/// <summary>
/// Calculates the <see cref="Fletcher16Checksum"/> of a <see cref="Byte"/>[]. This method implements the slow but straightforward way.
/// This algorithm is also known as the "8-bit Fletcher Checksum", since it consumes the data word in 8-bit chunks.
/// </summary>
/// <param name="data">The list whose elements shall be added to the checksum.</param>
/// <param name="offset">Location in the array.</param>
/// <param name="count">Number of elements.</param>
/// <param name="fletcher16">The <see cref="Fletcher16Checksum"/> to start from.</param>
/// <returns>A <see cref="Fletcher16Checksum"/>.</returns>
public static Fletcher16Checksum GetSlow(IList<byte> data, int offset=0, int count=0, Fletcher16Checksum fletcher16=new Fletcher16Checksum())
{
if(data==null) throw new ArgumentNullException("data");
if(offset<0||offset>data.Count)
throw new ArgumentOutOfRangeException("offset", "Must be non-negative and less than or equal to the length of data.");
if(count<0) throw new ArgumentOutOfRangeException("count", "Must be non-negative.");
if(offset+count>data.Count) throw new ArgumentOutOfRangeException("count", "Must be less than or equal to the length of data minus the offset argument.");
if(count==0)
{
count=data.Count-offset;
if(count==0) return fletcher16;
}
int c0=fletcher16.C0;
int c1=fletcher16.C1;
for(int i=offset; i<count; i++)
{
c0=(c0+data[i])%255;
c1=(c1+c0)%255;
}
return new Fletcher16Checksum() { C0=(byte)c0, C1=(byte)c1 };
}
/// <summary>
/// Calculates the <see cref="Fletcher16Checksum"/> of a <see cref="Byte"/>[].
/// This algorithm is also known as the "8-bit Fletcher Checksum", since it consumes the data word in 8-bit chunks.
/// </summary>
/// <param name="data">An array of bytes.</param>
/// <param name="offset">Location in the array.</param>
/// <param name="count">Number of elements.</param>
/// <param name="fletcher16">The <see cref="Fletcher16Checksum"/> to start from.</param>
/// <returns>A <see cref="Fletcher16Checksum"/>.</returns>
public static Fletcher16Checksum Get(byte[] data, int offset = 0, int count = 0, Fletcher16Checksum fletcher16 = new Fletcher16Checksum())
{
if (data == null) throw new ArgumentNullException("data");
if (offset < 0 || offset > data.Length) throw new ArgumentOutOfRangeException("offset", "Must be non-negative and less than or equal to the length of data.");
if (count < 0) throw new ArgumentOutOfRangeException("count", "Must be non-negative.");
if (offset + count > data.Length) throw new ArgumentOutOfRangeException("count", "Must be less than or equal to the length of data minus the offset argument.");
if (count == 0)
{
count = data.Length - offset;
if (count == 0) return fletcher16;
}
int c0 = fletcher16.C0;
int c1 = fletcher16.C1;
unsafe
{
// local copies of values and references make the code much faster
int lCount = count;
fixed(byte* lDataFixed=data)
{
byte* lData = lDataFixed + offset;
while (lCount > 0)
{
int c = lCount > 4101 ? 4101 : lCount;
lCount -= c;
do
{
c0 += *lData++;
c1 += c0;
}
while(--c > 0);
// 3 times, since we have to reduce it from 4 byte to 1 (with maybe one carry-over)
c1 = (c1 & 0xff) + (c1 >> 8);
c0 = (c0 & 0xff) + (c0 >> 8);
c1 = (c1 & 0xff) + (c1 >> 8);
c0 = (c0 & 0xff) + (c0 >> 8);
c1 = (c1 & 0xff) + (c1 >> 8);
//c0=(c0&0xff)+(c0>>8); // not really needed. The 4101*255+0x1fd don't need the fourth byte.
}
}
}
// once again, to reduce the sums to 8 bits, each
c0 = (c0 & 0xff) + (c0 >> 8);
c1 = (c1 & 0xff) + (c1 >> 8);
return new Fletcher16Checksum() { C0 = (byte)c0, C1 = (byte)c1 };
}
/// <summary>
/// Calculates the <see cref="Fletcher16Checksum"/> of a <see cref="IEnumerable{Byte}"/>. This method implements the slow but straightforward way.
/// This algorithm is also known as the "8-bit Fletcher Checksum", since it consumes the data word in 8-bit chunks.
/// </summary>
/// <param name="data">The collection whose elements shall be added to the checksum.</param>
/// <param name="fletcher16">The <see cref="Fletcher16Checksum"/> to start from.</param>
/// <returns>A <see cref="Fletcher16Checksum"/>.</returns>
public static Fletcher16Checksum GetSlow(IEnumerable<byte> data, Fletcher16Checksum fletcher16=new Fletcher16Checksum())
{
if(data==null) throw new ArgumentNullException("data");
int c0=fletcher16.C0;
int c1=fletcher16.C1;
foreach(byte value in data)
{
c0=(c0+value)%255;
c1=(c1+c0)%255;
}
return new Fletcher16Checksum() { C0=(byte)c0, C1=(byte)c1 };
}
/// <summary>
/// Calculates the <see cref="Fletcher16Checksum"/> of a <see cref="IEnumerable{Byte}"/>.
/// This algorithm is also known as the "8-bit Fletcher Checksum", since it consumes the data word in 8-bit chunks.
/// </summary>
/// <param name="data">The collection whose elements shall be added to the checksum.</param>
/// <param name="fletcher16">The <see cref="Fletcher16Checksum"/> to start from.</param>
/// <returns>A <see cref="Fletcher16Checksum"/>.</returns>
public static Fletcher16Checksum Get(IEnumerable<byte> data, Fletcher16Checksum fletcher16=new Fletcher16Checksum())
{
if(data==null) throw new ArgumentNullException("data");
int c0=fletcher16.C0;
int c1=fletcher16.C1;
// local copies of values and references make the code much faster
IEnumerable<byte> lData=data;
int c=0;
foreach(byte value in lData)
{
c0+=value;
c1+=c0;
if((++c)<=4101) continue;
c=0;
// 3 times, since we have to reduce it from 4 byte to 1 (with maybe one carry-over)
c1=(c1&0xff)+(c1>>8);
c0=(c0&0xff)+(c0>>8);
c1=(c1&0xff)+(c1>>8);
c0=(c0&0xff)+(c0>>8);
c1=(c1&0xff)+(c1>>8);
//c0=(c0&0xff)+(c0>>8); // not really needed. The 4101*255+0x1fd don't need the fourth byte.
}
if(c!=0)
{
// 3 times, since we have to reduce it from 4 byte to 1 (with maybe one carry-over)
c1=(c1&0xff)+(c1>>8);
c0=(c0&0xff)+(c0>>8);
c1=(c1&0xff)+(c1>>8);
c0=(c0&0xff)+(c0>>8);
c1=(c1&0xff)+(c1>>8);
//c0=(c0&0xff)+(c0>>8); // not really needed. The 4101*255+0x1fd don't need the fourth byte.
}
// once again, to reduce the sums to 8 bits, each
c0=(c0&0xff)+(c0>>8);
c1=(c1&0xff)+(c1>>8);
return new Fletcher16Checksum() { C0=(byte)c0, C1=(byte)c1 };
}
