using System.Buffers.Text;
using System.Runtime.InteropServices;

namespace EnotaryoPH.Web.Common.Extensions
{
    public static class GuidExtensions
    {
        private const char Dash = '-';
        private const char EqualsChar = '=';
        private const byte ForwardSlashByte = (byte)'/';
        private const char Plus = '+';
        private const byte PlusByte = (byte)'+';
        private const char Slash = '/';
        private const char Underscore = '_';

        public static string ToBase64String(this Guid guid)
        {
            Span<byte> guidBytes = stackalloc byte[16];
            Span<byte> encodedBytes = stackalloc byte[24];

            MemoryMarshal.TryWrite(guidBytes, ref guid); // write bytes from the Guid
            Base64.EncodeToUtf8(guidBytes, encodedBytes, out _, out _);

            Span<char> chars = stackalloc char[22];

            // replace any characters which are not URL safe
            // skip the final two bytes as these will be '==' padding we don't need
            for (var i = 0; i < 22; i++)
            {
                chars[i] = encodedBytes[i] switch
                {
                    ForwardSlashByte => Dash,
                    PlusByte => Underscore,
                    _ => (char)encodedBytes[i],
                };
            }

            var final = new string(chars);

            return final;
        }

        public static Guid ToGuidFromString(ReadOnlySpan<char> id)
        {
            Span<char> base64Chars = stackalloc char[24];
            for (var i = 0; i < 22; i++)
            {
                base64Chars[i] = id[i] switch
                {
                    '-' => Slash,
                    '_' => Plus,
                    _ => id[i]
                };
            }
            base64Chars[22] = EqualsChar;
            base64Chars[23] = EqualsChar;

            Span<byte> idBytes = stackalloc byte[16];
            Convert.TryFromBase64Chars(base64Chars, idBytes, out _);
            return new Guid(idBytes);
        }
    }
}