using System;
using System.Text.RegularExpressions;
class BinaryDivisibleBy7
{
// Check if a string contains only '0' and '1'
static bool IsBinary(string s) {
// Compiled once and reused
Regex binary = new Regex("^[01]+$");
return binary.IsMatch(s);
}
/*
In decimal: reading "123" means
remainder = remainder * 10 + digit
In binary: reading "1011" means
remainder = remainder * 2 + bit
*/
/*
0 (0*2 + 0) % 7 0
0 (0*2 + 0) % 7 0
0 (0*2 + 0) % 7 0
1 (0*2 + 1) % 7 1
1 (1*2 + 1) % 7 3
1 (3*2 + 1) % 7 0
0 (0*2 + 0) % 7 0
0 (0*2 + 0) % 7 0
*/
// Compute binary string modulo 7 without overflow
static bool DivisibleBy7(string s) {
int remainder = 0;
foreach (char c in s) {
remainder = (remainder * 2 + (c - '0')) % 7;
}
return remainder == 0;
}
static void Main()
{
string s = "00011100"; // 28
if (!IsBinary(s)) {
Console.WriteLine("Not a binary string");
return;
}
if (DivisibleBy7(s)) {
Console.WriteLine("Divisible by 7");
}
else {
Console.WriteLine("Not divisible by 7");
}
}
}
/*
run:
Divisible by 7
*/
