#include "UrlEncode2.h"

const char* const test PROGMEM = "Hello, this string should be \% encoded.";

void setup() 
{
  Serial.begin(9600);
  Serial.println(urlEncode(test, 256));
  //Hello%2C%20this%20string%20should%20be%20%25%20encoded. 
}

void loop(){}

Encode.ino

diagram.json

#include "UrlEncode2.h"

bool includeChar(char c)
{
  bool b1 = isAlphaNumeric(c);
  bool b2 = c == '-' || c == '_' || c == '.' || c == '~';
  return b1 || b2;
}

char* urlEncode(char *msg, int resultLength)
{
  int index = 0;
  int outputIndex = 0;
  char encodedMsg [resultLength];
  const char *hex = "0123456789ABCDEF";

  while (msg[index] != '\0')
  {
    if (includeChar(msg[index]))
    {
      encodedMsg[outputIndex] = msg[index];
      index += 1;
      outputIndex += 1;
    }
    else
    {
      encodedMsg[outputIndex] = '%';
      encodedMsg[outputIndex + 1] = hex[(unsigned char)msg[index] >> 4];
      encodedMsg[outputIndex + 2] = hex[msg[index] & 0xf];
      index += 1;
      outputIndex += 3;
    }
  }

  char *result = encodedMsg;
  return result;
}


UrlEncode2.cpp

#ifndef URLENCODE2_H
#define URLENCODE2_H

#include <Arduino.h>

/**
 * Determines when encoding is required for a character.
 * @param c, a character within the string.
 * @returns True if the character should be used without encoding.
 */
bool includeChar(char c);

/**
 * Percent-encodes a string.
 * @param msg, a UTF-8 string to encode.
 * @returns Percent-encoded string.
 */
char* urlEncode(char *msg, int resultLength);

#endif
