using System;

using System.Collections;

using System.Collections.Generic;

using System.Linq;

using System.Text;

using System.Runtime.Serialization.Formatters;

using System.IO;

using System.Data;

using System.ComponentModel.DataAnnotations;

using System.Globalization;

using System.Reflection;

using System.Collections.ObjectModel;

using System.Buffers.Text;

​

using System.Text.Json;

using System.Text.Json.Serialization;

​

using NUnit.Framework; // Throws NUnit.Framework.AssertionException

​

//https://stackoverflow.com/questions/66309150/how-do-i-write-a-float-like-82-0-with-the-0-intact-using-utf8jsonwriter

​

    public class DoubleConverter : JsonConverter<double>

    {

        // 2^49 is the largest power of 2 with fewer than 15 decimal digits.  

        // From experimentation casting to decimal does not lose precision for these values.

        const double MaxPreciselyRepresentedIntValue = (1L<<49);

        public override void Write(Utf8JsonWriter writer, double value, JsonSerializerOptions options)

        {

            bool written = false;

            // For performance check to see that the incoming double is an integer

            if ((value % 1) == 0)

            {

                if (value < MaxPreciselyRepresentedIntValue && value > -MaxPreciselyRepresentedIntValue)

                {

                    writer.WriteNumberValue(0.0m + (decimal)value);

                    written = true;

                }

                else

                {

                    // Directly casting these larger values from double to decimal seems to result in precision loss, as noted in  https://stackoverflow.com/q/7453900/3744182

                    // And also: https://docs.microsoft.com/en-us/dotnet/api/system.convert.todecimal?redirectedfrom=MSDN&view=net-5.0#System_Convert_ToDecimal_System_Double_

                    // > The Decimal value returned by Convert.ToDecimal(Double) contains a maximum of 15 significant digits.

                    // So if we want the full G17 precision we have to format and parse ourselves.

                    //

                    // Utf8Formatter and Utf8Parser should give the best performance for this, but, according to MSFT, 

                    // on frameworks earlier than .NET Core 3.0 Utf8Formatter does not produce roundtrippable strings.  For details see

                    // https://github.com/dotnet/runtime/blob/eb03e0f7bc396736c7ac59cf8f135d7c632860dd/src/libraries/System.Text.Json/src/System/Text/Json/Writer/Utf8JsonWriter.WriteValues.Double.cs#L103

                    // You may want format to string and parse in earlier frameworks -- or just use JsonDocument on these earlier versions.

                    Span<byte> utf8bytes = stackalloc byte[32];

                    if (Utf8Formatter.TryFormat(value, utf8bytes.Slice(0, utf8bytes.Length-2), out var bytesWritten)

                        && IsInteger(utf8bytes, bytesWritten))

                    {

                        utf8bytes[bytesWritten++] = (byte)'.';

                        utf8bytes[bytesWritten++] = (byte)'0';

                        if (Utf8Parser.TryParse(utf8bytes.Slice(0, bytesWritten), out decimal d, out var _))

                        {

                            writer.WriteNumberValue(d);

                            written = true;

                        }   

                    }

                }

            }

            if (!written)

            {

                if (double.IsFinite(value))

                    writer.WriteNumberValue(value);

                else

                    // Utf8JsonWriter does not take into account JsonSerializerOptions.NumberHandling so we have to make a recursive call to serialize

                    JsonSerializer.Serialize(writer, value, new JsonSerializerOptions { NumberHandling = options.NumberHandling });

            }

        }

        static bool IsInteger(Span<byte> utf8bytes, int bytesWritten)

        {

            if (bytesWritten <= 0)

                return false;

            var start = utf8bytes[0] == '-' ? 1 : 0;

            for (var i = start; i < bytesWritten; i++)

                if (!(utf8bytes[i] >= '0' && utf8bytes[i] <= '9'))

                    return false;

            return start < bytesWritten;

        }

        public override double Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options) => 

            // TODO: Handle "NaN", "Infinity", "-Infinity"

            reader.GetDouble();

    }

​

    class TestClass

    {

        public static void Test()

        {

            Console.WriteLine($"Math.Log2(1e15)={Math.Log2(1e15)}\n");

            var doubles = new []

            {

                82.0,

                82.1,

                -82.0,

                7e27,

                78e27,

                0, 1, 2,

                -1, -2,

                1 + 1e-14,

                1 + 1e-15,

                1 + 1e-16,

                9999999999999992,

                (1L<<52)-3,

                (1L<<52)-2,

                (1L<<52)-1,

                (1L<<52),

                (1L<<52)+1,

                (1L<<52)+2,

                (1L<<52)+3,

                (1L<<53)-3,

                (1L<<53)-2,

                (1L<<53)-1,

                (1L<<53),

                (1L<<53)+1,

                (1L<<53)+2,

                (1L<<53)+3,

                2L*(1L<<53),

                2L*(1L<<56),

                (double)decimal.MaxValue,

                (double)(0.99m*decimal.MaxValue),

                double.Epsilon,

                -double.Epsilon,

                long.MaxValue,

                -long.MaxValue,

                double.MinValue,

                double.MaxValue,

                double.PositiveInfinity,

                double.NegativeInfinity,

                double.NaN,

            };

            var powersOf2 = Enumerable.Range(0, 64)

                .Select(i => 1L<<i).ToList().SelectMany(l => Enumerable.Range(-25, 25).Select(i => (double)((ulong)l + (ulong)i)));

            foreach (var d in doubles.Concat(powersOf2))

            {

                var defaultOptions = new JsonSerializerOptions

                {

                    WriteIndented = true,

                    NumberHandling = JsonNumberHandling.AllowNamedFloatingPointLiterals,

                };

                var options = new JsonSerializerOptions

                {

                    Converters = { new DoubleConverter() },

                    WriteIndented = true,

                    NumberHandling = JsonNumberHandling.AllowNamedFloatingPointLiterals,

                };

                var jsonDefault = JsonSerializer.Serialize(d, defaultOptions);

                var json = JsonSerializer.Serialize(d, options);

                Console.WriteLine("fixed JSON = {0}, Original JSON = {1}", json, jsonDefault);

                // Newtonsoft always appends a .0 for doubles, so assert we are doing the same thing.

                Assert.AreEqual(Newtonsoft.Json.JsonConvert.SerializeObject(d), json, $"Json.NET and System.Text.Json returned inconsistent results for {d}");

            }

        }   

    }

​

    public class Program

    {

        public static void Main()

        {

            Console.WriteLine("Environment version: {0} ({1})", System.Runtime.InteropServices.RuntimeInformation.FrameworkDescription , GetNetCoreVersion());

            Console.WriteLine("System.Text.Json version: " + typeof(JsonSerializer).Assembly.FullName);

            Console.WriteLine();

​

            try

            {

                TestClass.Test();

            }

            catch (Exception ex)

            {

                Console.WriteLine("Failed with unhandled exception: ");

                Console.WriteLine(ex);

                throw;

            }

        }

        public static string GetNetCoreVersion()

        {

            //https://techblog.dorogin.com/how-to-detect-net-core-version-in-runtime-ecd65ad695be

            //

            var assembly = typeof(System.Runtime.GCSettings).GetTypeInfo().Assembly;

            var assemblyPath = assembly.Location.Split(new[] { '/', '\\' }, StringSplitOptions.RemoveEmptyEntries);

            int netCoreAppIndex = Array.IndexOf(assemblyPath, "Microsoft.NETCore.App");

            if (netCoreAppIndex > 0 && netCoreAppIndex < assemblyPath.Length - 2)

                return assemblyPath[netCoreAppIndex + 1];

            return null;

        }

    }

​