JSON.StringRule.EscapeSequence.swift

internal import Grammar

extension JSON.StringRule {
    /// Matches a sequence of escaped UTF-16 code units.
    ///
    /// A UTF-16 escape sequence consists of `\u`, followed by four hexadecimal digits.
    enum EscapeSequence {
    }
}
extension JSON.StringRule.EscapeSequence: ParsingRule {
    typealias Terminal = UInt8

    static func parse<Source>(
        _ input: inout ParsingInput<some ParsingDiagnostics<Source>>
    ) throws -> String
        where   Source.Element == Terminal,
        Source.Index == Location {
        typealias HexDigit = UnicodeDigit<Location, UInt8, UInt16>.Hex
        typealias ASCII = UnicodeEncoding<Location, UInt8>

        try input.parse(as: ASCII.Backslash.self)
        var unescaped: String = ""
        while true {
            if  let scalar: Unicode.Scalar = input.parse(
                    as: JSON.StringRule<Location>.EscapedCodeUnit?.self
                ) {
                unescaped.append(Character.init(scalar))
            } else {
                try input.parse(as: ASCII.LowercaseU.self)
                let value: UInt16 =
                (try input.parse(as: HexDigit.self) << 12) |
                (try input.parse(as: HexDigit.self) <<  8) |
                (try input.parse(as: HexDigit.self) <<  4) |
                (try input.parse(as: HexDigit.self))
                if let scalar: Unicode.Scalar = Unicode.Scalar.init(value) {
                    unescaped.append(Character.init(scalar))
                } else {
                    throw JSON.InvalidUnicodeScalarError.init(value: value)
                }
            }

            guard case _? = input.parse(as: ASCII.Backslash?.self) else {
                break
            }
        }
        return unescaped
    }
}