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Lead to inaccurate comparison, using EPSILON to correct. #350

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Lead to inaccurate comparison, using EPSILON to correct. #350

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4 changes: 3 additions & 1 deletion timing.cc
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Original file line number Diff line number Diff line change
Expand Up @@ -125,6 +125,8 @@

/* $Id$ */

#include <limits>

#include "timing.h"
#include "NmapOps.h"
#include "utils.h"
Expand Down Expand Up @@ -437,7 +439,7 @@ void RateMeter::update(double amount, const struct timeval *now) {
interval = MAX(current_rate_history, diff);
else
interval = TIMEVAL_SUBTRACT(*now, start_tv) / 1000000.0;
assert(diff <= interval);
assert(diff <= interval + std::numeric_limits<double>::epsilon());
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@ghost ghost Jul 8, 2016

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After investigation it seems like this is not an accurate method for comparaison between doubles less than 1.0, and it is possible to obtain interval == 0.0 so it may not be accurate enough for this comparaison. At https://randomascii.wordpress.com/2012/02/25/comparing-floating-point-numbers-2012-edition/, the author explains how float(pi)+sin(float(pi)) is more accurate than float(pi) to pi. I would suggest to compare double(interval)+sin(double(interval)) to double(diff)+sin(double(diff))... but I didn't check if the results would be in fact more accurate. What do you think?

/* If we record an amount in the very same instant that the timer is started,
there's no way to calculate meaningful rates. Ignore it. */
if (interval == 0.0)
Expand Down