#include <stdio.h>
#include <stdlib.h>
#include <ctype.h>
#define max 2048
int key[1024];

char * encode (int * string, const int count,const int n)
{
    int i,j;
    char ch, * encode_string = (char *)malloc(count);
    printf("The ciphertexts are :\n");
    for (i = 0,j = 0;i < count;i++)
    {
        if ((string[i] + 'a') == ' ')
        {
            putchar(' ');
            encode_string[i] = ' ';
        }
        else
        {
            if (j >= n)
                j %= n;
            string[i] = (string[i] + key[j++]) % 26;
            ch = string[i] + 'a';
            encode_string[i] = ch;
            putchar(ch);
        }
    }
    putchar('\n');
    return encode_string;
}
char * decode(int * string, const int count,const int n)
{
    int i,j;
    char ch,*decode_string = (char *)malloc(count);

    printf("\nThe plaintexts are :\n");
    for (i = 0,j = 0;i < count;i++)
    {
        if ((string[i] + 'a') == ' ')
        {
            decode_string[i] = ' ';
            putchar(' ');
        }
        else
        {
            if (j >= n)
                j %= n;
            string[i] = (string[i] - key[j++] + 26) % 26;
            ch = string[i] + 'a';
            decode_string[i] = ch;
            putchar(ch);
        }
    }
    putchar('\n');
    return decode_string;
}
void count_frequency(const int count,const char * string)
{
    int i, count_char = count;
    float fre[26],sum = 0, temp;
    for (i = 0;i < 26;i++)
        fre[i] = 0;
    for (i = 0;i < count;i++)
    {
        if (string[i] == ' ')
            count_char--;
        else
            fre[string[i] - 'a']++;
    }
    for (i = 0;i < 26;i++)
        if (fre[i] != 0)
        {
            printf("%c = %.3f\n",i + 'a',temp = fre[i] / count_char);
            sum += temp*temp;
        }
    printf("The index of coinindence is %.3f.\n",sum);
}
int main()
{
    int n;
    char ch, * encode_string, *decode_string,quit;
    int string_int[max],i,count;
    FILE * fp=fopen("in.txt","r");
    if (fp == NULL)
    {
        fprintf(stdout,"Can't open the file!");
        exit(1);
    }
    do
    {
        printf("Plesse input the number n:");
        scanf("%d",&n);
        for (i = 0;i < n;i++)
            key[i] = rand() % 26;
        i = 0;
        rewind(fp);
        while ((ch = fgetc(fp)) != EOF)//會有空格
        {
            if (ch == ' ' ||ch == '\n'||ch == '\r')
                ch = ' ';
            ch = tolower(ch);
            string_int[i++] = ch - 'a';
        }
        count = i;
        encode_string = (char *)malloc(count);
        decode_string = (char *)malloc(count);
        encode_string = encode(string_int,count,n);
        count_frequency(count,encode_string);
        decode_string = decode(string_int,count,n);
        count_frequency(count,decode_string);
        free(encode_string);
        free(decode_string);

        printf("Do you want to quit?(Enter the \'q\' to quit.\'n\' to continue.)\n");
        for (;(quit = getchar()) == '\r' || quit == '\n' || quit == ' ';);
    }
    while ( quit != 'q');

    fclose(fp);
    return 0;
}