tNMEA0183Msg::AddDoubleField requires Arduino dtostrf function

[clickworkorange]

I've successfully compiled your lovely NMEA0183 library with the arm-none-eabi toolchain for running on an ARM1176. While doing so I ran into an issue with the AddDoubleField method of tNMEA0183Msg; it depends on dtostrf which I'm pretty sure is an Arduino specific function. I solved this by creating an extern C library (noniso.c) which declares a substitute dtostrf as follows (shamelessly stolen from the ESP32 Arduino Core):

char *dtostrf(double number, signed int width, unsigned int prec, char *s) {
  bool negative = false;

  if (isnan(number)) {
    strcpy(s, "nan");
    return s;
  }
  if (isinf(number)) {
    strcpy(s, "inf");
    return s;
  }

  char *out = s;

  int fillme = width;  // how many cells to fill for the integer part
  if (prec > 0) {
    fillme -= (prec + 1);
  }

  // Handle negative numbers
  if (number < 0.0) {
    negative = true;
    fillme--;
    number = -number;
  }

  // Round correctly so that print(1.999, 2) prints as "2.00"
  // I optimized out most of the divisions
  double rounding = 2.0;
  for (unsigned int i = 0; i < prec; ++i) {
    rounding *= 10.0;
  }
  rounding = 1.0 / rounding;

  number += rounding;

  // Figure out how big our number really is
  double tenpow = 1.0;
  unsigned int digitcount = 1;
  while (number >= 10.0 * tenpow) {
    tenpow *= 10.0;
    digitcount++;
  }

  number /= tenpow;
  fillme -= digitcount;

  // Pad unused cells with spaces
  while (fillme-- > 0) {
    *out++ = ' ';
  }

  // Handle negative sign
  if (negative) {
    *out++ = '-';
  }

  // Print the digits, and if necessary, the decimal point
  digitcount += prec;
  int8_t digit = 0;
  while (digitcount-- > 0) {
    digit = (int8_t)number;
    if (digit > 9) {
      digit = 9;  // insurance
    }
    *out++ = (char)('0' | digit);
    if ((digitcount == prec) && (prec > 0)) {
      *out++ = '.';
    }
    number -= digit;
    number *= 10.0;
  }

  // make sure the string is terminated
  *out = 0;
  return s;
}

I haven't done any thorough testing (yet), but so far it seems to do what's expected. It might be worth providing something similar as a default substitute in the NMEA0183 library for non-Arduino builds. Alternatively it looks like dtostrf could be dropped in favour of a sprintf call?

Furthermore, I also needed the millis() and delay() functions - as is clearly explained in the NMEA0183Msg.cpp source (thank you!). The assumption is that these would be C library functions, but I'm using a C++ framework which already provides native ARM timer functions, so I changed the externs to C++ and created a small C++ wrapper (avr.cpp) for these two methods:

#include <circle/timer.h>
#include "avr.h"

unsigned millis(void) {
  return CTimer::Get ()->GetClockTicks () / (CLOCKHZ / 1000);
}

void delay(unsigned ms) {
  CTimer::SimpleMsDelay(ms);
}

In NMEA0183Msg.cpp:

...
#ifndef ARDUINO
#include <cstdio>
#include <cstdlib>
#include "noniso.h"
#include "avr.h"
#endif
...

This too seems to work, though it's entirely possible that I've made some mistake or that I've misunderstood something fundamental; C/C++ are not my first languages, or even my second :) Let me know what you think - and many thanks for all your hard work on this great library!

Edit: Having read up on extern I realised that's not how extern works, and that those stanzas were not needed.