using System.IO;
using System.Runtime.InteropServices;
using System.Threading.Tasks;

namespace Elwig.Helpers {
    static partial class Extensions {

        [LibraryImport("msvcrt.dll")]
        [UnmanagedCallConv(CallConvs = [typeof(System.Runtime.CompilerServices.CallConvCdecl)])]
        private static unsafe partial int memcmp(void* b1, void* b2, long count);

        public static unsafe int CompareBuffers(char[] buffer1, int offset1, char[] buffer2, int offset2, int count) {
            fixed (char* b1 = buffer1, b2 = buffer2) {
                return memcmp(b1 + offset1, b2 + offset2, count);
            }
        }

        public static string? ReadUntil(this StreamReader reader, string delimiter) {
            return ReadUntil(reader, delimiter.ToCharArray());
        }

        public static string? ReadUntil(this StreamReader reader, char delimiter) {
            return ReadUntil(reader, new char[] { delimiter });
        }

        public static string? ReadUntil(this StreamReader reader, char[] delimiter) {
            var buf = new char[512];
            int bufSize = 0, ret;
            while (!reader.EndOfStream && bufSize < buf.Length - 1) {
                if ((ret = reader.Read()) == -1)
                    return null;
                buf[bufSize++] = (char)ret;

                if (bufSize >= delimiter.Length && CompareBuffers(buf, bufSize - delimiter.Length, delimiter, 0, delimiter.Length) == 0)
                    return new string(buf, 0, bufSize);
            }
            return null;
        }

        public static Task<string?> ReadUntilAsync(this StreamReader reader, string delimiter) {
            return ReadUntilAsync(reader, delimiter.ToCharArray());
        }

        public static Task<string?> ReadUntilAsync(this StreamReader reader, char delimiter) {
            return ReadUntilAsync(reader, new char[] { delimiter });
        }

        public static async Task<string?> ReadUntilAsync(this StreamReader reader, char[] delimiter) {
            var buf = new char[512];
            int bufSize = 0;
            var tmpBuf = new char[1];
            while (!reader.EndOfStream && bufSize < buf.Length - 1) {
                if ((await reader.ReadAsync(tmpBuf, 0, 1)) != 1)
                    return null;
                buf[bufSize++] = tmpBuf[0];

                if (bufSize >= delimiter.Length && CompareBuffers(buf, bufSize - delimiter.Length, delimiter, 0, delimiter.Length) == 0)
                    return new string(buf, 0, bufSize);
            }
            return null;
        }
    }
}