public extension PureXML.Schema {
    /// Whitespace processing applied before a value is validated.
    enum WhiteSpace: Sendable {
        case preserve
        case replace
        case collapse
    }

    /// A built-in XSD datatype. Each carries its primitive value space, its
    /// intrinsic `whiteSpace`, and, for the bounded integer types, its inclusive
    /// bounds, exactly as XSD Part 2 derives them.
    enum BuiltinType: String, Sendable, CaseIterable {
        case string
        case normalizedString
        case token
        case language
        case name = "Name"
        case ncName = "NCName"
        case nmtoken = "NMTOKEN"
        case id = "ID"
        case idref = "IDREF"
        case entity = "ENTITY"
        case boolean
        case decimal
        case integer
        case long
        case int
        case short
        case byte
        case nonNegativeInteger
        case positiveInteger
        case nonPositiveInteger
        case negativeInteger
        case unsignedLong
        case unsignedInt
        case unsignedShort
        case unsignedByte
        case float
        case double
        case duration
        case dateTime
        case date
        case time
        case gYearMonth
        case gYear
        case gMonthDay
        case gDay
        case gMonth
        case hexBinary
        case base64Binary
        case anyURI
        case qName = "QName"
        case notation = "NOTATION"

        var whiteSpace: WhiteSpace {
            switch self {
            case .string: .preserve
            case .normalizedString: .replace
            default: .collapse
            }
        }

        /// The built-in this type is directly derived from in the XSD Part 2
        /// datatype lattice, or nil for the primitives whose base is the implicit
        /// root `anySimpleType`.
        var derivationBase: BuiltinType? {
            switch self {
            case .normalizedString: .string
            case .token: .normalizedString
            case .language, .name, .nmtoken: .token
            case .ncName: .name
            case .id, .idref, .entity: .ncName
            case .integer: .decimal
            case .long: .integer
            case .int: .long
            case .short: .int
            case .byte: .short
            case .nonNegativeInteger, .nonPositiveInteger: .integer
            case .positiveInteger: .nonNegativeInteger
            case .negativeInteger: .nonPositiveInteger
            case .unsignedLong: .nonNegativeInteger
            case .unsignedInt: .unsignedLong
            case .unsignedShort: .unsignedInt
            case .unsignedByte: .unsignedShort
            default: nil
            }
        }

        /// Whether this type is `other` or is derived from it through the built-in
        /// lattice. Used to check an `xsi:type` built-in validly substitutes for a
        /// built-in declared type: a derived type may always stand in for its base.
        func derives(from other: BuiltinType) -> Bool {
            var current: BuiltinType? = self
            while let type = current {
                if type == other {
                    return true
                }
                current = type.derivationBase
            }
            return false
        }

        /// Whether this type's value space is the `decimal` value space: `decimal`
        /// itself or any of the integer family (which derive from it). Values from
        /// two such types compare in the one shared value space, so `1` (decimal)
        /// equals `1` (unsignedByte). `float` and `double` are distinct primitive
        /// value spaces and are deliberately excluded.
        var isDecimalDerived: Bool {
            self == .decimal || Self.integerLike.contains(self)
        }

        var primitive: Primitive {
            if let primitive = Self.stringLike[self] {
                return primitive
            }
            if Self.integerLike.contains(self) {
                return .integer
            }
            return computedPrimitive
        }

        private var computedPrimitive: Primitive {
            switch self {
            case .boolean: .boolean
            case .decimal: .decimal
            case .float: .float
            case .double: .double
            case .duration: .duration
            case .dateTime: .dateKind(.dateTime)
            case .date: .dateKind(.date)
            case .time: .dateKind(.time)
            case .gYearMonth: .dateKind(.gYearMonth)
            case .gYear: .dateKind(.gYear)
            case .gMonthDay: .dateKind(.gMonthDay)
            case .gDay: .dateKind(.gDay)
            case .gMonth: .dateKind(.gMonth)
            case .hexBinary: .hexBinary
            case .base64Binary: .base64Binary
            case .anyURI: .anyURI
            case .qName: .qName
            case .notation: .notation
            default: .string
            }
        }

        private static let stringLike: [BuiltinType: Primitive] = [
            .string: .string, .normalizedString: .string, .token: .string,
            .language: .language, .name: .name,
            .ncName: .ncName, .id: .ncName, .idref: .ncName, .entity: .ncName,
            .nmtoken: .nmtoken,
        ]

        private static let integerLike: Set<BuiltinType> = [
            .integer, .long, .int, .short, .byte,
            .nonNegativeInteger, .positiveInteger, .nonPositiveInteger, .negativeInteger,
            .unsignedLong, .unsignedInt, .unsignedShort, .unsignedByte,
        ]

        /// The intrinsic inclusive bounds of the bounded integer types.
        var bounds: (lower: DecimalValue?, upper: DecimalValue?) {
            switch self {
            case .long: (Self.bound("-9223372036854775808"), Self.bound("9223372036854775807"))
            case .int: (Self.bound("-2147483648"), Self.bound("2147483647"))
            case .short: (Self.bound("-32768"), Self.bound("32767"))
            case .byte: (Self.bound("-128"), Self.bound("127"))
            case .unsignedLong: (Self.bound("0"), Self.bound("18446744073709551615"))
            case .unsignedInt: (Self.bound("0"), Self.bound("4294967295"))
            case .unsignedShort: (Self.bound("0"), Self.bound("65535"))
            case .unsignedByte: (Self.bound("0"), Self.bound("255"))
            case .nonNegativeInteger: (Self.bound("0"), nil)
            case .positiveInteger: (Self.bound("1"), nil)
            case .nonPositiveInteger: (nil, Self.bound("0"))
            case .negativeInteger: (nil, Self.bound("-1"))
            default: (nil, nil)
            }
        }

        private static func bound(_ lexical: String) -> DecimalValue? {
            DecimalValue(lexical, allowFraction: false)
        }
    }
}