using System;

using System.Collections;

using System.Collections.Generic;

using System.Linq;

using System.Text;

using System.Reflection;

​

using System.Text.Json;

using System.Text.Json.Serialization;

using System.Text.Json.Serialization.Metadata;

​

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

​

//https://stackoverflow.com/questions/75338574/indexof-char-within-an-readonlyspanbyte-of-utf8-bytes

​

#nullable enable

​

    public static class StringExtensions

    {

        // TODO: check behavior for invalid UTF8 chars.

        static readonly Encoding encoding = Encoding.GetEncoding(Encoding.UTF8.CodePage, new EncoderExceptionFallback(), new DecoderExceptionFallback());

        static readonly int maxBytes = encoding.GetMaxByteCount(1);

​

        public static int IndexOf(this ReadOnlySpan<byte> utf8Bytes, char @char)

        {

            Span<byte> charBytes = stackalloc byte[maxBytes];

            try

            {

                var n = encoding.GetBytes(stackalloc char[1] { @char }, charBytes);

                charBytes = charBytes.Slice(0, n);

            }

            catch (EncoderFallbackException)

            {

                return -1; // Or rethrow, if you prefer?

            }

            for (int i = 0; i <= utf8Bytes.Length - charBytes.Length; i++)

                if (charBytes.CommonPrefixLength(utf8Bytes.Slice(i)) == charBytes.Length)

                    return i;

            return -1;

        }       

        public static int IndexOf(this ReadOnlySpan<byte> utf8Bytes, Rune @char)

        {

            Span<byte> charBytes = stackalloc byte[@char.Utf8SequenceLength];

            try

            {

                var n = @char.EncodeToUtf8(charBytes);

                charBytes = charBytes.Slice(0, n);

            }

            catch (EncoderFallbackException)

            {

                return -1; // Or rethrow, if you prefer?

            }

            for (int i = 0; i <= utf8Bytes.Length - charBytes.Length; i++)

                if (charBytes.CommonPrefixLength(utf8Bytes.Slice(i)) == charBytes.Length)

                    return i;

            return -1;

        }       

    }

​

    class TestClass

    {

        public static void Test()

        {

            Test("?hello", 'z');

            Test("?hello", '\a');

            Test("", 'o');

            Test("o", 'o');

            Test("?hello", 'o');

            Test("?hello", '\xFF');

            Test("?hello", '\xfe');

            Test("?hello", '\x01');

            Test("?hello", '\xFFFF');

            Test("?hello", '\xDC00');

        }   

        public static void Test(string s, char c)

        {

            ReadOnlySpan<byte> bytes = Encoding.UTF8.GetBytes(s);

​

            var index = bytes.IndexOf(c);

            var charIndex = s.IndexOf(c);

            var charFound = charIndex >= 0;

            var found = index >= 0;

            Console.WriteLine($"For s={s} and c={c}, index={index}, charIndex={charIndex}");

            Assert.That(charFound == found, $"For s={s} and c={c}, {charFound} != {found}");

            var charBytes = Encoding.UTF8.GetBytes(c.ToString());

​

            if (charBytes.Length == 1 && bytes.Length == s.Length) // All 1-byte ASCII

                Assert.AreEqual(charIndex, index, $"{charIndex} == {index}");

        }   

    }

​

    public class Program

    {

        public static void Main()

        {

            Console.WriteLine("Environment version: {0} ({1}), {2}", System.Runtime.InteropServices.RuntimeInformation.FrameworkDescription, Environment.Version, Environment.OSVersion);

            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;

            }

        }

    }

​