base64.c

#include <stdio.h>
#include <stdbool.h>
#include <stdint.h>

static const size_t num_of_chunk_bits = 6;
static const char base64_conversion_table[64] = {
  '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', '+', '/'
};

bool base64_quadruplicate(char buf[4], const char input_data[3], const size_t length_of_data) {
  if(length_of_data == 0 || length_of_data > 4) {
    return false;
  }

  char data[3] = {0, 0, 0};
  for(size_t i = 0; i < length_of_data; ++i) {
    data[i] = input_data[i];
  }

  buf[0] = base64_conversion_table[(0xfc & data[0]) >> 2];
  buf[1] = base64_conversion_table[((0x03 & data[0]) << 4) + ((0xf0 & data[1]) >> 4)];
  buf[2] = base64_conversion_table[((0x0f & data[1]) << 2) + ((0xc0 & data[2]) >> 6)];
  buf[3] = base64_conversion_table[0x3f & data[2]];

  for(int i = 3; length_of_data < i; --i) {
    buf[i] = '=';
  }
  return true;
}

bool base64(char *buf, const char *data, const size_t length) {
  int buf_count = 0;
  for(size_t data_count = 0; data_count < length; data_count += 3) {
    if((data_count != 0) && ((((data_count/3) * 4) % 76) == 0)) {
      buf[buf_count] = '\n';
      buf_count++;
    }
    int rest = length - data_count;
    int n = rest < 3 ? rest : 3;
    base64_quadruplicate(&buf[buf_count], &data[data_count], n);
    buf_count +=4;
  }
  buf[buf_count] = '\0';
  return true;
}

int main() {
  // char xs[] = "ABCDEFG";
  char xs[] = "111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111";
  char buf[BUFSIZ];

  base64(buf, xs, sizeof(xs) - 1);
  printf("%s\n", buf);

  return 0;
}