Converting C++ into C for memory efficiency (with a side effect of increased performance too!)

urlDecode.cpp

//OLD C++ STYLE

String ESP8266WebServer::urlDecode(const String& text)
{
	String decoded = "";
	char temp[] = "0x00";
	unsigned int len = text.length();
	unsigned int i = 0;
	while (i < len)
	{
		char decodedChar;
		char encodedChar = text.charAt(i++);
		if ((encodedChar == '%') && (i + 1 < len))
		{
			temp[2] = text.charAt(i++);
			temp[3] = text.charAt(i++);

			decodedChar = strtol(temp, NULL, 16);
		}
		else {
			if (encodedChar == '+')
			{
				decodedChar = ' ';
			}
			else {
				decodedChar = encodedChar;  // normal ascii char
			}
		}
		decoded += decodedChar;
	}
	return decoded;
}




//NEW C STYLE

char *ESP8266WebServer::urlDecode(char *text, int len) {
	if (!text) return text;

	char *src	= text;
	char *dst	= text;
	char temp[]	= "00";

	while (*src) {
		if (len-- < 1) break;

		if (*src == '%') {
			if (!src[1]  ||  !src[2]) break;

			temp[0] = src[1];
			temp[1] = src[2];

			len -= 2;
			src += 2;
			*dst = (char) strtol(temp, nullptr, 16);

		} else if (*src == '+') {
			*dst = ' ';

		} else {
			*dst = *src;
		}

		src++;
		dst++;
	}

	*dst = NULL;

	return text;
}