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

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

/**
 * Encodes three input chars into four output chars based on the base64 scheme.
 * @param in chararray with input characters
 * @param out chararray where the output is written to
 * @param len number of chars to be used from input
 */
void encode_block( unsigned char in[3], unsigned char out[4], int len )
{
    out[0] = cb64[ in[0] >> 2 ];
    out[1] = cb64[ ((in[0] & 0x03) << 4) | (len > 1 ? (( in[1] & 0xf0) >> 4) : 0)];
    out[2] = (unsigned char) (len > 1 ? cb64[((in[1] & 0x0f) << 2) | (len > 2 ? ((in[2] & 0xc0) >> 6) : 0)] : '=');
    out[3] = (unsigned char) (len > 2 ? cb64[in[2] & 0x3f] : '=');
}

/**
 * Encodes the input string into an output string based on the base64 scheme.
 * @param length size of input string
 * @param input string with input characters
 * @param output string where the output is written to. It has to be long enough to hold the output
 */
void encode(int length, char *input, char *output)
{
    unsigned char in[3], out[4];

    int iterator = 0;
    int blocksize = 3;
    for(iterator = 0; iterator <= (length / 3); ++iterator )
    {
        if (iterator == (length / 3))
        {
            blocksize = length % 3;
        }
        int i;
        for (i=0; i<blocksize; ++i)
        {
            in[i] = input[iterator*3 + i];
        }
        if (0 < blocksize)
        {
            encode_block( in, out, blocksize );
            for (i=0; i<4; ++i)
            {
                output[iterator*4 + i] = out[i];
            }
        }
    }
}

int main(int argc, char **argv)
{
    if (2 != argc){
        printf("please give exactly one parameter\n");
        return 0;
    }
    unsigned char *input_string = argv[1];
    int length = strlen(argv[1]);    
    
    int output_length = (((length / 3) *4)); /* 3 bytes input will become 4 bytes output */
    
    if (length % 3)
    {
        output_length += 4;
    }

    char base64string[output_length+1];
    base64string[output_length] = '\0';
    
    encode(length, input_string, base64string);

    printf("%s\n", base64string);

}