/// <summary>
/// Calculate the checksum for the whole font by setting checksum adjustment in the head table to 0.
/// </summary>
public static uint CalculateWholeFontChecksum(IInputBytes bytes, TrueTypeHeaderTable headerTable)
{
if (bytes == null)
{
throw new ArgumentNullException(nameof(bytes));
}
if (!IsHeadTable(headerTable))
{
throw new ArgumentException($"Can only calculate checksum for the whole font when the head table is provided. Got: {headerTable}.");
}
bytes.Seek(0);
return(Calculate(ToChecksumSkippedEnumerable(bytes, headerTable)));
}
/// <summary>
/// Calculate the checksum for the specific table.
/// </summary>
public static uint Calculate(IInputBytes bytes, TrueTypeHeaderTable table)
{
bytes.Seek(table.Offset);
if (IsHeadTable(table))
{
// To calculate the checkSum for the 'head' table which itself includes the
// checkSumAdjustment entry for the entire font, do the following:
// Set the checkSumAdjustment to 0.
// Calculate the checksum as normal.
var fullTableBytes = new byte[table.Length];
var read = bytes.Read(fullTableBytes);
if (read != table.Length)
{
throw new InvalidOperationException();
}
// Zero out the checksum adjustment
fullTableBytes[ChecksumAdjustmentPosition] = 0;
fullTableBytes[ChecksumAdjustmentPosition + 1] = 0;
fullTableBytes[ChecksumAdjustmentPosition + 2] = 0;
fullTableBytes[ChecksumAdjustmentPosition + 3] = 0;
return(Calculate(fullTableBytes));
}
var result = 0u;
unchecked
{
while (TryReadUInt(bytes, table.Offset + table.Length, out var next))
{
result += next;
}
}
return(result);
}
private static bool IsHeadTable(TrueTypeHeaderTable table) => string.Equals(HeaderTableTag, table.Tag, StringComparison.OrdinalIgnoreCase);
private static IEnumerable <byte> ToChecksumSkippedEnumerable(IInputBytes bytes, TrueTypeHeaderTable table)
{
while (bytes.MoveNext())
{
// Skip checksum adjustment
if (bytes.CurrentOffset > table.Offset + ChecksumAdjustmentPosition && bytes.CurrentOffset <= table.Offset + ChecksumAdjustmentPosition + 4)
{
continue;
}
yield return(bytes.CurrentByte);
}
}
