source: OniSplit/SubtitleImporter.cs@ 1123

using System;
using System.Collections.Generic;
using System.IO;
using System.Text;

namespace Oni
{
    internal sealed class SubtitleImporter : Importer
    {
        #region Private data
        private List<byte> subtitles;
        private List<int> offsets;
        #endregion

        public override void Import(string filePath, string outputDirPath)
        {
            ReadTextFile(filePath);

            string name = Path.GetFileNameWithoutExtension(filePath);

            BeginImport();
            WriteSUBT(name, subtitles.ToArray());
            Write(outputDirPath);
        }

        private void ReadTextFile(string filePath)
        {
            subtitles = new List<byte>();
            offsets = new List<int>();

            var data = File.ReadAllBytes(filePath);
            int i = SkipPreamble(data);

            while (i < data.Length)
            {
                int nameStart = i;

                while (!IsNewLine(data, i) && data[i] != '=')
                    i++;

                int nameEnd = i;

                if (IsNewLine(data, i))
                {
                    //
                    // This was an empty line or a line that does not contain a =. Skip it.
                    //

                    continue;
                }

                i++;

                int textStart = i;

                while (!IsNewLine(data, i))
                    i++;

                int textEnd = i;

                if (nameEnd > nameStart)
                {
                    offsets.Add(subtitles.Count);

                    for (int j = nameStart; j < nameEnd; j++)
                        subtitles.Add(data[j]);

                    subtitles.Add(0);

                    for (int j = textStart; j < textEnd; j++)
                        subtitles.Add(data[j]);

                    subtitles.Add(0);
                }

                i = SkipNewLine(data, i);
            }
        }

        private static int SkipPreamble(byte[] data)
        {
            int offset = CheckPreamble(data, Encoding.UTF8.GetPreamble());

            if (offset > 0)
                return offset;

            if (CheckPreamble(data, Encoding.Unicode.GetPreamble()) != 0
                || CheckPreamble(data, Encoding.BigEndianUnicode.GetPreamble()) != 0
                || CheckPreamble(data, Encoding.UTF32.GetPreamble()) != 0)
            {
                throw new InvalidDataException("UTF16/32 input text files are not supported.");
            }

            if (data.Length >= 4)
            {
                if ((data[1] == 0 && data[3] == 0)
                    || (data[0] == 0 && data[2] == 0))
                {
                    throw new InvalidDataException("UTF16/32 input text files are not supported.");
                }
            }

            return 0;
        }

        private static int CheckPreamble(byte[] data, byte[] preamble)
        {
            if (data.Length < preamble.Length)
                return 0;

            for (int i = 0; i < preamble.Length; i++)
            {
                if (data[i] != preamble[i])
                    return 0;
            }

            return preamble.Length;
        }

        private static bool IsNewLine(byte[] data, int offset)
        {
            if (offset >= data.Length)
                return true;

            return (SkipNewLine(data, offset) > offset);
        }

        private static int SkipNewLine(byte[] data, int offset)
        {
            if (offset < data.Length)
            {
                if (data[offset] == '\n')
                {
                    offset++;
                }
                else if (data[offset] == '\r')
                {
                    offset++;

                    if (offset < data.Length && data[offset] == '\n')
                        offset++;
                }
            }

            return offset;
        }

        private void WriteSUBT(string name, byte[] subtitles)
        {
            var subt = CreateInstance(TemplateTag.SUBT, name);

            using (var writer = subt.OpenWrite(16))
            {
                writer.Write(WriteRawPart(subtitles));
                writer.Write(offsets.Count);
                writer.Write(offsets.ToArray());
            }
        }
    }
}