bug report: [ #447945 ] time.time() is not non-decreasing

[zooko at zooko.com]

I have committed a cleaned up version of the IncreasingTimer which does less
floating point arithmetic and which contains comments showing exactly what 
I don't know about floating point.

http://cvs.sourceforge.net/cgi-bin/viewcvs.cgi/~checkout~/mojonation/evil/common/timeutil.py?content-type=text/plain

The relevant excerpt of the source is appended.

Regards,

Zooko


class IncreasingTimer:
    def __init__(self):
        self.lasttime = time.time() # This stores the most recent answer that we returned from `time()'.
        self.delta = 0 # We add this to the result from the underlying `time.time()'.

        # How big of an increment do we need to add in order to make the new float greater than the old float?
        trye = 1.0
        while (self.lasttime + trye) > self.lasttime:
            olde = trye
            trye = trye / 2.0
        self._TIME_EPSILON = olde

    def time(self):
        t = time.time() + self.delta
        lasttime = self.lasttime

        if t <= lasttime:
            self.delta = self.delta + (lasttime - t) + self._TIME_EPSILON
            t = lasttime + self._TIME_EPSILON

        # XXX if you were sure that you could generate a bigger float in one pass, you could change
        # this `while' to an `if' and optimize out a test.
        while t <= lasttime:
            # We can get into here only if self._TIME_EPSILON is too small to make the time float "tick over" to a new higher value.
            # So we (permanently) double self._TIME_EPSILON.
            # XXX is doubling epsilon the best way to quickly get a minimally bigger float?
            self._TIME_EPSILON = self._TIME_EPSILON * 2.0
            # Delta, having smaller magnitude than t, can be incremented by more than t was incremented.  (Up to the old epsilon more.)  That's OK.
            self.delta = self.delta + self._TIME_EPSILON
            t = t + self._TIME_EPSILON

        self.lasttime = t
        return t