shoc-backend/SeaHaven.Services/Helpers/VideoDurationProbe.cs

using System.Buffers.Binary;
using System.Text;

namespace SeaHaven.Services.Helpers
{
    /// <summary>
    /// Reads a video's duration from the ISO base media (MP4 / QuickTime) movie header:
    /// top-level <c>moov</c> box → <c>mvhd</c> timescale and duration. It seeks over box
    /// headers only (the <c>moov</c> box may sit after a large <c>mdat</c>), never decodes
    /// media and never allocates more than a few bytes. Returns null whenever the structure
    /// cannot be read, so callers can let unreadable files through (SH-116: unreadable
    /// metadata never blocks an upload).
    /// </summary>
    public static class VideoDurationProbe
    {
        private const int MaxBoxesPerLevel = 1024;

        public static double? TryReadDurationSeconds(Stream? stream)
        {
            if (stream == null || !stream.CanSeek || !stream.CanRead)
                return null;

            try
            {
                var moov = FindBox(stream, 0, stream.Length, "moov");
                if (moov == null)
                    return null;

                var mvhd = FindBox(stream, moov.Value.BodyStart, moov.Value.End, "mvhd");
                return mvhd == null ? null : ReadMovieHeaderSeconds(stream, mvhd.Value);
            }
            catch (IOException)
            {
                return null;
            }
        }

        private readonly record struct Box(long BodyStart, long End);

        private static Box? FindBox(Stream stream, long start, long end, string type)
        {
            var header = new byte[8];
            var position = start;
            for (var i = 0; i < MaxBoxesPerLevel && position + 8 <= end; i++)
            {
                stream.Position = position;
                if (!ReadExactly(stream, header, 8))
                    return null;

                long size = BinaryPrimitives.ReadUInt32BigEndian(header);
                var boxType = Encoding.ASCII.GetString(header, 4, 4);
                var headerLength = 8;
                if (size == 1)
                {
                    // 64-bit "largesize" follows the type.
                    var large = new byte[8];
                    if (!ReadExactly(stream, large, 8))
                        return null;
                    var largeSize = BinaryPrimitives.ReadUInt64BigEndian(large);
                    if (largeSize > long.MaxValue)
                        return null;
                    size = (long)largeSize;
                    headerLength = 16;
                }
                else if (size == 0)
                {
                    size = end - position;
                }

                if (size < headerLength || position + size > end)
                    return null;

                if (boxType == type)
                    return new Box(position + headerLength, position + size);

                position += size;
            }

            return null;
        }

        private static double? ReadMovieHeaderSeconds(Stream stream, Box mvhd)
        {
            // version(1) flags(3), then v0: creation(4) modification(4) timescale(4) duration(4)
            //                          v1: creation(8) modification(8) timescale(4) duration(8)
            var body = new byte[32];
            stream.Position = mvhd.BodyStart;
            var available = (int)Math.Min(body.Length, mvhd.End - mvhd.BodyStart);
            if (available < 20 || !ReadExactly(stream, body, available))
                return null;

            uint timescale;
            ulong duration;
            if (body[0] == 0)
            {
                timescale = BinaryPrimitives.ReadUInt32BigEndian(body.AsSpan(12, 4));
                duration = BinaryPrimitives.ReadUInt32BigEndian(body.AsSpan(16, 4));
            }
            else if (body[0] == 1 && available >= 32)
            {
                timescale = BinaryPrimitives.ReadUInt32BigEndian(body.AsSpan(20, 4));
                duration = BinaryPrimitives.ReadUInt64BigEndian(body.AsSpan(24, 8));
            }
            else
            {
                return null;
            }

            // All-ones duration means "unknown" in the spec.
            if (timescale == 0 || duration == uint.MaxValue || duration == ulong.MaxValue)
                return null;

            return (double)duration / timescale;
        }

        private static bool ReadExactly(Stream stream, byte[] buffer, int count)
        {
            var read = 0;
            while (read < count)
            {
                var n = stream.Read(buffer, read, count - read);
                if (n <= 0)
                    return false;
                read += n;
            }
            return true;
        }
    }
}