PureYAML/Sources/PureYAMLJSON/JSON.Source.swift

import PureYAML

extension PureYAML.JSON {
    /// JSON text with the characters the YAML reader cannot be given held back.
    ///
    /// Each U+007F to U+009F, U+2028, U+2029, U+FFFE and U+FFFF that is inside a string is replaced, one scalar for one scalar, by a private-use character that is not
    /// in the text, so that the reader sees a plain character and every position of the text is the position it had. The replacement is undone in the value that
    /// comes out of the reader. A private-use character that the text itself holds is never used as a replacement, so what comes back is never mistaken for it.
    struct Source {
        let masked: String
        /// What each replacement stands for.
        private let originals: [Unicode.Scalar: Unicode.Scalar]

        init(_ text: String) throws {
            let present = Set(text.unicodeScalars.filter { Self.privateUse.contains($0.value) })
            var free = Self.privateUse.lazy.compactMap(Unicode.Scalar.init).filter { !present.contains($0) }.makeIterator()
            var replacement: [Unicode.Scalar: Unicode.Scalar] = [:]
            var originals: [Unicode.Scalar: Unicode.Scalar] = [:]
            var output = String.UnicodeScalarView()
            var inString = false
            var escaped = false
            for scalar in text.unicodeScalars {
                guard inString else {
                    inString = scalar == "\""
                    output.append(scalar)
                    continue
                }
                if escaped {
                    escaped = false
                } else if scalar == "\\" {
                    escaped = true
                } else if scalar == "\"" {
                    inString = false
                } else if Self.isHeldBack(scalar) {
                    if replacement[scalar] == nil {
                        guard let next = free.next() else {
                            throw PureYAML.JSON.Convert.Failure.sourceInvalid("the text holds too many private-use characters to read it as JSON")
                        }
                        replacement[scalar] = next
                        originals[next] = scalar
                    }
                    output.append(replacement[scalar] ?? scalar)
                    continue
                }
                output.append(scalar)
            }
            masked = String(output)
            self.originals = originals
        }

        private static let privateUse: ClosedRange<UInt32> = 0xE000 ... 0xF8FF

        /// The characters RFC 8259 lets a string hold as they are and the YAML reader folds or refuses.
        static func isHeldBack(_ scalar: Unicode.Scalar) -> Bool {
            (0x7F ... 0x9F).contains(scalar.value) || [0x2028, 0x2029, 0xFFFE, 0xFFFF].contains(scalar.value)
        }

        func restoring(_ text: String) -> String {
            guard !originals.isEmpty else { return text }
            var output = String.UnicodeScalarView()
            for scalar in text.unicodeScalars {
                output.append(originals[scalar] ?? scalar)
            }
            return String(output)
        }

        func restoring(_ value: PureYAML.Model.Value) -> PureYAML.Model.Value {
            guard !originals.isEmpty else { return value }
            switch value {
            case .null, .bool, .int, .double:
                return value
            case let .string(text):
                return .string(restoring(text))
            case let .sequence(items):
                return .sequence(items.map(restoring))
            case let .mapping(mapping):
                return .mapping(restoring(mapping))
            }
        }

        func restoring(_ located: PureYAML.Model.LocatedValue) -> PureYAML.Model.LocatedValue {
            guard !originals.isEmpty else { return located }
            switch located {
            case let .scalar(value, range):
                return .scalar(restoring(value), range)
            case let .sequence(items, range):
                return .sequence(items.map(restoring), range)
            case let .mapping(pairs, range):
                return .mapping(pairs.map { .init(keyNode: restoring($0.keyNode), keyRange: $0.keyRange, value: restoring($0.value)) }, range)
            }
        }

        private func restoring(_ mapping: PureYAML.Model.Mapping) -> PureYAML.Model.Mapping {
            .init(mapping.pairs.map { .init(keyNode: restoring($0.keyNode), value: restoring($0.value)) })
        }

        private func restoring(_ key: PureYAML.Model.Key) -> PureYAML.Model.Key {
            switch key {
            case let .string(text): .string(restoring(text))
            case let .sequence(items): .sequence(items.map(restoring))
            case let .mapping(mapping): .mapping(restoring(mapping))
            }
        }
    }
}