Convert CRC16 from C to Ruby using truncated polynomial 800516

Question

I am very new to Ruby, please help.

This is a C code which I need to convert into Ruby.

Passing values [1,2,3,4,5] it gives me B059 in HEX

    unsigned short CalcCrc16(const unsigned char *Data,unsigned short DataLen)
    {
      unsigned short Temp;
      unsigned short Crc;
      Crc = 0;
      while (DataLen--)
      {
             Temp = (unsigned short)((*Data++) ^ (Crc >> 8));
             Temp ^= (Temp >> 4);
             Temp ^= (Temp >> 2);
             Temp ^= (Temp >> 1);
             Crc = (Crc << 8) ^ (Temp << 15) ^ (Temp << 2) ^ Temp;
      }
        return Crc; 
   }

This is the Ruby code I have tried:

class CRC16 
  def CRC16.CalculateCrc16(data) 
    crc = 0x0000 
    temp = 0x0000 
    i = 0 
    while i < data.Length 
      value = data[i] 
      temp = (value ^ (crc >> 8)) 
      temp = (temp ^ (temp >> 4)) 
      temp = (temp ^ (temp >> 2)) 
      temp = (temp ^ (temp >> 1)) 
      crc = (((crc << 8) ^ (temp << 15) ^ (temp << 2) ^ temp)) 
      i += 1 
    end 
    return crc 
  end 
end

Please help me to convert this code into Ruby. Thanks Deepak

What have you tried? Nobody will just do the work for you if you don't show at least what you've tried or where your specific problem is. — Patrick Oscity, Aug 20 '13 at 11:19

score 4 · Accepted Answer · answered Aug 20 '13 at 12:00

4

You are almost there.

Here is the fix:

class CRC16 
  def CRC16.CalculateCrc16(data) 
    crc = 0
    temp = 0
    i = 0 
    while i < data.length    # Modified from OP version
      value = data[i] 
      temp = (value ^ (crc >> 8)) 
      temp = (temp ^ (temp >> 4)) 
      temp = (temp ^ (temp >> 2)) 
      temp = (temp ^ (temp >> 1)) 
      crc = (((crc << 8) ^ (temp << 15) ^ (temp << 2) ^ temp))
      crc &= 0xffff          # New - keep integer in "unsigned short" bit space
      i += 1 
    end 
    return crc 
  end 
end

I changed just two things to make it work as per the C version:

Length -> length, a typo
Ruby doesn't do short, or any other kind of restriction on integer size. You have to add it. That is what crc &= 0xffff is doing. Without it, bits shifted "out" of the short come back to haunt you and give a nonsense result.

In addition, I replaced 0x0000 with 0, as it looked like an attempt to get Ruby to treat the integers as "short", which is not possible this way.

answered Aug 20 '13 at 12:00

Neil Slater

26,512
6
76
94

1

`crc &= 0xffff` makes sense to add in the C version to deal with variant `short` sizes. (uint32_t be better.) – chux - Reinstate Monica Aug 20 '13 at 12:07
thank you very much...i got my answer from your solution...:) – Deepak Kumar Jha Aug 20 '13 at 12:13
one more query if i want to pass '12345' in place of [1,2,3,4,5]. To get the same o/p . can you please help me where to change in the code to get the desired result b059. – Deepak Kumar Jha Aug 20 '13 at 12:32
@Deepak Kumar Jha: The character `"1"` is different to the number `1`, are you sure you want this? Does your input really consist of an array of values 0 to 9? If so, a Ruby one-liner would be `"12345".chars.map { |c| c.to_i }` or more generally `numbers = str.chars.map { |c| c.to_i }` where `str` is your input string. – Neil Slater Aug 20 '13 at 13:06

Convert CRC16 from C to Ruby using truncated polynomial 800516

1 Answers1