stream

    fn stream(r: *Io.Reader, w: *Io.Writer, limit: Io.Limit) Io.Reader.StreamError!usize {
        const ms: *MsfStream = @alignCast(@fieldParentPtr("interface", r));

        var block_id: usize = @intCast(ms.next_read_pos / ms.block_size);
        if (block_id >= ms.blocks.len) return error.EndOfStream;
        var block = ms.blocks[block_id];
        var offset = ms.next_read_pos % ms.block_size;

        ms.file_reader.seekTo(block * ms.block_size + offset) catch |err| {
            ms.err = err;
            return error.ReadFailed;
        };

        var remaining = @backingInt(limit);
        while (remaining != 0) {
            const stream_len: usize = @min(remaining, ms.block_size - offset);
            const n = try ms.file_reader.interface.stream(w, .limited(stream_len));
            remaining -= n;
            offset += n;

            // If we're at the end of a block, go to the next one.
            if (offset == ms.block_size) {
                offset = 0;
                block_id += 1;
                if (block_id >= ms.blocks.len) break; // End of Stream
                block = ms.blocks[block_id];
                ms.file_reader.seekTo(block * ms.block_size) catch |err| {
                    ms.err = err;
                    return error.ReadFailed;
                };
            }
        }

        const total = @backingInt(limit) - remaining;
        ms.next_read_pos += total;
        return total;
    }