实现 CHECKSUM 的 C/C++ 程序

校验和是一种错误检测方法，可在数据/消息从发送方传输到接收方时检测数据/消息中的错误。此方法由更高层协议使用，并利用发送方的校验和生成器和接收方的校验和检查器。

例子：

Input: sent_message = “10101111”, rec_message = “10101101”, block_size = 8
Output: Error
Explaination: Since the 7th bit in the sent_message and the rec_message is different, the final checksum value is not equal to zero denoting that some error has occurred during transmission.

Input: sent_message = “10000101011000111001010011101101”, rec_message = “10000101011000111001010011101101”, block_size = 8
Output: No Error

方法：给定的问题可以分为以下两个部分：

• 可以使用以下步骤生成发送方消息的校验和值：
  • 将消息分成给定块大小的二进制字符串。
  • 所有二进制字符串相加得到总和。
  • 表示和的二进制字符串的 One's Complement 是所需的校验和值。
• 检查接收到的消息的值（即rec_message + senders_checksum ）是否等于0 。
  • 接收消息的校验和可以与上述过程中计算的校验和类似地计算。
  • 如果校验和值为0 ，则消息正确传输，没有错误，否则，在传输过程中发生了错误。

下面是上述方法的实现：

C++

// C++ implementation of the above approach
#include 
using namespace std;
 
// Function to find the One's complement
// of the given binary string
string Ones_complement(string data)
{
    for (int i = 0; i < data.length(); i++) {
        if (data[i] == '0')
            data[i] = '1';
        else
            data[i] = '0';
    }
 
    return data;
}
 
// Function to return the checksum value of
// the give string when divided in K size blocks
string checkSum(string data, int block_size)
{
    // Check data size is divisible by block_size
    // Otherwise add '0' front of the data
    int n = data.length();
    if (n % block_size != 0) {
        int pad_size = block_size - (n % block_size);
        for (int i = 0; i < pad_size; i++) {
            data = '0' + data;
        }
    }
 
    // Binary addition of all blocks with carry
    string result = "";
 
    // First block of data stored in result variable
    for (int i = 0; i < block_size; i++) {
        result += data[i];
    }
 
    // Loop to calculate the block
    // wise addition of data
    for (int i = block_size; i < n; i += block_size) {
 
        // Stores the data of the next bloack
        string next_block = "";
 
        for (int j = i; j < i + block_size; j++) {
            next_block += data[j];
        }
 
        // Stores the binary addition of two blocks
        string additions = "";
        int sum = 0, carry = 0;
 
        // Loop to calculate the binary addition of
        // the current two blocls of k size
        for (int k = block_size - 1; k >= 0; k--) {
            sum += (next_block[k] - '0')
                   + (result[k] - '0');
            carry = sum / 2;
            if (sum == 0) {
                additions = '0' + additions;
                sum = carry;
            }
            else if (sum == 1) {
                additions = '1' + additions;
                sum = carry;
            }
            else if (sum == 2) {
                additions = '0' + additions;
                sum = carry;
            }
            else {
                additions = '1' + additions;
                sum = carry;
            }
        }
 
        // After binary add of two blocks with carry,
        // if carry is 1 then apply binary addition
        string final = "";
 
        if (carry == 1) {
            for (int l = additions.length() - 1; l >= 0;
                 l--) {
                if (carry == 0) {
                    final = additions[l] + final;
                }
                else if (((additions[l] - '0') + carry) % 2
                         == 0) {
                    final = "0" + final;
                    carry = 1;
                }
                else {
                    final = "1" + final;
                    carry = 0;
                }
            }
 
            result = final;
        }
        else {
            result = additions;
        }
    }
 
    // Return One's complements of result value
    // which represents the required checksum value
    return Ones_complement(result);
}
 
// Function to check if the received message
// is same as the senders message
bool checker(string sent_message,
             string rec_message,
             int block_size)
{
 
    // Checksum Value of the senders message
    string sender_checksum
        = checkSum(sent_message, block_size);
 
    // Checksum value for the receivers message
    string receiver_checksum = checkSum(
        rec_message + sender_checksum, block_size);
 
    // If receivers checksum value is 0
    if (count(receiver_checksum.begin(),
              receiver_checksum.end(), '0')
        == block_size) {
        return true;
    }
    else {
        return false;
    }
}
 
// Driver Code
int main()
{
    string sent_message
        = "10000101011000111001010011101101";
    string recv_message
        = "10000101011000111001010011101101";
    int block_size = 8;
 
    if (checker(sent_message,
                recv_message,
                block_size)) {
        cout << "No Error";
    }
    else {
        cout << "Error";
    }
 
    return 0;
}

输出

No Error

时间复杂度： O(N)
辅助空间： O(block_size)