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

namespace Foodsoft.Alpm.Interop
{
    internal static class CFormatter
    {
        private static readonly int vaListCopyNWords;

        private static readonly Vsnprintf? vsnprintfFunc;

        static CFormatter()
        {
            if (RuntimeInformation.IsOSPlatform(OSPlatform.Linux) ||
                RuntimeInformation.IsOSPlatform(OSPlatform.FreeBSD))
            {
                vaListCopyNWords = RuntimeInformation.ProcessArchitecture switch
                {
                    Architecture.X86 => 1,
                    Architecture.X64 => 3,
                    Architecture.Arm => 1,
                    Architecture.Arm64 => 4,
                    _ => vaListCopyNWords
                };
                vsnprintfFunc = vsnprintfLibc;
            }
            else if (RuntimeInformation.IsOSPlatform(OSPlatform.Windows))
            {
                vaListCopyNWords = 1;
                vsnprintfFunc = vsnprintfUCrt;
            }
            else if (RuntimeInformation.IsOSPlatform(OSPlatform.OSX))
            {
                vaListCopyNWords = RuntimeInformation.ProcessArchitecture switch
                {
                    Architecture.X86 => 1,
                    Architecture.X64 => 3,
                    Architecture.Arm => 1,
                    Architecture.Arm64 => 1,
                    _ => vaListCopyNWords
                };
                vsnprintfFunc = vsnprintfLibc;
            }
        }

        internal static unsafe string Format(IntPtr format, IntPtr vaList)
        {
            const int bufSize = 256;
            if (vsnprintfFunc == null || vaListCopyNWords <= 0)
                return "[Formatter unsupported for platform]";

            var vaListBuf = (void**) vaList;
            var vaListCopy = stackalloc void*[vaListCopyNWords];
            for (var i = 0; i < vaListCopyNWords; i++) vaListCopy[i] = vaListBuf[i];
            
            var strBuf = stackalloc byte[bufSize];
            var strBufPtr = (IntPtr) strBuf;
            
            var nbNeed = vsnprintfFunc(strBufPtr, (UIntPtr) bufSize, format, vaList);

            if (nbNeed < bufSize)
                return Marshal.PtrToStringUTF8(strBufPtr, nbNeed);

            var pool = ArrayPool<byte>.Shared;

            var buffer = pool.Rent(nbNeed + 1);
            fixed (byte* pb = buffer)
            {
                nbNeed = vsnprintfFunc((IntPtr) pb, (UIntPtr) buffer.Length, format, (IntPtr) vaListCopy);
            }

            var result = Encoding.UTF8.GetString(buffer, 0, nbNeed);
            pool.Return(buffer);
            return result;
        }

        private delegate int Vsnprintf(IntPtr buffer, UIntPtr size, IntPtr format, IntPtr args);

        
        [DllImport("libc", EntryPoint = "vsnprintf", CallingConvention = CallingConvention.Cdecl)]
        internal static extern int vsnprintfLibc(IntPtr buffer, UIntPtr size, IntPtr format, IntPtr args);

        [DllImport("ucrt", EntryPoint = "vsnprintf", CallingConvention = CallingConvention.Cdecl)]
        internal static extern int vsnprintfUCrt(IntPtr buffer, UIntPtr size, IntPtr format, IntPtr args);
    }
}