentKey: Hashable {
        let kind: String
        let targetNamespace: String?
        let name: String
    }

    private static func checkDuplicateGlobals(
        in containers: [XSDTree],
        indices: [Int],
        namespaceMap: [Int: String?],
        mainTargetNamespace: String?,
    ) -> [String] {
        var errors: [String] = []
        var seenComponents: Set<ComponentKey> = []
        for index in indices {
            let container = containers[index]
            guard PureXML.Schema.XSDNode.localName(container) != "redefine" else { continue }
            let namespaceURI = namespaceMap[index] ?? mainTargetNamespace
            let globals = PureXML.Schema.XSDNode.elementChildren(container).filter {
                $0.name?.namespaceURI == xsdNamespace
            }
            for global in globals {
                let localName = PureXML.Schema.XSDNode.localName(global) ?? ""
                guard let kind = componentKind(for: localName),
                      let name = PureXML.Schema.XSDNode.attribute(global, "name")?.trimmingXMLWhitespace()
                else {
                    continue
                }
                let key = ComponentKey(kind: kind, targetNamespace: namespaceURI, name: name)
                if !seenComponents.insert(key).inserted {
                    let nsStr = namespaceURI.map { " in namespace '\($0)'" } ?? " in no namespace"
                    let kindLabel = kind == "group" ? "model group" : (kind == "attributeGroup" ? "attribute group" : kind)
                    errors.append("duplicate \(kindLabel) name '\(name)'\(nsStr)")
                }
            }
        }
        return errors
    }

    private static func componentKind(for localName: String) -> String? {
        switch localName {
        case "simpleType", "complexType": "type"
        case "element": "element"
        case "attribute": "attribute"
        case "group": "group"
        case "attributeGroup": "attributeGroup"
        case "notation": "notation"
        default: nil
        }
    }

    private struct KeyrefInfo {
        let name: String
        let refer: String
        let arity: Int
    }

    /// Findings for a `keyref` whose `refer` does not name a `key` or `unique` in
    /// the document, or whose field arity does not match the referenced key/unique's.
    /// Skipped when the document pulls in external definitions
    /// (`import`/`include`/`redefine`), which the default compile does not load,
    /// so a `refer` into them is never flagged.
    private static func keyrefReferErrors(_ schema: XSDTree, _ containers: [XSDTree], _ context: PureXML.Schema.XSDContext) -> [String] {
        if skipsCrossDocumentRules(schema, compositionLoaded: context.compositionLoaded) { return [] }
        var keyArities: [String: Int] = [:]
        var keyrefs: [KeyrefInfo] = []
        let sources = context.compositionLoaded ? containers : [schema]
        for source in sources where PureXML.Schema.XSDNode.localName(source) != "redefine" {
            collectKeysAndRefers(source, keyArities: &keyArities, keyrefs: &keyrefs)
        }

        var errors: [String] = []
        for keyref in keyrefs {
            if let keyArity = keyArities[keyref.refer] {
                if keyArity != keyref.arity {
                    errors.append("keyref '\(keyref.name)' and its referenced key/unique '\(keyref.refer)' must have the same number of fields")
                }
            } else {
                errors.append("keyref refers to undeclared key or unique '\(keyref.refer)'")
            }
        }
        return errors
    }

    private static func collectKeysAndRefers(
        _ node: XSDTree,
        keyArities: inout [String: Int],
        keyrefs: inout [KeyrefInfo],
    ) {
        if let local = PureXML.Schema.XSDNode.localName(node), local == "appinfo" || local == "documentation" { return }
        if node.name?.namespaceURI == xsdNamespace, let local = PureXML.Schema.XSDNode.localName(node) {
            if local == "key" || local == "unique", let name = PureXML.Schema.XSDNode.attribute(node, "name")?.trimmingXMLWhitespace() {
                let arity = PureXML.Schema.XSDNode.elementChildren(node)
                    .count(where: { $0.name?.namespaceURI == xsdNamespace && PureXML.Schema.XSDNode.localName($0) == "field" })

                keyArities[name] = arity
            }
            if local == "keyref" {
                let name = PureXML.Schema.XSDNode.attribute(node, "name")?.trimmingXMLWhitespace()
                let refer = PureXML.Schema.XSDNode.attribute(node, "refer")?.trimmingXMLWhitespace()
                if let name, let refer {
                    let arity = PureXML.Schema.XSDNode.elementChildren(node)
                        .count(where: { $0.name?.namespaceURI == xsdNamespace && PureXML.Schema.XSDNode.localName($0) == "field" })
                    keyrefs.append(KeyrefInfo(name: name, refer: PureXML.Schema.XSDNode.stripPrefix(refer), arity: arity))
                }
            }
        }
        for child in PureXML.Schema.XSDNode.elementChildren(node) {
            collectKeysAndRefers(child, keyArities: &keyArities, keyrefs: &keyrefs)
        }
    }

    private static func identityConstraintNameErrors(
        _ containers: [XSDTree],
        _ uniqueIndices: [Int],
        namespaceMap: [Int: String?],
        mainTargetNamespace: String?,
    ) -> [String] {
        struct KeyrefNameKey: Hashable {
            let targetNamespace: String?
            let name: String
        }
        var seen: Set<KeyrefNameKey> = []
        var errors: [String] = []
        func walk(_ node: XSDTree, namespaceURI: String?) {
            let local = PureXML.Schema.XSDNode.localName(node)
            if local == "appinfo" || local == "documentation" { return }
            if let name = identityConstraintName(node) {
                let key = KeyrefNameKey(targetNamespace: namespaceURI, name: name)
                if !seen.insert(key).inserted {
                    let nsStr = namespaceURI.map { " in namespace '\($0)'" } ?? " in no namespace"
                    errors.append("duplicate identity constraint name '\(name)'\(nsStr)")
                }
            }
            for child in PureXML.Schema.XSDNode.elementChildren(node) {
                walk(child, namespaceURI: namespaceURI)
            }
        }
        for index in uniqueIndices {
            let container = containers[index]
            guard PureXML.Schema.XSDNode.localName(container) != "redefine" else { continue }
            let namespaceURI = namespaceMap[index] ?? mainTargetNamespace
            walk(container, namespaceURI: namespaceURI)
        }
        return errors
    }

    /// The `name` of `node` when it is an identity constraint (`unique`/`key`/
    /// `keyref`) in the XSD namespace, or nil.
    private static func identityConstraintName(_ node: XSDTree) -> String? {
        guard node.name?.namespaceURI == xsdNamespace,
              let local = PureXML.Schema.XSDNode.localName(node),
              ["unique", "key", "keyref"].contains(local)
        else { return nil }
        return PureXML.Schema.XSDNode.attribute(node, "name")?.trimmingXMLWhitespace()
    }

    private static func isStructurallyEqual(_ lhs: XSDTree, _ rhs: XSDTree) -> Bool {
        if lhs.name != rhs.name { return false }
        if lhs.attributes.count != rhs.attributes.count { return false }
        for attr in lhs.attributes {
            guard let rhsVal = rhs.attributes.first(where: { $0.name == attr.name })?.value,
                  rhsVal == attr.value
            else { return false }
        }
        let lhsChildren = PureXML.Schema.XSDNode.elementChildren(lhs)
        let rhsChildren = PureXML.Schema.XSDNode.elementChildren(rhs)
        if lhsChildren.count != rhsChildren.count { return false }
        guard !zip(lhsChildren, rhsChildren).contains(where: { !isStructurallyEqual($0, $1) }) else { return false }
        return true
    }

    static func resolveContainerNamespaces(_ containers: [XSDTree], mainTargetNamespace: String?) -> [Int: String?] {
        var resolved: [Int: String?] = [:]
        var resolvedIndices = Set<Int>()
        guard !containers.isEmpty else { return resolved }
        let mainIndex = containers.count - 1
        resolved[mainIndex] = mainTargetNamespace
        resolvedIndices.insert(mainIndex)
        for parentIndex in containers.indices.reversed() {
            guard resolvedIndices.contains(parentIndex) else { continue }
            let parentNamespace = resolvedNamespace(at: parentIndex, in: resolved, fallback: mainTargetNamespace)
            let parent = containers[parentIndex]
            for child in PureXML.Schema.XSDNode.elementChildren(parent) {
                let kind = PureXML.Schema.XSDNode.localName(child)
                guard kind == "include" || kind == "import" || kind == "redefine",
                      let childKind = kind
                else {
                    continue
                }
                let targetNamespaceURI: String? = if childKind == "import" {
                    PureXML.Schema.XSDNode.attribute(child, "namespace")
                } else {
                    parentNamespace
                }
                let expectedAttrNamespace: String? = childKind == "import" ? PureXML.Schema.XSDNode.attribute(child, "namespace") : parentNamespace
                let foundIndex = findMatchingContainerIndex(
                    in: containers,
                    before: parentIndex,
                    kind: childKind,
                    namespaces: (parent: parentNamespace, expected: expectedAttrNamespace),
                    resolvedIndices: resolvedIndices,
                )
                if let foundIndex {
                    resolved[foundIndex] = targetNamespaceURI
                    resolvedIndices.insert(foundIndex)
                }
            }
        }
        for index in containers.indices where !resolvedIndices.contains(index) {
            let container = containers[index]
            if let attrNS = PureXML.Schema.XSDNode.attribute(container, "targetNamespace") {
                resolved[index] = attrNS
            } else {
                resolved[index] = mainTargetNamespace
            }
            resolvedIndices.insert(index)
        }
        return resolved
    }

    /// Schema-validity findings for `xs:include` when external schemas load: the
    /// included document must be chameleon (no `targetNamespace`) or declare the
    /// same target namespace as the includer (XSD Structures §4.2.3).
    static func includeCompositionErrors(
        _ containers: [XSDTree],
        mainTargetNamespace: String?,
        compositionLoaded: Bool,
        containerLocations: [ObjectIdentifier: String?],
    ) -> [String] {
        includeCompositionFindings(
            containers,
            mainTargetNamespace: mainTargetNamespace,
            compositionLoaded: compositionLoaded,
            containerLocations: containerLocations,
        ).map(\.reason)
    }

    static func includeCompositionFindings(
        _ containers: [XSDTree],
        mainTargetNamespace: String?,
        compositionLoaded: Bool,
        containerLocations: [ObjectIdentifier: String?],
    ) -> [PureXML.Schema.SchemaLocatedFinding] {
        guard compositionLoaded else { return [] }
        let namespaceMap = resolveContainerNamespaces(containers, mainTargetNamespace: mainTargetNamespace)
        var findings: [PureXML.Schema.SchemaLocatedFinding] = []
        for (parentIndex, parent) in containers.enumerated() {
            guard PureXML.Schema.XSDNode.localName(parent) == "schema" else { continue }
            let parentNamespace = resolvedNamespace(at: parentIndex, in: namespaceMap, fallback: mainTargetNamespace)
            for child in PureXML.Schema.XSDNode.elementChildren(parent) {
                guard PureXML.Schema.XSDNode.localName(child) == "include",
                      let location = PureXML.Schema.XSDNode.attribute(child, "schemaLocation")
                else { continue }
                guard let includedIndex = containers.firstIndex(where: {
                    containerLocations[ObjectIdentifier($0)] == location
                }) else { continue }
                let includedNamespace = PureXML.Schema.XSDNode.attribute(containers[includedIndex], "targetNamespace")
                if let includedNamespace, includedNamespace != parentNamespace {
                    let parentLabel = parentNamespace ?? "no namespace"
                    findings.append(PureXML.Schema.SchemaLocatedFinding(
                        reason: "included schema targetNamespace '\(includedNamespace)' must match includer targetNamespace '\(parentLabel)' or be chameleon (no targetNamespace)",
                        node: child,
                    ))
                }
            }
        }
        return findings
    }

    private static func resolvedNamespace(at index: Int, in map: [Int: String?], fallback: String?) -> String? {
        switch map[index] {
        case nil: fallback
        case let .some(namespace): namespace
        }
    }

    private static func findMatchingContainerIndex(
        in containers: [XSDTree],
        before parentIndex: Int,
        kind: String,
        namespaces: (parent: String?, expected: String?),
        resolvedIndices: Set<Int>,
    ) -> Int? {
        for index in (0 ..< parentIndex).reversed() where !resolvedIndices.contains(index) {
            let container = containers[index]
            let attrNS = PureXML.Schema.XSDNode.attribute(container, "targetNamespace")
            let isMatch: Bool = if kind == "import" {
                attrNS == namespaces.expected
            } else {
                attrNS == nil || attrNS == namespaces.parent
            }
            if isMatch {
                return index
            }
        }
        return nil
    }
}