Add Writer::SetMaxDecimalPlaces()

[miloyip]

Fixes #362

[DavidRJohns]

Great new feature. Great library in general! Is there a way to add the ability for it to round as opposed to truncating? I tried to read through the paper with the Grisu2 algorithm and saw that the paper also has a Grisu2b algorithm that rounds. Is there a way to modify dtoa to use this rounding algorithm? Maybe both algoritms could be implemented and a class member could be added to distinguish between rounding or truncating.

[jdemeyer]

+1 to the comment of @DavidRJohns

Unfortunately, the current implementation is lossy: reading a floating point number with a given number of decimal places and then writing that number back with the same number of decimal places can result in a different number being written. Example: reading 50.907667 and writing it back with 6 decimal places gives 50.907666.

[Llerd]

I needed rounding rather than truncating, so I added this block in the initial part of Prettify function. Use as you see fit.

inline char* Prettify(char* buffer, int length, int k, int maxDecimalPlaces) {
  int kk = length + k;  // 10^(kk-1) <= v < 10^kk

  if (kk > -6 && kk <= 21) {
    // 29.9995xxx -> 30.000
    int rounder =  kk + maxDecimalPlaces;
    if (length > rounder && buffer[rounder] >= '5') {
      for (int i = rounder - 1; i >= 0; i--) {
        buffer[i]++;
        if (buffer[i] <= '9')
          break;

        // overflow to next digit
        if (i != 0) {
          buffer[i] = '0';
        } else {
          // 9.9995xxx -> 10.000
          // Need to insert an initial '1', shift decimal and set rounder to 0
          buffer[0] = '1';
          buffer[rounder] = '0';
          kk++;
          k--;
        }
      }
    }
  }

Also, tested previous commenters 50.907667 value, but that seems fixed already.