text: #include "base64url.h"
  
  
  static 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 char *decoding_table = NULL;
  static int mod_table[] = {0, 2, 1};
  
  
  char *base64url_encode(const unsigned char *data,
                      int input_length,
                      int *output_length,
                      int tail_padding)
  {
      char *encoded_data;
      int padding, i, j;
  
      *output_length = 4 * ((input_length + 2) / 3);
  
      encoded_data = malloc(*output_length + 1);
  
      if (encoded_data == NULL) return NULL;
  
      for (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];
      }
  
      padding = mod_table[input_length % 3];
  
      if (tail_padding) {
          for (i = 0; i < padding; i++) {
              encoded_data[*output_length - i - 1] = '=';
          }
      }
      else {
          *output_length -= padding;
      }
  
  
      encoded_data[*output_length] = '\0';
  
      return encoded_data;
  }
  
  