/// <summary>Write the header to the file</summary>
/// <param name="headerFlags"></param>
/// <param name="uid"></param>
/// <param name="sequence"></param>
/// <param name="count"></param>
/// <param name="timestamp"></param>
/// <param name="attributes"></param>
/// <remarks>This needs to be called before writing any level to the file</remarks>
public Task WriteHeaderAsync(SnapshotFormat.Flags headerFlags, Uuid128 uid, ulong sequence, long count, long timestamp, IDictionary <string, IFdbTuple> attributes)
{
// The header will be use on ore more "pages", to simplify the job of loading / peeking at a stream content (no need for fancy buffering, just need to read 4K pages)
// > The last page is padded with 0xAAs to detect corruption.
m_uid = uid;
m_sequence = sequence;
m_itemCount = count;
m_timestamp = timestamp;
// HEADER
// - DB_HEADER (64 bytes)
// - DB ATTRIBUTES (variable size list of k/v)
// - END_MARKER + HEADER_CRC
// - PADDING (to fill last page)
// DB Header
// "PNDB"
m_writer.WriteFixed32(SnapshotFormat.HEADER_MAGIC_NUMBER);
// v1.0
m_writer.WriteFixed16(1); // major
m_writer.WriteFixed16(0); // minor
// FLAGS
m_writer.WriteFixed64((ulong)headerFlags);
// Database ID
m_writer.WriteBytes(uid.ToSlice());
// Database Version
m_writer.WriteFixed64(sequence);
// Number of items in the database
m_writer.WriteFixed64((ulong)count);
// Database Timestamp
m_writer.WriteFixed64((ulong)timestamp);
// Page Size
m_writer.WriteFixed32(SnapshotFormat.PAGE_SIZE);
// Header Size (not known yet and will be filled in later)
int offsetToHeaderSize = m_writer.Skip(4);
// we should be at the 64 byte mark
Contract.Assert(m_writer.Position == SnapshotFormat.HEADER_METADATA_BYTES);
// DB Attributes
m_writer.WriteFixed32((uint)attributes.Count);
foreach (var kvp in attributes)
{
// Name
m_writer.WriteVarbytes(Slice.FromString(kvp.Key));
// Value
m_writer.WriteVarbytes(kvp.Value.ToSlice());
}
// Mark the end of the header
m_writer.WriteFixed32(uint.MaxValue);
// we now have the size of the header, and can fill in the blank
var headerEnd = m_writer.Position;
m_writer.Position = offsetToHeaderSize;
// write the header size (includes the CRC)
m_writer.WriteFixed32((uint)checked (headerEnd + SnapshotFormat.HEADER_CRC_SIZE));
m_writer.Position = headerEnd;
// now we can compute the actual CRC
uint headerChecksum = SnapshotFormat.ComputeChecksum(m_writer.ToSlice());
m_writer.WriteFixed32(headerChecksum);
m_headerChecksum = headerChecksum;
// optional padding to fill the rest of the page
PadPageIfNeeded(SnapshotFormat.PAGE_SIZE, 0xFD);
return(TaskHelpers.CompletedTask);
}