divmod10() : a fast replacement for /10 and %10 (unsigned)

Development Libraries
81

Many users need faster decimal print for logging data to SD cards. The big problem with Print is this line

  return write(str);

A class derived from Print may have a fast write for strings but it is not uses since Print defines these members

size_t write(const char *str) {
      if (str == NULL) return 0;
      return write((const uint8_t *)str, strlen(str));
    }

size_t Print::write(const uint8_t *buffer, size_t size)
{
  size_t n = 0;
  while (size--) {
    n += write(*buffer++);
  }
  return n;
}

So printNumber uses the above write(const char *str) and the string is written character at a time.

I did a test with the new version of print. It is faster but not as much as one would expect.

I timed this loop

  for (int i = 0; i < 20000; i++) {
    file.println(i);
  }

The result for the old print is:

Time 9.38 sec
File size 128.89 KB
Write 13.74 KB/sec

For the new print with Stimmer's optimization.

Time 6.00 sec
File size 128.89 KB
Write 21.50 KB/sec

For a simple printField() function with no optimized divide that uses SdFat's write(const char*)

Time 2.66 sec
File size 128.89 KB
Write 48.44 KB/sec

So the improvement in Print will not be fully realized as long as printNumber uses Print::write(const char*).

I believe there is a way to fix this in a class derived from Print since size_t Print::write(const char *str) is not pure virtual. Correct me if I am wrong.