poj1317

简单模拟

#include <iostream>
#include <cstdio>
#include <cstdlib>
#include <cstring>
using namespace std;

#define maxl 75

int key;
char cipher_text[maxl];
char plain_text[maxl];

bool is_letter(char ch)
{
    return ch <= 'z' && ch >= 'a';
}

int get_code(char ch)
{
    if (is_letter(ch))
        return ch - 'a' + 1;
    if (ch == '_')
        return 0;
    if (ch == '.')
        return 27;
    return -1;
}

char get_text(int a)
{
    if (a <= 26 && a >= 1)
        return a + 'a' - 1;
    if (a == 0)
        return '_';
    if (a == 27)
        return '.';
    return -1;
}

int unify(int a, int w)
{
    return (a % w + w) % w;
}

int main()
{
    //freopen("t.txt", "r", stdin);
    while (scanf("%d", &key), key)
    {
        scanf("%s", cipher_text);
        int len = strlen(cipher_text);
        for (int i = 0; i < len; i++)
            plain_text[unify(key * i, len)] = get_text(unify(get_code(cipher_text[i]) + i, 28));
        plain_text[len] = 0;
        puts(plain_text);
    }
    return 0;
}