private ulong CalculateTableChecksum(FontStreamReader reader, OffsetTable.Entry offsetTable)
{
reader.Push();
reader.Seek(offsetTable.Offset, SeekOrigin.Begin);
var sum = 0u;
var count = (offsetTable.Length + 3) / 4;
while (count-- > 0)
{
var value = reader.ReadUInt32();
unchecked { sum += value; }
}
reader.Pop();
return(sum);
}
internal OffsetTable ReadOffsetTable(FontStreamReader reader)
{
reader.Seek(0, SeekOrigin.Begin);
var scalarType = reader.ReadUInt32();
var numberOfTables = reader.ReadUInt16();
var searchRange = reader.ReadUInt16();
var entrySelector = reader.ReadUInt16();
var rangeShift = reader.ReadUInt16();
var entries = new Dictionary <string, OffsetTable.Entry>();
for (var i = 0; i < numberOfTables; i++)
{
var tag = reader.ReadString(4);
var checkSum = reader.ReadUInt32();
var offset = reader.ReadUInt32();
var length = reader.ReadUInt32();
var entry = new OffsetTable.Entry(tag, checkSum, offset, length);
//TODO assert in bounds
if (tag != "head")
{
var calculatedCheckSum = CalculateTableChecksum(reader, entry);
if (calculatedCheckSum != checkSum)
{
throw new FormatException(
$"CheckSum for '{tag}' table was {calculatedCheckSum:X} but was expected to be {checkSum:X}");
}
}
entries.Add(tag, entry);
}
return(new OffsetTable(scalarType, entries));
}