int l[] = { 0, 2, 4, 6, 8, 1, 3, 5, 7, 9 }; int c_luhn2( char *s ) { int total = 0; total += l[ s[ 0] - '0' ]; total += s[ 1] - '0'; total += l[ s[ 2] - '0' ]; total += s[ 3] - '0'; total += l[ s[ 4] - '0' ]; total += s[ 5] - '0'; total += l[ s[ 6] - '0' ]; total += s[ 7] - '0'; total += l[ s[ 8] - '0' ]; total += s[ 9] - '0'; total += l[ s[10] - '0' ]; total += s[11] - '0'; total += l[ s[12] - '0' ]; total += s[13] - '0'; total += l[ s[14] - '0' ]; total *= 9; return total % 10; }