/*****************************************************
* Section : 15
* Machine No. : N
* Roll No. : 19CS100XY
* Name : Aritra Hazra
* Assignment No : 6A
* Description : Encrypt and Decrypt Character Strings
*****************************************************/

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

#define MAX 100000
#define ASCII_BIT_WIDTH 8
#define CHAR_BIT_WIDTH 6
#define ENC_MAX MAX * ASCII_BIT_WIDTH / CHAR_BIT_WIDTH + 1

/* function to print charaters using given encodings */
void printEncryptString (int code)
{
    if ( (code >= 0) && (code < 26) )
        printf("%c", code+'A');
    else if ( (code > 25) && (code < 52) )
        printf("%c", code-26+'a');
    else if ( (code > 51) && (code < 62) )
        printf("%c", code-52+'0');
    else if (code == 62)
        printf("+");
    else if (code == 63)
        printf("-");
    else /* unreached part */
        printf("?");
}

/* function to compute specified encodings for a given character */
int getCodeChar (char c)
{
    if ( (c >= 'A') && (c <='Z') )
        return (c-'A');
    else if ( (c >= 'a') && (c <='z') )
        return (c-'a'+26);
    else if ( (c >= '0') && (c <='9') )
        return (c-'0'+52);
    else if (c=='+')
        return (62);
    else if (c=='-')
        return (63);
    else /* unreached part */
        return (-1);
}

int main ()
{
    char str[MAX], encStr[ENC_MAX];
    int i, j, k, strBin[ASCII_BIT_WIDTH * MAX], encStrBin[CHAR_BIT_WIDTH * MAX], asciiChar, lengthStr, fullPartLength, lastPartLength, codeChar;

    printf("\nEnter Original String: ");
    gets(str);
    lengthStr = strlen(str);
    /* compute Binary encodings from the ascii values of each charater of a string */
    for (i=0; i<lengthStr; i++) {
        asciiChar = str[i];
        j = ASCII_BIT_WIDTH * (i+1) - 1;
        while (asciiChar > 0) {
            strBin[j] = asciiChar % 2;
            asciiChar /= 2;
            j--;
        }
        while (j >= ASCII_BIT_WIDTH * i) {
            strBin[j] = 0;
            j--;
        }
    }

    /* encode in own format from Binary code using specified encoding scheme */
    fullPartLength = (lengthStr * ASCII_BIT_WIDTH) / CHAR_BIT_WIDTH;
    lastPartLength = (lengthStr * ASCII_BIT_WIDTH) % CHAR_BIT_WIDTH;
    printf("The Encrypted String: ");
    for (i=0; i<fullPartLength; i++) {
        k = 1; codeChar = 0;
        for (j = CHAR_BIT_WIDTH * (i+1) - 1; j>=CHAR_BIT_WIDTH * i; j--) {
            codeChar += strBin[j] * k;
            k *= 2;
        }
        printEncryptString(codeChar);
    }
    if (lastPartLength) {
        for (j = CHAR_BIT_WIDTH * fullPartLength + lastPartLength - 1, k=1, codeChar = 0; j>=CHAR_BIT_WIDTH * fullPartLength; j--, k*=2)
            codeChar += strBin[j] * k;
            printEncryptString(codeChar);
    }
    printf("\n\n");


    printf("Enter Encrypted String: ");
    gets(encStr);
    lengthStr = strlen(encStr);
    /* compute Binary encodings from own encode values of each charater of a string */
    for (i=0; i<lengthStr; i++) {
        codeChar = getCodeChar(encStr[i]);
        j = CHAR_BIT_WIDTH * (i+1) - 1;
        while (codeChar > 0) {
            encStrBin[j] = codeChar % 2;
            codeChar /= 2;
            j--;
        }
        while (j >= CHAR_BIT_WIDTH * i) {
            encStrBin[j] = 0;
            j--;
        }
    }
    /* decode to string format from Binary code using ascii values */
    fullPartLength = (lengthStr * CHAR_BIT_WIDTH) / ASCII_BIT_WIDTH;
    lastPartLength = (lengthStr * CHAR_BIT_WIDTH) % ASCII_BIT_WIDTH;
    printf("The Original String: ");
    for (i=0; i<fullPartLength; i++) {
        k = 1; codeChar = 0;
        for (j = ASCII_BIT_WIDTH * (i+1) - 1; j>=ASCII_BIT_WIDTH * i; j--) {
            codeChar += encStrBin[j] * k;
            k *= 2;
        }
        printf("%c", codeChar);
    }
    if (lastPartLength) {
        for (j = ASCII_BIT_WIDTH * fullPartLength + lastPartLength - 1, k=1, codeChar = 0; j>=ASCII_BIT_WIDTH * fullPartLength; j--, k*=2)
            codeChar += encStrBin[j] * k;
        printf("%c", codeChar);
    }
    printf("\n\n");

    return 0;
}