Modified copy of Apple's Swift implementation of the JSONSerialization class that parses numbers as Decimal's instead of Int's or Double's

JSONSerialization2.swift

// This source file is part of the Swift.org open source project
//
// Copyright (c) 2014 - 2016 Apple Inc. and the Swift project authors
// Licensed under Apache License v2.0 with Runtime Library Exception
//
// See http://swift.org/LICENSE.txt for license information
// See http://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
//

// This is a modified copy which does deserialization only (no serialization) and
// parses numbers as Decimal's instead of Int's or Double's.
// By Ahmad Alhashemi

// Original version can be found here:
// https://github.com/apple/swift-corelibs-foundation/blob/master/Foundation/NSJSONSerialization.swift

import Foundation

extension JSONSerialization2 {
    public struct ReadingOptions : OptionSet {
        public let rawValue : UInt
        public init(rawValue: UInt) { self.rawValue = rawValue }
        
        public static let mutableContainers = ReadingOptions(rawValue: 1 << 0)
        public static let mutableLeaves = ReadingOptions(rawValue: 1 << 1)
        public static let allowFragments = ReadingOptions(rawValue: 1 << 2)
    }

    public struct WritingOptions : OptionSet {
        public let rawValue : UInt
        public init(rawValue: UInt) { self.rawValue = rawValue }
        
        public static let prettyPrinted = WritingOptions(rawValue: 1 << 0)
    }
}


/* A class for converting JSON to Foundation/Swift objects and converting Foundation/Swift objects to JSON.
   
   An object that may be converted to JSON must have the following properties:
    - Top level object is a `Swift.Array` or `Swift.Dictionary`
    - All objects are `Swift.String`, `Foundation.NSNumber`, `Swift.Array`, `Swift.Dictionary`,
      or `Foundation.NSNull`
    - All dictionary keys are `Swift.String`s
    - `NSNumber`s are not NaN or infinity
*/

open class JSONSerialization2 : NSObject {
    /* Create a Foundation object from JSON data. Set the NSJSONReadingAllowFragments option if the parser should allow top-level objects that are not an NSArray or NSDictionary. Setting the NSJSONReadingMutableContainers option will make the parser generate mutable NSArrays and NSDictionaries. Setting the NSJSONReadingMutableLeaves option will make the parser generate mutable NSString objects. If an error occurs during the parse, then the error parameter will be set and the result will be nil.
       The data must be in one of the 5 supported encodings listed in the JSON specification: UTF-8, UTF-16LE, UTF-16BE, UTF-32LE, UTF-32BE. The data may or may not have a BOM. The most efficient encoding to use for parsing is UTF-8, so if you have a choice in encoding the data passed to this method, use UTF-8.
     */
    open class func jsonObject(with data: Data, options opt: ReadingOptions = []) throws -> Any {
        return try data.withUnsafeBytes { (bytes: UnsafePointer<UInt8>) -> Any in
            let encoding: String.Encoding
            let buffer: UnsafeBufferPointer<UInt8>
            if let detected = parseBOM(bytes, length: data.count) {
                encoding = detected.encoding
                buffer = UnsafeBufferPointer(start: bytes.advanced(by: detected.skipLength), count: data.count - detected.skipLength)
            }
            else {
                encoding = detectEncoding(bytes, data.count)
                buffer = UnsafeBufferPointer(start: bytes, count: data.count)
            }
            
            let source = JSONReader.UnicodeSource(buffer: buffer, encoding: encoding)
            let reader = JSONReader(source: source)
            if let (object, _) = try reader.parseObject(0) {
                return object
            }
            else if let (array, _) = try reader.parseArray(0) {
                return array
            }
            else if opt.contains(.allowFragments), let (value, _) = try reader.parseValue(0) {
                return value
            }
            throw NSError(domain: NSCocoaErrorDomain, code: CocoaError.propertyListReadCorrupt.rawValue, userInfo: [
                "NSDebugDescription" : "JSON text did not start with array or object and option to allow fragments not set."
            ])
        }
        
    }
}

//MARK: - Encoding Detection

internal extension JSONSerialization2 {
    
    /// Detect the encoding format of the NSData contents
    class func detectEncoding(_ bytes: UnsafePointer<UInt8>, _ length: Int) -> String.Encoding {

        if length >= 4 {
            switch (bytes[0], bytes[1], bytes[2], bytes[3]) {
            case (0, 0, 0, _):
                return .utf32BigEndian
            case (_, 0, 0, 0):
                return .utf32LittleEndian
            case (0, _, 0, _):
                return .utf16BigEndian
            case (_, 0, _, 0):
                return .utf16LittleEndian
            default:
                break
            }
        }
        else if length >= 2 {
            switch (bytes[0], bytes[1]) {
            case (0, _):
                return .utf16BigEndian
            case (_, 0):
                return .utf16LittleEndian
            default:
                break
            }
        }
        return .utf8
    }
    
    static func parseBOM(_ bytes: UnsafePointer<UInt8>, length: Int) -> (encoding: String.Encoding, skipLength: Int)? {
        if length >= 2 {
            switch (bytes[0], bytes[1]) {
            case (0xEF, 0xBB):
                if length >= 3 && bytes[2] == 0xBF {
                    return (.utf8, 3)
                }
            case (0x00, 0x00):
                if length >= 4 && bytes[2] == 0xFE && bytes[3] == 0xFF {
                    return (.utf32BigEndian, 4)
                }
            case (0xFF, 0xFE):
                if length >= 4 && bytes[2] == 0 && bytes[3] == 0 {
                    return (.utf32LittleEndian, 4)
                }
                return (.utf16LittleEndian, 2)
            case (0xFE, 0xFF):
                return (.utf16BigEndian, 2)
            default:
                break
            }
        }
        return nil
    }
}

//MARK: - JSONDeserializer
private struct JSONReader {

    static let whitespaceASCII: [UInt8] = [
        0x09, // Horizontal tab
        0x0A, // Line feed or New line
        0x0D, // Carriage return
        0x20, // Space
    ]

    struct Structure {
        static let BeginArray: UInt8     = 0x5B // [
        static let EndArray: UInt8       = 0x5D // ]
        static let BeginObject: UInt8    = 0x7B // {
        static let EndObject: UInt8      = 0x7D // }
        static let NameSeparator: UInt8  = 0x3A // :
        static let ValueSeparator: UInt8 = 0x2C // ,
        static let QuotationMark: UInt8  = 0x22 // "
        static let Escape: UInt8         = 0x5C // \
    }

    typealias Index = Int
    typealias IndexDistance = Int

    struct UnicodeSource {
        let buffer: UnsafeBufferPointer<UInt8>
        let encoding: String.Encoding
        let step: Int

        init(buffer: UnsafeBufferPointer<UInt8>, encoding: String.Encoding) {
            self.buffer = buffer
            self.encoding = encoding

            self.step = {
                switch encoding {
                case String.Encoding.utf8:
                    return 1
                case String.Encoding.utf16BigEndian, String.Encoding.utf16LittleEndian:
                    return 2
                case String.Encoding.utf32BigEndian, String.Encoding.utf32LittleEndian:
                    return 4
                default:
                    return 1
                }
            }()
        }

        func takeASCII(_ input: Index) -> (UInt8, Index)? {
            guard hasNext(input) else {
                return nil
            }

            let index: Int
            switch encoding {
            case String.Encoding.utf8:
                index = input
            case String.Encoding.utf16BigEndian where buffer[input] == 0:
                index = input + 1
            case String.Encoding.utf32BigEndian where buffer[input] == 0 && buffer[input+1] == 0 && buffer[input+2] == 0:
                index = input + 3
            case String.Encoding.utf16LittleEndian where buffer[input+1] == 0:
                index = input
            case String.Encoding.utf32LittleEndian where buffer[input+1] == 0 && buffer[input+2] == 0 && buffer[input+3] == 0:
                index = input
            default:
                return nil
            }
            return (buffer[index] < 0x80) ? (buffer[index], input + step) : nil
        }

        func takeString(_ begin: Index, end: Index) throws -> String {
            let byteLength = begin.distance(to: end)
            
            guard let chunk = String(data: Data(bytes: buffer.baseAddress!.advanced(by: begin), count: byteLength), encoding: encoding) else {
                throw NSError(domain: NSCocoaErrorDomain, code: CocoaError.propertyListReadCorrupt.rawValue, userInfo: [
                    "NSDebugDescription" : "Unable to convert data to a string using the detected encoding. The data may be corrupt."
                    ])
            }
            return chunk
        }

        func hasNext(_ input: Index) -> Bool {
            return input + step <= buffer.endIndex
        }
        
        func distanceFromStart(_ index: Index) -> IndexDistance {
            return buffer.startIndex.distance(to: index) / step
        }
    }

    let source: UnicodeSource

    func consumeWhitespace(_ input: Index) -> Index? {
        var index = input
        while let (char, nextIndex) = source.takeASCII(index), JSONReader.whitespaceASCII.contains(char) {
            index = nextIndex
        }
        return index
    }

    func consumeStructure(_ ascii: UInt8, input: Index) throws -> Index? {
        return try consumeWhitespace(input).flatMap(consumeASCII(ascii)).flatMap(consumeWhitespace)
    }

    func consumeASCII(_ ascii: UInt8) -> (Index) throws -> Index? {
        return { (input: Index) throws -> Index? in
            switch self.source.takeASCII(input) {
            case .none:
                throw NSError(domain: NSCocoaErrorDomain, code: CocoaError.propertyListReadCorrupt.rawValue, userInfo: [
                    "NSDebugDescription" : "Unexpected end of file during JSON parse."
                    ])
            case let (taken, index)? where taken == ascii:
                return index
            default:
                return nil
            }
        }
    }

    func consumeASCIISequence(_ sequence: String, input: Index) throws -> Index? {
        var index = input
        for scalar in sequence.unicodeScalars {
            guard let nextIndex = try consumeASCII(UInt8(scalar.value))(index) else {
                return nil
            }
            index = nextIndex
        }
        return index
    }

    func takeMatching(_ match: @escaping (UInt8) -> Bool) -> ([Character], Index) -> ([Character], Index)? {
        return { input, index in
            guard let (byte, index) = self.source.takeASCII(index), match(byte) else {
                return nil
            }
            return (input + [Character(UnicodeScalar(byte))], index)
        }
    }

    //MARK: - String Parsing

    func parseString(_ input: Index) throws -> (String, Index)? {
        guard let beginIndex = try consumeWhitespace(input).flatMap(consumeASCII(Structure.QuotationMark)) else {
            return nil
        }
        var chunkIndex: Int = beginIndex
        var currentIndex: Int = chunkIndex

        var output: String = ""
        while source.hasNext(currentIndex) {
            guard let (ascii, index) = source.takeASCII(currentIndex) else {
                currentIndex += source.step
                continue
            }
            switch ascii {
            case Structure.QuotationMark:
                output += try source.takeString(chunkIndex, end: currentIndex)
                return (output, index)
            case Structure.Escape:
                output += try source.takeString(chunkIndex, end: currentIndex)
                if let (escaped, nextIndex) = try parseEscapeSequence(index) {
                    output += escaped
                    chunkIndex = nextIndex
                    currentIndex = nextIndex
                    continue
                }
                else {
                    throw NSError(domain: NSCocoaErrorDomain, code: CocoaError.propertyListReadCorrupt.rawValue, userInfo: [
                        "NSDebugDescription" : "Invalid escape sequence at position \(source.distanceFromStart(currentIndex))"
                    ])
                }
            default:
                currentIndex = index
            }
        }
        throw NSError(domain: NSCocoaErrorDomain, code: CocoaError.propertyListReadCorrupt.rawValue, userInfo: [
            "NSDebugDescription" : "Unexpected end of file during string parse."
        ])
    }

    func parseEscapeSequence(_ input: Index) throws -> (String, Index)? {
        guard let (byte, index) = source.takeASCII(input) else {
            throw NSError(domain: NSCocoaErrorDomain, code: CocoaError.propertyListReadCorrupt.rawValue, userInfo: [
                "NSDebugDescription" : "Early end of unicode escape sequence around character"
            ])
        }
        let output: String
        switch byte {
        case 0x22: output = "\""
        case 0x5C: output = "\\"
        case 0x2F: output = "/"
        case 0x62: output = "\u{08}" // \b
        case 0x66: output = "\u{0C}" // \f
        case 0x6E: output = "\u{0A}" // \n
        case 0x72: output = "\u{0D}" // \r
        case 0x74: output = "\u{09}" // \t
        case 0x75: return try parseUnicodeSequence(index)
        default: return nil
        }
        return (output, index)
    }

    func parseUnicodeSequence(_ input: Index) throws -> (String, Index)? {

        guard let (codeUnit, index) = parseCodeUnit(input) else {
            return nil
        }

        if !UTF16.isLeadSurrogate(codeUnit) {
            return (String(UnicodeScalar(codeUnit)!), index)
        }

        guard let (trailCodeUnit, finalIndex) = try consumeASCIISequence("\\u", input: index).flatMap(parseCodeUnit) , UTF16.isTrailSurrogate(trailCodeUnit) else {
            throw NSError(domain: NSCocoaErrorDomain, code: CocoaError.propertyListReadCorrupt.rawValue, userInfo: [
                "NSDebugDescription" : "Unable to convert unicode escape sequence (no low-surrogate code point) to UTF8-encoded character at position \(source.distanceFromStart(input))"
            ])
        }

        let highValue = (UInt32(codeUnit  - 0xD800) << 10)
        let lowValue  =  UInt32(trailCodeUnit - 0xDC00)
        return (String(UnicodeScalar(highValue + lowValue + 0x10000)!), finalIndex)
    }

    func isHexChr(_ byte: UInt8) -> Bool {
        return (byte >= 0x30 && byte <= 0x39)
            || (byte >= 0x41 && byte <= 0x46)
            || (byte >= 0x61 && byte <= 0x66)
    }
    func parseCodeUnit(_ input: Index) -> (UTF16.CodeUnit, Index)? {
        let hexParser = takeMatching(isHexChr)
        guard let (result, index) = hexParser([], input).flatMap(hexParser).flatMap(hexParser).flatMap(hexParser),
            let value = Int(String(result), radix: 16) else {
                return nil
        }
        return (UTF16.CodeUnit(value), index)
    }
    
    //MARK: - Number parsing
    static let numberCodePoints: [UInt8] = [
        0x30, 0x31, 0x32, 0x33, 0x34, 0x35, 0x36, 0x37, 0x38, 0x39, // 0...9
        0x2E, 0x2D, 0x2B, 0x45, 0x65, // . - + E e
    ]
    func parseNumber(_ input: Index) throws -> (Any, Index)? {
        var index = input
        
        while let (ascii, nextIndex) = source.takeASCII(index), JSONReader.numberCodePoints.contains(ascii) {
            index = nextIndex
        }
        
        let numberCharacters = try source.takeString(input, end: index)
        
        guard let decimal = Decimal(string: numberCharacters) else {
            return nil
        }
        
        return (decimal, index)
    }

    //MARK: - Value parsing
    func parseValue(_ input: Index) throws -> (Any, Index)? {
        if let (value, parser) = try parseString(input) {
            return (value, parser)
        }
        else if let parser = try consumeASCIISequence("true", input: input) {
            return (true, parser)
        }
        else if let parser = try consumeASCIISequence("false", input: input) {
            return (false, parser)
        }
        else if let parser = try consumeASCIISequence("null", input: input) {
            return (NSNull(), parser)
        }
        else if let (object, parser) = try parseObject(input) {
            return (object, parser)
        }
        else if let (array, parser) = try parseArray(input) {
            return (array, parser)
        }
        else if let (number, parser) = try parseNumber(input) {
            return (number, parser)
        }
        return nil
    }

    //MARK: - Object parsing
    func parseObject(_ input: Index) throws -> ([String: Any], Index)? {
        guard let beginIndex = try consumeStructure(Structure.BeginObject, input: input) else {
            return nil
        }
        var index = beginIndex
        var output: [String: Any] = [:]
        while true {
            if let finalIndex = try consumeStructure(Structure.EndObject, input: index) {
                return (output, finalIndex)
            }
    
            if let (key, value, nextIndex) = try parseObjectMember(index) {
                output[key] = value
    
                if let finalParser = try consumeStructure(Structure.EndObject, input: nextIndex) {
                    return (output, finalParser)
                }
                else if let nextIndex = try consumeStructure(Structure.ValueSeparator, input: nextIndex) {
                    index = nextIndex
                    continue
                }
                else {
                    return nil
                }
            }
            return nil
        }
    }
    
    func parseObjectMember(_ input: Index) throws -> (String, Any, Index)? {
        guard let (name, index) = try parseString(input) else {
            throw NSError(domain: NSCocoaErrorDomain, code: CocoaError.propertyListReadCorrupt.rawValue, userInfo: [
                "NSDebugDescription" : "Missing object key at location \(source.distanceFromStart(input))"
            ])
        }
        guard let separatorIndex = try consumeStructure(Structure.NameSeparator, input: index) else {
            throw NSError(domain: NSCocoaErrorDomain, code: CocoaError.propertyListReadCorrupt.rawValue, userInfo: [
                "NSDebugDescription" : "Invalid separator at location \(source.distanceFromStart(index))"
            ])
        }
        guard let (value, finalIndex) = try parseValue(separatorIndex) else {
            throw NSError(domain: NSCocoaErrorDomain, code: CocoaError.propertyListReadCorrupt.rawValue, userInfo: [
                "NSDebugDescription" : "Invalid value at location \(source.distanceFromStart(separatorIndex))"
            ])
        }
        
        return (name, value, finalIndex)
    }

    //MARK: - Array parsing
    func parseArray(_ input: Index) throws -> ([Any], Index)? {
        guard let beginIndex = try consumeStructure(Structure.BeginArray, input: input) else {
            return nil
        }
        var index = beginIndex
        var output: [Any] = []
        while true {
            if let finalIndex = try consumeStructure(Structure.EndArray, input: index) {
                return (output, finalIndex)
            }
    
            if let (value, nextIndex) = try parseValue(index) {
                output.append(value)
    
                if let finalIndex = try consumeStructure(Structure.EndArray, input: nextIndex) {
                    return (output, finalIndex)
                }
                else if let nextIndex = try consumeStructure(Structure.ValueSeparator, input: nextIndex) {
                    index = nextIndex
                    continue
                }
            }
            throw NSError(domain: NSCocoaErrorDomain, code: CocoaError.propertyListReadCorrupt.rawValue, userInfo: [
                "NSDebugDescription" : "Badly formed array at location \(source.distanceFromStart(index))"
            ])
        }
    }
}