text: //
  // Created by gj21798 on 2019/10/11.
  //
  
  #include<stdlib.h>
  #include <ctype.h>
  #include "base64.h"
  
  char *b64_encode(const unsigned char *src, size_t len) {
      int i = 0;
      int j = 0;
      char *enc = NULL;
      size_t size = 0;
      unsigned char buf[4];
      unsigned char tmp[3];
  
      // alloc
      enc = (char *) malloc(0);
      if (NULL == enc) { return NULL; }
  
      // parse until end of source
      while (len--) {
          // read up to 3 bytes at a time into `tmp'
          tmp[i++] = *(src++);
  
          // if 3 bytes read then encode into `buf'
          if (3 == i) {
              buf[0] = (tmp[0] & 0xfc) >> 2;
              buf[1] = ((tmp[0] & 0x03) << 4) + ((tmp[1] & 0xf0) >> 4);
              buf[2] = ((tmp[1] & 0x0f) << 2) + ((tmp[2] & 0xc0) >> 6);
              buf[3] = tmp[2] & 0x3f;
  
              // allocate 4 new byts for `enc` and
              // then translate each encoded buffer
              // part by index from the base 64 index table
              // into `enc' unsigned char array
              enc = (char *) realloc(enc, size + 4);
              for (i = 0; i < 4; ++i) {
                  enc[size++] = b64_table[buf[i]];
              }
  
              // reset index
              i = 0;
          }
      }
  
      // remainder
      if (i > 0) {
          // fill `tmp' with `\0' at most 3 times
          for (j = i; j < 3; ++j) {
              tmp[j] = '\0';
          }
  
          // perform same codec as above
          buf[0] = (tmp[0] & 0xfc) >> 2;
          buf[1] = ((tmp[0] & 0x03) << 4) + ((tmp[1] & 0xf0) >> 4);
          buf[2] = ((tmp[1] & 0x0f) << 2) + ((tmp[2] & 0xc0) >> 6);
          buf[3] = tmp[2] & 0x3f;
  
          // perform same write to `enc` with new allocation
          for (j = 0; (j < i + 1); ++j) {
              enc = (char *) realloc(enc, size + 1);
              enc[size++] = b64_table[buf[j]];
          }
  
          // while there is still a remainder
          // append `=' to `enc'
          while ((i++ < 3)) {
              enc = (char *) realloc(enc, size + 1);
              enc[size++] = '=';
          }
      }
  
      // Make sure we have enough space to add '\0' character at end.
      enc = (char *) realloc(enc, size + 1);
      enc[size] = '\0';
  
      return enc;
  }
  
  unsigned char *b64_decode(const char *src, size_t len) {
      return b64_decode_ex(src, len, NULL);
  }
  
  unsigned char *b64_decode_ex(const char *src, size_t len, size_t *decsize) {
      int i = 0;
      int j = 0;
      int l = 0;
      size_t size = 0;
      unsigned char *dec = NULL;
      unsigned char buf[3];
      unsigned char tmp[4];
  
      // alloc
      dec = (unsigned char *) malloc(0);
      if (NULL == dec) { return NULL; }
  
      // parse until end of source
      while (len--) {
          // break if char is `=' or not base64 char
          if ('=' == src[j]) { break; }
          if (!(isalnum(src[j]) || '+' == src[j] || '/' == src[j])) { break; }
  
          // read up to 4 bytes at a time into `tmp'
          tmp[i++] = src[j++];
  
          // if 4 bytes read then decode into `buf'
          if (4 == i) {
              // translate values in `tmp' from table
              for (i = 0; i < 4; ++i) {
                  // find translation char in `b64_table'
                  for (l = 0; l < 64; ++l) {
                      if (tmp[i] == b64_table[l]) {
                          tmp[i] = l;
                          break;
                      }
                  }
              }
  
              // decode
              buf[0] = (tmp[0] << 2) + ((tmp[1] & 0x30) >> 4);
              buf[1] = ((tmp[1] & 0xf) << 4) + ((tmp[2] & 0x3c) >> 2);
              buf[2] = ((tmp[2] & 0x3) << 6) + tmp[3];
  
              // write decoded buffer to `dec'
              dec = (unsigned char *) realloc(dec, size + 3);
              for (i = 0; i < 3; ++i) {
                  dec[size++] = buf[i];
              }
  
              // reset
              i = 0;
          }
      }
  
      // remainder
      if (i > 0) {
          // fill `tmp' with `\0' at most 4 times
          for (j = i; j < 4; ++j) {
              tmp[j] = '\0';
          }
  
          // translate remainder
          for (j = 0; j < 4; ++j) {
              // find translation char in `b64_table'
              for (l = 0; l < 64; ++l) {
                  if (tmp[j] == b64_table[l]) {
                      tmp[j] = l;
                      break;
                  }
              }
          }
  
          // decode remainder
          buf[0] = (tmp[0] << 2) + ((tmp[1] & 0x30) >> 4);
          buf[1] = ((tmp[1] & 0xf) << 4) + ((tmp[2] & 0x3c) >> 2);
          buf[2] = ((tmp[2] & 0x3) << 6) + tmp[3];
  
          // write remainer decoded buffer to `dec'
          dec = (unsigned char *) realloc(dec, size + (i - 1));
          for (j = 0; (j < i - 1); ++j) {
              dec[size++] = buf[j];
          }
      }
  
      // Make sure we have enough space to add '\0' character at end.
      dec = (unsigned char *) realloc(dec, size + 1);
      dec[size] = '\0';
  
      // Return back the size of decoded string if demanded.
      if (decsize != NULL) *decsize = size;
  
      return dec;
  }