/// The per-facet validation checks, split out so SimpleType's own body stays
/// within the type-body length budget.
extension PureXML.Schema.SimpleType {
    private func boundsError(_ value: String, _: PureXML.Schema.Primitive) -> String? {
        let (lower, upper) = base.bounds
        guard lower != nil || upper != nil, let decimal = PureXML.Schema.DecimalValue(value, allowFraction: false) else {
            return nil
        }
        if let lower, decimal < lower {
            return "\(value) is below the minimum for \(base.rawValue)"
        }
        if let upper, decimal > upper {
            return "\(value) is above the maximum for \(base.rawValue)"
        }
        return nil
    }

    private func lengthError(_ value: String, _ primitive: PureXML.Schema.Primitive) -> String? {
        // length/minLength/maxLength measure characters, octets, or list
        // items depending on the type; for QName the spec leaves the unit
        // unspecified (XSD 1.0 Datatypes 4.3.1), so like Xerces and the XSTS
        // NIST oracle we do not constrain QName values by length.
        if case .qName = primitive {
            return nil
        }
        if case .notation = primitive {
            return nil
        }
        let length = primitive.measuredLength(value)
        if let exact = facets.length, length != exact {
            return "length \(length) is not \(exact)"
        }
        if let minimum = facets.minLength, length < minimum {
            return "length \(length) is below \(minimum)"
        }
        if let maximum = facets.maxLength, length > maximum {
            return "length \(length) is above \(maximum)"
        }
        return nil
    }

    private func digitsError(_ value: String) -> String? {
        guard facets.totalDigits != nil || facets.fractionDigits != nil,
              let decimal = PureXML.Schema.DecimalValue(value, allowFraction: true) else { return nil }
        if let total = facets.totalDigits, decimal.totalDigits > total {
            return "more than \(total) total digits"
        }
        if let fraction = facets.fractionDigits, decimal.fractionDigits.count > fraction {
            return "more than \(fraction) fraction digits"
        }
        return nil
    }

    func patternError(_ value: String) -> String? {
        for group in facets.patternGroups {
            let matched = group.contains { pattern in
                guard let regex = try? PureXML.Regex.Pattern(pattern) else { return false }
                return regex.matches(value)
            }
            guard matched else {
                let label = group.joined(separator: "|")
                return "'\(value)' does not match pattern '\(label)'"
            }
        }
        return nil
    }

    private func enumerationError(_ value: String, _ primitive: PureXML.Schema.Primitive) -> String? {
        guard let enumeration = facets.enumeration else { return nil }
        let whiteSpace = facets.whiteSpace ?? base.whiteSpace
        let matched = enumeration.contains { candidate in
            let normalized = Self.process(candidate, whiteSpace: whiteSpace)
            if let lhs = primitive.ordered(value), let rhs = primitive.ordered(normalized) {
                return lhs == rhs
            }
            return normalized == value
        }
        return matched ? nil : "'\(value)' is not in the enumeration"
    }

    private func rangeError(_ value: String, _ primitive: PureXML.Schema.Primitive) -> String? {
        if case .duration = primitive {
            return durationRangeError(value)
        }
        guard let actual = primitive.ordered(value) else { return nil }
        if let bound = facets.minInclusive, let limit = primitive.ordered(bound), actual < limit {
            return "\(value) is below the inclusive minimum"
        }
        if let bound = facets.maxInclusive, let limit = primitive.ordered(bound), actual > limit {
            return "\(value) is above the inclusive maximum"
        }
        if let bound = facets.minExclusive, let limit = primitive.ordered(bound), !(limit < actual) {
            return "\(value) is not above the exclusive minimum"
        }
        if let bound = facets.maxExclusive, let limit = primitive.ordered(bound), !(actual < limit) {
            return "\(value) is not below the exclusive maximum"
        }
        return nil
    }

    /// Ordering facets on `xs:duration`, which is a partial order: a value
    /// that is incomparable to the bound does not satisfy it (it is not in
    /// the ordered range), so an incomparable result is a violation.
    private func durationRangeError(_ value: String) -> String? {
        guard let actual = PureXML.Schema.DurationValue(value) else { return nil }
        func order(_ bound: String?) -> PureXML.Schema.DurationOrder? {
            bound.flatMap(PureXML.Schema.DurationValue.init).map(actual.compare(to:))
        }
        if let order = order(facets.minInclusive), order == .lessThan || order == .incomparable {
            return "\(value) is below the inclusive minimum"
        }
        if let order = order(facets.maxInclusive), order == .greaterThan || order == .incomparable {
            return "\(value) is above the inclusive maximum"
        }
        if let order = order(facets.minExclusive), order != .greaterThan {
            return "\(value) is not above the exclusive minimum"
        }
        if let order = order(facets.maxExclusive), order != .lessThan {
            return "\(value) is not below the exclusive maximum"
        }
        return nil
    }

    func validateAtomic(_ lexical: String) -> String? {
        let value = Self.process(lexical, whiteSpace: facets.whiteSpace ?? base.whiteSpace)
        let primitive = base.primitive
        guard primitive.isValid(value) else {
            return "'\(lexical)' is not a valid \(base.rawValue)"
        }
        return boundsError(value, primitive)
            ?? lengthError(value, primitive)
            ?? digitsError(value)
            ?? patternError(value)
            ?? enumerationError(value, primitive)
            ?? rangeError(value, primitive)
    }
}