base64.h

// https://en.wikibooks.org/wiki/Algorithm_Implementation/Miscellaneous/Base64

#include <inttypes.h>
#include <string.h>
 
// TODO: split on header and implementation
static int base64encode(const void* data_buf, size_t dataLength, char* result, size_t resultSize)
{
   const char base64chars[] = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/";
   const uint8_t *data = (const uint8_t *)data_buf;
   size_t resultIndex = 0;
   size_t x;
   uint32_t n = 0;
   int padCount = dataLength % 3;
   uint8_t n0, n1, n2, n3;
 
   /* increment over the length of the string, three characters at a time */
   for (x = 0; x < dataLength; x += 3) 
   {
      /* these three 8-bit (ASCII) characters become one 24-bit number */
      n = ((uint32_t)data[x]) << 16; //parenthesis needed, compiler depending on flags can do the shifting before conversion to uint32_t, resulting to 0
 
      if((x+1) < dataLength)
         n += ((uint32_t)data[x+1]) << 8;//parenthesis needed, compiler depending on flags can do the shifting before conversion to uint32_t, resulting to 0
 
      if((x+2) < dataLength)
         n += data[x+2];
 
      /* this 24-bit number gets separated into four 6-bit numbers */
      n0 = (uint8_t)(n >> 18) & 63;
      n1 = (uint8_t)(n >> 12) & 63;
      n2 = (uint8_t)(n >> 6) & 63;
      n3 = (uint8_t)n & 63;
 
      /*
       * if we have one byte available, then its encoding is spread
       * out over two characters
       */
      if(resultIndex >= resultSize) return 1;   /* indicate failure: buffer too small */
      result[resultIndex++] = base64chars[n0];
      if(resultIndex >= resultSize) return 1;   /* indicate failure: buffer too small */
      result[resultIndex++] = base64chars[n1];
 
      /*
       * if we have only two bytes available, then their encoding is
       * spread out over three chars
       */
      if((x+1) < dataLength)
      {
         if(resultIndex >= resultSize) return 1;   /* indicate failure: buffer too small */
         result[resultIndex++] = base64chars[n2];
      }
 
      /*
       * if we have all three bytes available, then their encoding is spread
       * out over four characters
       */
      if((x+2) < dataLength)
      {
         if(resultIndex >= resultSize) return 1;   /* indicate failure: buffer too small */
         result[resultIndex++] = base64chars[n3];
      }
   }  
 
   /*
    * create and add padding that is required if we did not have a multiple of 3
    * number of characters available
    */
   if (padCount > 0) 
   { 
      for (; padCount < 3; padCount++) 
      { 
         if(resultIndex >= resultSize) return 1;   /* indicate failure: buffer too small */
         result[resultIndex++] = '=';
      } 
   }
   if(resultIndex >= resultSize) return 1;   /* indicate failure: buffer too small */
   result[resultIndex] = 0;
   return 0;   /* indicate success */
}