Roblox-File-Format/BinaryFormat/BinaryFileReader.cs

using System;
using System.IO;
using System.Linq;
using System.Collections.Generic;
using System.Runtime.InteropServices;
using System.Text;

namespace RobloxFiles.BinaryFormat
{
    public class BinaryRobloxFileReader : BinaryReader
    {
        public readonly BinaryRobloxFile File;
        private byte[] lastStringBuffer = Array.Empty<byte>();

        public BinaryRobloxFileReader(BinaryRobloxFile file, Stream stream) : base(stream)
        {
            File = file;
        }

        // Reads 'count * sizeof(T)' interleaved bytes
        public T[] ReadInterleaved<T>(int count, Func<byte[], int, T> transform) where T : struct
        {
            int sizeof_T = Marshal.SizeOf<T>();
            int blobSize = count * sizeof_T;

            var blob = ReadBytes(blobSize);
            var work = new byte[sizeof_T];
            var values = new T[count];

            for (int offset = 0; offset < count; offset++)
            {
                for (int i = 0; i < sizeof_T; i++)
                {
                    int index = (i * count) + offset;
                    work[sizeof_T - i - 1] = blob[index];
                }

                values[offset] = transform(work, 0);
            }

            return values;
        }
        
        // Rotates the sign bit of an int32 buffer.
        public int RotateInt32(byte[] buffer, int startIndex)
        {
            int value = BitConverter.ToInt32(buffer, startIndex);
            return (int)((uint)value >> 1) ^ (-(value & 1));
        }

        // Rotates the sign bit of an int64 buffer.
        public long RotateInt64(byte[] buffer, int startIndex)
        {
            long value = BitConverter.ToInt64(buffer, startIndex);
            return (long)((ulong)value >> 1) ^ (-(value & 1));
        }

        // Rotates the sign bit of a float buffer.
        public float RotateFloat(byte[] buffer, int startIndex)
        {
            uint u = BitConverter.ToUInt32(buffer, startIndex);
            uint i = (u >> 1) | (u << 31);

            byte[] b = BitConverter.GetBytes(i);
            return BitConverter.ToSingle(b, 0);
        }
        
        // Reads and accumulates an interleaved int32 buffer.
        public List<int> ReadInstanceIds(int count)
        {
            int[] values = ReadInterleaved(count, RotateInt32);

            for (int i = 1; i < count; ++i)
                values[i] += values[i - 1];

            return values.ToList();
        }

        public override string ReadString()
        {
            int length = ReadInt32();
            byte[] buffer = ReadBytes(length);

            lastStringBuffer = buffer;
            return Encoding.UTF8.GetString(buffer);
        }

        public float ReadFloat()
        {
            return ReadSingle();
        }

        public byte[] GetLastStringBuffer()
        {
            return lastStringBuffer;
        }
    }
}