C++: Luhn Test of Credit Card Numbers

The Luhn test is used by some credit card companies to distinguish valid credit card numbers from what could be a random selection of digits.

Those companies using credit card numbers that can be validated by the Luhn test have numbers that pass the following test:

Reverse the order of the digits in the number.
Take the first, third, … and every other odd digit in the reversed digits and sum them to form the partial sum s1
Taking the second, fourth … and every other even digit in the reversed digits:

Multiply each digit by two and sum the digits if the answer is greater than nine to form partial sums for the even digits
Sum the partial sums of the even digits to form s2

If s1 + s2 ends in zero then the original number is in the form of a valid credit card number as verified by the Luhn test.

For example, if the trial number is 49927398716:

Reverse the digits:
  61789372994
Sum the odd digits:
  6 + 7 + 9 + 7 + 9 + 4 = 42 = s1
The even digits:
    1,  8,  3,  2,  9
  Two times each even digit:
    2, 16,  6,  4, 18
  Sum the digits of each multiplication:
    2,  7,  6,  4,  9
  Sum the last:
    2 + 7 + 6 + 4 + 9 = 28 = s2

s1 + s2 = 70 which ends in zero which means that 49927398716 passes the Luhn test

The task is to write a function/method/procedure/subroutine that will validate a number with the Luhn test, and use it to validate the following numbers:

49927398716

49927398717

1234567812345678

1234567812345670

#include <iostream>
using namespace std;
 
int toInt(const char c)
{
    return c-'0';
}
 
int confirm( const char *id)
{
    bool is_odd_dgt = true;
    int s = 0;
    const char *cp;
 
    for(cp=id; *cp; cp++);
    while(cp > id) {
        --cp;
        int k = toInt(*cp);
        if (is_odd_dgt) {
            s += k;
        }
        else {
            s += (k!=9)? (2*k)%9 : 9;
        }
	is_odd_dgt = !is_odd_dgt;
    }
    return 0 == s%10;
}
 
int main( )
{
    const char * t_cases[] = {
        "49927398716",
        "49927398717",
        "1234567812345678",
        "1234567812345670",
        NULL,
    };
    for ( const char **cp = t_cases; *cp; cp++) {
        cout << *cp << ": " << confirm(*cp) << endl;
    }
    return 0;
}

C++11

#include <iostream>
#include <vector>
#include <algorithm>
using namespace std;
 
bool luhn( const string& id)
{
  static const int m[10]  = {0,2,4,6,8,1,3,5,7,9}; // mapping for rule 3
  bool is_odd_dgt = false;
  auto lambda = [&](int a, char c) {return a + ((is_odd_dgt = !is_odd_dgt) ? c-'0' : m[c-'0']);};
  int s = std::accumulate(id.rbegin(), id.rend(), 0, lambda);
  return 0 == s%10;
}
 
int main( )
{
  auto t_cases = {"49927398716", "49927398717", "1234567812345678", "1234567812345670"};
  auto print = [](const string & s) {cout << s << ": " << luhn(s) << endl;};
  for_each(t_cases.begin(), t_cases.end(), print);
  return 0;
}