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


static char encoding_table[] = {'A', 'B', 'C', 'D', 'E', 'F', 'G', '!',
                                'I', 'J', 'K', 'L', 'M', 'N', '@', 'P',
                                'Q', 'R', 'S', 'T', 'U', '#', 'W', 'X',
                                'Y', 'Z', 'a', 'b', '$', 'd', 'e', 'f',
                                'g', 'h', 'i', '%', 'k', 'l', 'm', 'n',
                                'o', 'p', '^', 'r', 's', 't', 'u', 'v',
                                'w', '&', 'y', 'z', '0', '1', '2', '3',
                                '*', '5', '6', '7', '8', '9', '+', '/'};
static char *decoding_table = NULL;
static int mod_table[] = {0, 2, 1};
void build_decoding_table();


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

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

    char *encoded_data = malloc(*output_length);
    if (encoded_data == NULL) return NULL;

    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,
                             size_t *output_length) {

    if (decoding_table == NULL) build_decoding_table();

    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)--;

    unsigned char *decoded_data = malloc(*output_length);
    if (decoded_data == NULL) return NULL;

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

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


void build_decoding_table() {

    decoding_table = malloc(256);

    for (int i = 0; i < 64; i++)
        decoding_table[(unsigned char) encoding_table[i]] = i;
}


void base64_cleanup() {
    free(decoding_table);
}

int main(int argc, char* argv[]) {
  // get file
  size_t oc;
  char *res;
  FILE * ifp = fopen(argv[1], "r");
  fseek(ifp, 0, SEEK_END);
  size_t bc = ftell(ifp);
  fseek(ifp, 0, SEEK_SET);
  unsigned char *dat = malloc(bc+1);
  fread(dat, bc, 1, ifp);
  fclose(ifp);

  // encode
  res = base64_encode(dat, bc, &oc);

  // write to file
  FILE * ofp = fopen(argv[2], "w");
  fputs(res, ofp);
  fclose(ofp);

  return 0;
}