NOIP2012 Day1 - vigenere

vigenere.c

/* NOIP2012 Day1 - Vigenere */
#include <stdio.h>
#include <string.h>
#include <ctype.h>
#define Rni(ci, ki) ( (ci)-(ki)+((ci)<(ki)?(26):0) )

char M[100];
char K[100];
char C[100];

int main(void)
{
   int i = 0;
   int mn = 0, kn = 0;

   scanf("%s\n%s", K, C);
   kn = strlen(K);
   mn = strlen(C);

   for(i = 0;i < mn;i++)
   {
      int islow = islower(C[i]);
      K[i%kn] = toupper(K[i%kn]);
      C[i] = toupper(C[i]);
      M[i] = Rni(C[i]-'A', K[i%kn]-'A')+'A';
      if(islow) M[i] = tolower(M[i]);
   }
   M[i] = '\0';

   printf("%s", M);   
   return 0;
}