#include <stdio.h>
#include <string.h>

#define MAX_LINE_LEN 256
#define MIN_PRINTABLE_ASCII 32
#define MAX_PRINTABLE_ASCII 126
#define NUM_PRINTABLE (MAX_PRINTABLE_ASCII - MIN_PRINTABLE_ASCII + 1)

typedef enum { false = 0, true } bool;
typedef unsigned char uchar;

void encode(uchar *,int);
void decode(uchar *,int);

int main(int argc, char *argv[]) {
    uchar line[MAX_LINE_LEN];
    int n;
    bool inputOk;

    for (inputOk = false; inputOk == false; ) {
        printf("Enter offset: ");
        fgets((char *)line,MAX_LINE_LEN,stdin);
        inputOk = (sscanf((char *)line,"%d",&n) == 1);
    }
    n %= NUM_PRINTABLE;
    printf("Enter line of text: ");
    fgets((char *)line,MAX_LINE_LEN,stdin);
    *strchr((char *)line,'\n') = '\0';
    encode(line,n);
    printf("\nEncoded: \"%s\"\n",line);
    decode(line,n);
    printf("Decoded: \"%s\"\n",line);

    return 0;
}

void encode(uchar *s,int x) {
    uchar *p = s;
    int z;

    while (*p != '\0') {
        z = (int)*p + x;
        if (z > MAX_PRINTABLE_ASCII) {
            z -= NUM_PRINTABLE;
        } else if (z < MIN_PRINTABLE_ASCII) {
            z += NUM_PRINTABLE;
        }
        *p++ = (uchar)z;
    }
}

void decode(uchar *s, int x) {
    encode(s,x *= -1);
}