projekt kivitendo

  # Rounding like "Kaufmannsrunden" (see http://de.wikipedia.org/wiki/Rundung )

  # If you search for rounding in Perl, you'll likely get the first version of

  # this algorithm:

  #

  # ($amount <=> 0) * int(abs($amount) * 10**$places) + .5) / 10**$places

  #

  # That doesn't work. It falls apart for certain values that are exactly 0.5

  # over the cutoff, because the internal IEEE754 representation is slightly

  # below the cutoff. Perl makes matters worse in that it really, really tries to

  # recognize exact values for presentation to you, even if they are not.

  #

  # Example: take the value 64.475 and round to 2 places.

  #

  # printf("%.20f\n", 64.475) gives you 64.47499999999999431566

  #

  # Then 64.475 * 100 + 0.5 is 6447.99999999999909050530, and

  # int(64.475 * 100 + 0.5) / 100 = 64.47

  #

  # Trying to round with more precision first only shifts the problem to rarer

  # cases, which nevertheless exist.

  #

  # Now we exploit the presentation rounding of Perl. Since it really tries hard

  # to recognize integers, we double $amount, and let Perl give us a representation.

  # If Perl recognizes it as a slightly too small integer, and rounds up to the

  # next odd integer, we follow suit and treat the fraction as .5 or greater.

  my $sign               = $amount <=> 0;

  $amount                = abs $amount;

  my $shift              = 10 ** ($places);

  my $shifted_and_double = $amount * $shift * 2;

  my $rounding_bias      = sprintf('%f', $shifted_and_double) % 2;

  $amount                = int($amount * $shift) + $rounding_bias;

  $amount                = $amount / $shift * $sign;