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Convert CRC calculation from C# to C

I am currently working on a serial monitor, to ensure some data integrity I am trying to implement a CRC8 checksum, below is the calculation i do on any messages before i send them.
public byte Checksum(params byte[] val)
{
if (val == null)
throw new ArgumentNullException("val");
byte c = 0;
foreach (byte b in val)
{
c = table[c ^ b];
}
return c;
}
I generate a table using 0xD8:
public byte[] GenerateTable(CRC8_POLY polynomial)
{
byte[] csTable = new byte[256];
for (int i = 0; i < 256; ++i)
{
int curr = i;
for (int j = 0; j < 8; ++j)
{
if ((curr & 0x80) != 0)
{
curr = (curr << 1) ^ (int)polynomial;
}
else
{
curr <<= 1;
}
}
csTable[i] = (byte)curr;
}
return csTable;
}
This is a code i have used for testing the setup:
private void btnSend_Click(object sender, EventArgs e)
{
ProtoFrame rxFrame = new ProtoFrame();
if (cboParam.Text == "test")
{
rxFrame.Start = 0x73;
rxFrame.Size = 9;
rxFrame.Command = 01;
rxFrame.Unused = 0;
rxFrame.ParamId = 0x0100;
rxFrame.Param = 8000;
}
byte[] rxBuffer = getBytes(rxFrame); //call to byte array formatter
rxBuffer[rxBuffer.Length-1] = Checksum(rxBuffer); //append crc at end of array
ComPort.Write(rxBuffer, 0, rxBuffer.Length);
}
static byte[] getBytes(object str) //input struct
{
int size = Marshal.SizeOf(str) + 1;
byte[] arr = new byte[size];
IntPtr ptr = Marshal.AllocHGlobal(size);
Marshal.StructureToPtr(str, ptr, true);
Marshal.Copy(ptr, arr, 0, size);
Marshal.FreeHGlobal(ptr);
return arr;
}
As far as i know this code work as intendended, and im using the table generator to implement a hardcoded table in my microcontroller, to speed up the process.
What i dont quite get is how i implement a function to calculate the CRC in a similar way as i do here.
Any help or guides in the right direction is aprreciated.
So far i have come up with this function:
uint8_t crc8(uint8_t *crc)
{
uint8_t crcVal;
int m;
for (m = 0; m < PacketSize ;m++ )startbyte
{
*crc = crc8_table[(*crc) ^ m];
*crc &= 0xFF;
}
}
where table is:
uint8_t crc8_table[256] = {0,24,48,40,96,120,80,72,192,216,240,232,160,184,144,136,88,64,104,112,56,32,8,16,
152,128,168,176,248,224,200,208,176,168,128,152,208,200,224,248,112,104,64,88,16,8,
32,56,232,240,216,192,136,144,184,160,40,48,24,0,72,80,120,96,184,160,136,144,216,
192,232,240,120,96,72,80,24,0,40,48,224,248,208,200,128,152,176,168,32,56,16,8,64,
88,112,104,8,16,56,32,104,112,88,64,200,208,248,224,168,176,152,128,80,72,96,120,48,
40,0,24,144,136,160,184,240,232,192,216,168,176,152,128,200,208,248,224,104,112,88,
64,8,16,56,32,240,232,192,216,144,136,160,184,48,40,0,24,80,72,96,120,24,0,40,48,120,
96,72,80,216,192,232,240,184,160,136,144,64,88,112,104,32,56,16,8,128,152,176,168,224,
248,208,200,16,8,32,56,112,104,64,88,208,200,224,248,176,168,128,152,72,80,120,96,40,
48,24,0,136,144,184,160,232,240,216,192,160,184,144,136,192,216,240,232,96,120,80,72,
0,24,48,40,248,224,200,208,152,128,168,176,56,32,8,16,88,64,104,112
};
and PacketSize is found from rxFrame.Size

So you simply need to port your C# function to C
public byte Checksum(params byte[] val)
{
if (val == null)
throw new ArgumentNullException("val");
byte c = 0;
foreach (byte b in val)
{
c = table[c ^ b];
}
return c;
}
I. there are no exceptions in C. Use return value to indicate errors and add an argument that you'll use as a return value. It's up to you to decide whether to pass a message length as a parameter or leave it at the global scope:
int checksum(uint8_t const *msg, size_t msglen, uint8_t *result)
II. foreach loop is converted to for loop, where i is index and msg[i] is the b from the foreach:
int checksum(uint8_t const *msg, size_t msglen, uint8_t *result)
{
if (msg == NULL || msglen == 0)
return 0;
uint8_t crc = 0;
for (int i = 0; i < msglen; i++)
{
crc = table[crc ^ msg[i]];
}
III. Store the result and return a success code:
int checksum(uint8_t const *msg, size_t msglen, uint8_t *result)
{
if (msg == NULL || msglen == 0)
return 0;
uint8_t crc = 0;
for (int i = 0; i < msglen; i++)
{
crc = table[crc ^ msg[i]];
}
*result = crc;
return 1;
}
IV. Usage:
uint8_t crc;
if (!checksum(message, PacketSize, &crc))
report_error();

Looking at the C# code should be:
uint8_t crc8(uint8_t const *crc, size_t size)
{
unit8_t c = 0;
size_t m;
for (m = 0; m < size; m++ )
{
c = crc8_table[c ^ *crc];
crc++;
}
return c;
}

Don't use pointers, convert to arrays
public static byte crc8(byte[] crc)
{
byte crcVal;
int m;
for (m = 0; m < PacketSize; m++)
{
crc[m] = crc8_table[crc[m] ^ m];
crc[m] &= 0xFF;
}
return crcVal;
}

How to transform C# byte[] to struct[]

In C# I have a struct like this:
[StructLayout(LayoutKind.Sequential,Size = 3)]
public struct int24
{
private byte a;
private byte b;
private byte c;
public int24(byte a, byte b, byte c)
{
this.a = a;
this.b = b;
this.c = c;
}
public Int32 getInt32()
{
byte[] bytes = {this.a, this.b, this.c , 0};
// if we want to put the struct into int32, need a method, not able to type cast directly
return BitConverter.ToInt32(bytes, 0);
}
public void display()
{
Console.WriteLine(" content is ： " + a.ToString() + b.ToString() + c.ToString());
}
}
For byte[] to struct[] transformation, I use:
public static int24[] byteArrayToStructureArrayB(byte[] input) {
int dataPairNr = input.Length / 3;
int24[] structInput = new int24[dataPairNr];
var reader = new BinaryReader(new MemoryStream(input));
for (int i = 0; i < dataPairNr; i++) {
structInput[i] = new int24(reader.ReadByte(), reader.ReadByte(), reader.ReadByte());
}
return structInput;
}
I feel really bad about the code.
Questions are:
What can I do to improve the function byteArrayToStructureArrayB?
As you can see in the int24 struct, I have a function called getInt32(). This function is only for bit shift operation of the struct. Is there a more efficient way?

Something like this should work:
public struct int24 {
public int24(byte[] array, int index) {
// TODO: choose what to do if out of bounds
a = array[index];
b = array[index + 1];
c = array[index + 2];
}
...
}
public static int24[] byteArrayToStructureArrayB(byte[] input) {
var count = input.Length / 3;
var result = new int24[count];
for (int i = 0; i < count; i++)
result[i] = new int24(input, i * 3);
return result;
}

Removing Leading Zeros in a Char Array

I'm attempting to subtract two strings (of theoretically infinite length) without the use of libraries like BigIntbut I was wondering if anybody has any good ideas on how to remove the leading zeros in the corner cases like the one below?
static void Main(string[] args)
{
Console.WriteLine(Subtract("10", "10005"));
}
static string ReverseInput(string inputString)
{
char[] charArray = inputString.ToCharArray();
Array.Reverse(charArray);
return new string(charArray);
}
static string Subtract(string firstNumInput, string secondNumInput)
{
string firstNum = String.Empty;
string secondNum = String.Empty;
bool negative = false;
// Reverse order of string input
if (firstNumInput.Length > secondNumInput.Length)
{
firstNum = ReverseInput(firstNumInput);
secondNum = ReverseInput(secondNumInput);
}
else if (firstNumInput.Length < secondNumInput.Length)
{
negative = true;
firstNum = ReverseInput(secondNumInput);
secondNum = ReverseInput(firstNumInput);
}
else if (firstNumInput.Length == secondNumInput.Length)
{
// iterate through string to find largest
}
char[] result = new char[firstNum.Length + 1];
int resultLength = 0;
int carry = 0;
for (int i = 0; i < firstNum.Length; i++)
{
int an = (i < firstNum.Length) ? int.Parse(firstNum[i].ToString()) : 0;
int bn = (i < secondNum.Length) ? int.Parse(secondNum[i].ToString()) : 0;
int rn = an - bn - carry;
if (rn < 0)
{
carry = 1;
rn += 10;
}
else
{
carry = 0;
}
result[resultLength++] = (char)(rn + '0');
}
// create the result string from the char array
string finalResult = ReverseInput(new string(result, 0, resultLength));
if (negative)
{
finalResult = '-' + finalResult;
}
return finalResult;
}

Are you looking for TrimStart?
// create the result string from the char array
string finalResult = ReverseInput(new string(result, 0, resultLength)).TrimStart('0');

how can i find lcs length between two large strings

I've written the following code in C# for obtaining the length of longest common subsequence of two texts given by use, but it doesn't work with large strings. Could you please help me. I'm really confused.
public Form1()
{
InitializeComponent();
}
public int lcs(char[] s1, char[] s2, int s1size, int s2size)
{
if (s1size == 0 || s2size == 0)
{
return 0;
}
else
{
if (s1[s1size - 1] == s2[s2size - 1])
{
return (lcs(s1, s2, s1size - 1, s2size - 1) + 1);
}
else
{
int x = lcs(s1, s2, s1size, s2size - 1);
int y = lcs(s1, s2, s1size - 1, s2size);
if (x > y)
{
return x;
}
else
return y;
}
}
}
private void button1_Click(object sender, EventArgs e)
{
string st1 = textBox2.Text.Trim(' ');
string st2 = textBox3.Text.Trim(' ');
char[] a = st1.ToCharArray();
char[] b = st2.ToCharArray();
int s1 = a.Length;
int s2 = b.Length;
textBox1.Text = lcs(a, b, s1, s2).ToString();
}

Here you are using the Recursion method. So it leads the program to occur performance problems as you mentioned.
Instead of recursion, use the dynamic programming approach.
Here is the C# Code.
public static void LCS(char[] str1, char[] str2)
{
int[,] l = new int[str1.Length, str2.Length];
int lcs = -1;
string substr = string.Empty;
int end = -1;
for (int i = 0; i < str1.Length; i++)
{
for (int j = 0; j < str2.Length; j++)
{
if (str1[i] == str2[j])
{
if (i == 0 || j == 0)
{
l[i, j] = 1;
}
else
l[i, j] = l[i - 1, j - 1] + 1;
if (l[i, j] > lcs)
{
lcs = l[i, j];
end = i;
}
}
else
l[i, j] = 0;
}
}
for (int i = end - lcs + 1; i <= end; i++)
{
substr += str1[i];
}
Console.WriteLine("Longest Common SubString Length = {0}, Longest Common Substring = {1}", lcs, substr);
}

Here is a solution how to find the longest common substring in C#:
public static string GetLongestCommonSubstring(params string[] strings)
{
var commonSubstrings = new HashSet<string>(strings[0].GetSubstrings());
foreach (string str in strings.Skip(1))
{
commonSubstrings.IntersectWith(str.GetSubstrings());
if (commonSubstrings.Count == 0)
return string.Empty;
}
return commonSubstrings.OrderByDescending(s => s.Length).DefaultIfEmpty(string.Empty).First();
}
private static IEnumerable<string> GetSubstrings(this string str)
{
for (int c = 0; c < str.Length - 1; c++)
{
for (int cc = 1; c + cc <= str.Length; cc++)
{
yield return str.Substring(c, cc);
}
}
}
I found it here: http://www.snippetsource.net/Snippet/75/longest-common-substring

Just for fun, here is one example using Queue<T>:
string LongestCommonSubstring(params string[] strings)
{
return LongestCommonSubstring(strings[0], new Queue<string>(strings.Skip(1)));
}
string LongestCommonSubstring(string x, Queue<string> strings)
{
if (!strings.TryDequeue(out var y))
{
return x;
}
var output = string.Empty;
for (int i = 0; i < x.Length; i++)
{
for (int j = x.Length - i; j > -1; j--)
{
string common = x.Substring(i, j);
if (y.IndexOf(common) > -1 && common.Length > output.Length) output = common;
}
}
return LongestCommonSubstring(output, strings);
}
It's still using recursion though, but it's a nice example of Queue<T>.

I refactored the C++ code from Ashutosh Singh at https://iq.opengenus.org/longest-common-substring-using-rolling-hash/ to create a rolling hash approach in C# - this will find the substring in O(N * log(N)^2) time and O(N) space
using System;
using System.Collections.Generic;
public class RollingHash
{
private class RollingHashPowers
{
// _mod = prime modulus of polynomial hashing
// any prime number over a billion should suffice
internal const int _mod = (int)1e9 + 123;
// _hashBase = base (point of hashing)
// this should be a prime number larger than the number of characters used
// in my use case I am only interested in ASCII (256) characters
// for strings in languages using non-latin characters, this should be much larger
internal const long _hashBase = 257;
// _pow1 = powers of base modulo mod
internal readonly List<int> _pow1 = new List<int> { 1 };
// _pow2 = powers of base modulo 2^64
internal readonly List<long> _pow2 = new List<long> { 1L };
internal void EnsureLength(int length)
{
if (_pow1.Capacity < length)
{
_pow1.Capacity = _pow2.Capacity = length;
}
for (int currentIndx = _pow1.Count - 1; currentIndx < length; ++currentIndx)
{
_pow1.Add((int)(_pow1[currentIndx] * _hashBase % _mod));
_pow2.Add(_pow2[currentIndx] * _hashBase);
}
}
}
private class RollingHashedString
{
readonly RollingHashPowers _pows;
readonly int[] _pref1; // Hash on prefix modulo mod
readonly long[] _pref2; // Hash on prefix modulo 2^64
// Constructor from string:
internal RollingHashedString(RollingHashPowers pows, string s, bool caseInsensitive = false)
{
_pows = pows;
_pref1 = new int[s.Length + 1];
_pref2 = new long[s.Length + 1];
const long capAVal = 'A';
const long capZVal = 'Z';
const long aADif = 'a' - 'A';
unsafe
{
fixed (char* c = s)
{
// Fill arrays with polynomial hashes on prefix
for (int i = 0; i < s.Length; ++i)
{
long v = c[i];
if (caseInsensitive && capAVal <= v && v <= capZVal)
{
v += aADif;
}
_pref1[i + 1] = (int)((_pref1[i] + v * _pows._pow1[i]) % RollingHashPowers._mod);
_pref2[i + 1] = _pref2[i] + v * _pows._pow2[i];
}
}
}
}
// Rollingnomial hash of subsequence [pos, pos+len)
// If mxPow != 0, value automatically multiply on base in needed power.
// Finally base ^ mxPow
internal Tuple<int, long> Apply(int pos, int len, int mxPow = 0)
{
int hash1 = _pref1[pos + len] - _pref1[pos];
long hash2 = _pref2[pos + len] - _pref2[pos];
if (hash1 < 0)
{
hash1 += RollingHashPowers._mod;
}
if (mxPow != 0)
{
hash1 = (int)((long)hash1 * _pows._pow1[mxPow - (pos + len - 1)] % RollingHashPowers._mod);
hash2 *= _pows._pow2[mxPow - (pos + len - 1)];
}
return Tuple.Create(hash1, hash2);
}
}
private readonly RollingHashPowers _rhp;
public RollingHash(int longestLength = 0)
{
_rhp = new RollingHashPowers();
if (longestLength > 0)
{
_rhp.EnsureLength(longestLength);
}
}
public string FindCommonSubstring(string a, string b, bool caseInsensitive = false)
{
// Calculate max neede power of base:
int mxPow = Math.Max(a.Length, b.Length);
_rhp.EnsureLength(mxPow);
// Create hashing objects from strings:
RollingHashedString hash_a = new RollingHashedString(_rhp, a, caseInsensitive);
RollingHashedString hash_b = new RollingHashedString(_rhp, b, caseInsensitive);
// Binary search by length of same subsequence:
int pos = -1;
int low = 0;
int minLen = Math.Min(a.Length, b.Length);
int high = minLen + 1;
var tupleCompare = Comparer<Tuple<int, long>>.Default;
while (high - low > 1)
{
int mid = (low + high) / 2;
List<Tuple<int, long>> hashes = new List<Tuple<int, long>>(a.Length - mid + 1);
for (int i = 0; i + mid <= a.Length; ++i)
{
hashes.Add(hash_a.Apply(i, mid, mxPow));
}
hashes.Sort(tupleCompare);
int p = -1;
for (int i = 0; i + mid <= b.Length; ++i)
{
if (hashes.BinarySearch(hash_b.Apply(i, mid, mxPow), tupleCompare) >= 0)
{
p = i;
break;
}
}
if (p >= 0)
{
low = mid;
pos = p;
}
else
{
high = mid;
}
}
// Output answer:
return pos >= 0
? b.Substring(pos, low)
: string.Empty;
}
}

Help me with XOR encryption

I wrote this code in C# to encrypt a string with a key:
private static int Bin2Dec(string num)
{
int _num = 0;
for (int i = 0; i < num.Length; i++)
_num += (int)Math.Pow(2, num.Length - i - 1) * int.Parse(num[i].ToString());
return _num;
}
private static string Dec2Bin(int num)
{
if (num < 2) return num.ToString();
return Dec2Bin(num / 2) + (num % 2).ToString();
}
public static string StrXor(string str, string key)
{
string _str = "";
string _key = "";
string _xorStr = "";
string _temp = "";
for (int i = 0; i < str.Length; i++)
{
_temp = Dec2Bin(str[i]);
for (int j = 0; j < 8 - _temp.Length + 1; j++)
_temp = '0' + _temp;
_str += _temp;
}
for (int i = 0; i < key.Length; i++)
{
_temp = Dec2Bin(key[i]);
for (int j = 0; j < 8 - _temp.Length + 1; j++)
_temp = '0' + _temp;
_key += _temp;
}
while (_key.Length < _str.Length) _key += _key;
if (_key.Length > _str.Length) _key = _key.Substring(0, _str.Length);
for (int i = 0; i < _str.Length; i++)
if (_str[i] == _key[i]) { _xorStr += '0'; } else { _xorStr += '1'; }
_str = "";
for (int i = 0; i < _xorStr.Length; i += 8)
{
char _chr = (char)0;
_chr = (char)Bin2Dec(_xorStr.Substring(i, 8)); //ERROR : (Index and length must refer to a location within the string. Parameter name: length)
_str += _chr;
}
return _str;
}
The problem is that I always get error when I want to decrypt an encryted text with this code:
string enc_text = ENCRYPT.XORENC("abc","a"); // enc_text = " ♥☻"
string dec_text = ENCRYPT.XORENC(enc_text,"a"); // ArgumentOutOfRangeException
Any clues?

If you have a character, a char, you can convert it to an integer, an int.
And then you can use the ^ operator to perform XOR on it. You don't appear to be using that operator at the moment, which might be the source of your problem.
string EncryptOrDecrypt(string text, string key)
{
var result = new StringBuilder();
for (int c = 0; c < text.Length; c++)
result.Append((char)((uint)text[c] ^ (uint)key[c % key.Length]));
return result.ToString();
}
That kind of thing. Here's a longer version with comments that does the same thing in steps, to make it easier to learn from:
string EncryptOrDecrypt(string text, string key)
{
var result = new StringBuilder();
for (int c = 0; c < text.Length; c++)
{
// take next character from string
char character = text[c];
// cast to a uint
uint charCode = (uint)character;
// figure out which character to take from the key
int keyPosition = c % key.Length; // use modulo to "wrap round"
// take the key character
char keyChar = key[keyPosition];
// cast it to a uint also
uint keyCode = (uint)keyChar;
// perform XOR on the two character codes
uint combinedCode = charCode ^ keyCode;
// cast back to a char
char combinedChar = (char)combinedCode;
// add to the result
result.Append(combineChar);
}
return result.ToString();
}
The short version is the same but with the intermediate variables removed, substituting expressions directly into where they're used.

// Code
public static byte[] EncryptOrDecrypt(byte[] text, byte[] key)
{
byte[] xor = new byte[text.Length];
for (int i = 0; i < text.Length; i++)
{
xor[i] = (byte)(text[i] ^ key[i % key.Length]);
}
return xor;
}
// Test
static void Main(string[] args){
string input;
byte[] inputBytes;
string inputKey;
byte[] key;
do
{
input = System.Console.ReadLine();
inputBytes = Encoding.Unicode.GetBytes(input);
inputKey = System.Console.ReadLine();
key = Encoding.Unicode.GetBytes(inputKey);
//byte[] key = { 0, 0 }; if key is 0, encryption will not happen
byte[] encryptedBytes = EncryptOrDecrypt(inputBytes, key);
string encryptedStr = Encoding.Unicode.GetString(encryptedBytes);
byte[] decryptedBytes = EncryptOrDecrypt(encryptedBytes, key);
string decryptedStr = Encoding.Unicode.GetString(decryptedBytes);
System.Console.WriteLine("Encrypted string:");
System.Console.WriteLine(encryptedStr);
System.Console.WriteLine("Decrypted string:");
System.Console.WriteLine(decryptedStr);
} while (input != "-1" && inputKey != "-1");
//test:
//pavle
//23
//Encrypted string:
//BRD_W
//Decrypted string:
//pavle
}

Here is some simple code to encrypt and decrypt
class CEncryption
{
public static string Encrypt(string strIn, string strKey)
{
string sbOut = String.Empty;
for (int i = 0; i < strIn.Length; i++)
{
sbOut += String.Format("{0:00}", strIn[i] ^ strKey[i % strKey.Length]);
}
return sbOut;
}
public static string Decrypt(string strIn, string strKey)
{
string sbOut = String.Empty;
for (int i = 0; i < strIn.Length; i += 2)
{
byte code = Convert.ToByte(strIn.Substring(i, 2));
sbOut += (char)(code ^ strKey[(i/2) % strKey.Length]);
}
return sbOut;
}
}