#include <stdio.h>

char cb64[] = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/"; /* lookup table */

unsigned char encoding_string[] = "Das ist ein Test!?!"; /* what do you want to encode today?*/
int length = 19; /* number of characters entered above*/

unsigned char encode_first_output(unsigned char first){ 
	return first >> 2;
}

unsigned char encode_second_output(unsigned char first, unsigned char second)
{
	return ((first & 0x03) << 4) | ((second)>> 4);
}

unsigned char encode_third_output(unsigned char second, unsigned char third)
{
	return ((second & 0x0f) << 2) | ((third) >> 6);
}

unsigned char encode_fourth_output(unsigned char third)
{
	return (third & 0x3f);
}


int main(void)
{
	int walker = 0;
	int no_padding_length = (length / 3) * 3; /* the biggest by 3 dividable integer smaller than length*/
	int num_padding = length % 3;
	int output_length = ((((no_padding_length)/3 )*4) +1); /* 3 bytes input will become 4 bytes output, 1 char as string terminator*/
	char last_char, penultimate_char;
	
	if (num_padding) /* our output will be 4 bytes larger, if there was a remainder in num_padding */
	{
		output_length += 4;
	}

	char base64string[output_length]; /* the output will be stored here*/
	base64string[output_length-1] = '\0'; /* to be able to use printf we need a \0 at the end of our output*/
	
	for(walker = 0; walker < no_padding_length / 3; walker = walker+1 )
	{	
		unsigned char first_input = encoding_string[walker*3];
		unsigned char second_input = encoding_string[walker*3 + 1];
		unsigned char third_input = encoding_string[walker*3 + 2];

		/* calculate 4 bytes of output and lookup char */
		base64string[walker*4] = cb64[encode_first_output(first_input)]; 
		base64string[(walker*4) + 1] = cb64[encode_second_output(first_input, second_input)];
		base64string[(walker*4) + 2] = cb64[encode_third_output(second_input, third_input)];
		base64string[(walker*4) + 3] = cb64[encode_fourth_output(third_input)];
	}
	
	switch(num_padding){ /* in case the string was not divideable by 3, we need to do padding */
		case 0: break;
		case 1: last_char = encoding_string[length-1]; 
			base64string[output_length-5] = cb64[encode_first_output(last_char)];
			base64string[output_length-4] = cb64[encode_second_output(last_char, 0)];
			base64string[output_length-3] = '=';
			base64string[output_length-2] = '=';
			break;
		case 2: penultimate_char = encoding_string[length-2];
			last_char = encoding_string[length-1];
			base64string[output_length-5] = cb64[encode_first_output(penultimate_char)];
			base64string[output_length-4] = cb64[encode_second_output(penultimate_char, last_char)];
			base64string[output_length-3] = cb64[encode_third_output(last_char, 0)];
			base64string[output_length-2] = '=';
			break;
	}

	printf(base64string); /* print the result*/
	printf("\n"); /* a new line makes the output nicer*/

}