Competitive/Collaborative Programming Class

#include <iostream>
#include <cstring>
#include <string>

#define DEBUG if(false)
using namespace std;

char cipherText[100];
int plain[100];
		
int i, key, length, temp;
int moreToDo = 1;
		

void process()
{
	cin >> cipherText;
	length = strlen(cipherText);

	for(i=0; i<length; i++) 
	{
		if		(cipherText[i] == '_') temp = 0;
		else if	(cipherText[i] == '.') temp = 27;
		else	temp = cipherText[i] - 'a' + 1;

		plain[key*i % length] = (temp+i) % 28;
	}
	
	for(i=0; i<length; i++) 
	{
        if		(plain[i] == 0)	cout << "_";
		else if	(plain[i] == 27)cout << ".";
		else	 cout << (char)(plain[i] + 'a' - 1);
	}
	cout << endl;
}

int main()
{
	while(moreToDo) 
	{
		cin >> key;
            
		if(key != 0) process();
		else moreToDo = 0;
	}
	return 0;
}

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