#include <stdint.h>
#include <string.h>

static const char encoding_table[] = {'A', 'B', 'C', 'D', 'E', 'F', 'G', 'H',
                                      'I', 'J', 'K', 'L', 'M', 'N', 'O', 'P',
                                      'Q', 'R', 'S', 'T', 'U', 'V', 'W', 'X',
                                      'Y', 'Z', 'a', 'b', 'c', 'd', 'e', 'f',
                                      'g', 'h', 'i', 'j', 'k', 'l', 'm', 'n',
                                      'o', 'p', 'q', 'r', 's', 't', 'u', 'v',
                                      'w', 'x', 'y', 'z', '0', '1', '2', '3',
                                      '4', '5', '6', '7', '8', '9', '+', '/'};

static const char decoding_table[256] = {['A'] = 0, // using GCC initialiser style
                                         ['B'] = 1,
                                         ['C'] = 2,
                                         ['D'] = 3,
                                         ['E'] = 4,
                                         ['F'] = 5,
                                         ['G'] = 6,
                                         ['H'] = 7,
                                         ['I'] = 8,
                                         ['J'] = 9,
                                         ['K'] = 10,
                                         ['L'] = 11,
                                         ['M'] = 12,
                                         ['N'] = 13,
                                         ['O'] = 14,
                                         ['P'] = 15,
                                         ['Q'] = 16,
                                         ['R'] = 17,
                                         ['S'] = 18,
                                         ['T'] = 19,
                                         ['U'] = 20,
                                         ['V'] = 21,
                                         ['W'] = 22,
                                         ['X'] = 23,
                                         ['Y'] = 24,
                                         ['Z'] = 25,
                                         ['a'] = 26,
                                         ['b'] = 27,
                                         ['c'] = 28,
                                         ['d'] = 29,
                                         ['e'] = 30,
                                         ['f'] = 31,
                                         ['g'] = 32,
                                         ['h'] = 33,
                                         ['i'] = 34,
                                         ['j'] = 35,
                                         ['k'] = 36,
                                         ['l'] = 37,
                                         ['m'] = 38,
                                         ['n'] = 39,
                                         ['o'] = 40,
                                         ['p'] = 41,
                                         ['q'] = 42,
                                         ['r'] = 43,
                                         ['s'] = 44,
                                         ['t'] = 45,
                                         ['u'] = 46,
                                         ['v'] = 47,
                                         ['w'] = 48,
                                         ['x'] = 49,
                                         ['y'] = 50,
                                         ['z'] = 51,
                                         ['0'] = 52,
                                         ['1'] = 53,
                                         ['2'] = 54,
                                         ['3'] = 55,
                                         ['4'] = 56,
                                         ['5'] = 57,
                                         ['6'] = 58,
                                         ['7'] = 59,
                                         ['8'] = 60,
                                         ['9'] = 61,
                                         ['+'] = 62,
                                         ['/'] = 63};

static const int mod_table[] = {0, 2, 1};

unsigned char *base64_encode(const unsigned char *data,
                             size_t input_length,
                             unsigned char *encoded_data,
                             size_t *output_length)
{

    *output_length = 4 * ((input_length + 2) / 3);

    for (int i = 0, j = 0; i < input_length;)
    {

        uint32_t octet_a = i < input_length ? (unsigned char)data[i++] : 0;
        uint32_t octet_b = i < input_length ? (unsigned char)data[i++] : 0;
        uint32_t octet_c = i < input_length ? (unsigned char)data[i++] : 0;

        uint32_t triple = (octet_a << 0x10) + (octet_b << 0x08) + octet_c;

        encoded_data[j++] = encoding_table[(triple >> 3 * 6) & 0x3F];
        encoded_data[j++] = encoding_table[(triple >> 2 * 6) & 0x3F];
        encoded_data[j++] = encoding_table[(triple >> 1 * 6) & 0x3F];
        encoded_data[j++] = encoding_table[(triple >> 0 * 6) & 0x3F];
    }

    for (int i = 0; i < mod_table[input_length % 3]; i++)
        encoded_data[*output_length - 1 - i] = '=';

    return encoded_data;
}

unsigned char *base64_decode(const char *data,
                             size_t input_length,
                             unsigned char *decoded_data,
                             size_t *output_length)
{
     if (input_length % 4 != 0)
        return NULL;

    *output_length = input_length / 4 * 3;
    if (data[input_length - 1] == '=')
        (*output_length)--;
    if (data[input_length - 2] == '=')
        (*output_length)--;

    for (int i = 0, j = 0; i < input_length;)
    {

        uint32_t sextet_a = data[i] == '=' ? 0 & i++ : decoding_table[(int)data[i++]];
        uint32_t sextet_b = data[i] == '=' ? 0 & i++ : decoding_table[(int)data[i++]];
        uint32_t sextet_c = data[i] == '=' ? 0 & i++ : decoding_table[(int)data[i++]];
        uint32_t sextet_d = data[i] == '=' ? 0 & i++ : decoding_table[(int)data[i++]];

        uint32_t triple = (sextet_a << 3 * 6) + (sextet_b << 2 * 6) + (sextet_c << 1 * 6) + (sextet_d << 0 * 6);

        if (j < *output_length)
            decoded_data[j++] = (triple >> 2 * 8) & 0xFF;
        if (j < *output_length)
            decoded_data[j++] = (triple >> 1 * 8) & 0xFF;
        if (j < *output_length)
            decoded_data[j++] = (triple >> 0 * 8) & 0xFF;
    }

    return decoded_data;
}