Floating point comparison is typically done with +- delta as Mike suggested.
Sincerely, Harinder Sood Senior Program Mnager hs...@owlcyberdefense.com 240 805 4219 owlcyberdefense.com The information contained in this transmission is for the personal and confidential use of the individual or entity to which it is addressed. If the reader is not the intended recipient, you are hereby notified that any review, dissemination, or copying of this communication is strictly prohibited. If you have received this transmission in error, please notify the sender immediately. -----Original Message----- From: Steve Lawrence <slawre...@apache.org> Sent: Monday, September 25, 2023 10:01 AM To: dev@daffodil.apache.org Subject: Re: Comparing Floating Point numbers Based on what I've learned today from reading that bug, I think in Java at least, float -> string -> float will give you back the exact same float. Note that string -> float -> string could give a different result, since multiple strings can map to the same float, but a float maps to only a single string. Java doesn't actually round when converting a float to a string. Instead it finds the shortest string representation that can be uniquely mapped back to the original float. The bug I linked below is about how Java didn't find the shortest. For example, "1.0E23" and "9.999999999999999E22" both map to the exact same float. Older versions of Java output the latter, newer version of Java output the former. But even though it didn't output the shortest, and are different decimal representations, both of those strings uniquely map back to the exact same float. So although users must be aware that float values in the infoset might not be accurate to the original value, we can be guaranteed that if an infoset string is converted to a Java float, it will at least be the exact same float value that Daffodil wrote to the infoset. And so for the purposes of TDML comparisons, it should all be exactly the same. For these reasons, I don't actually think creating a special raw representation actually fixes anything. The decimal string already maps back to the original float exactly. The real issues with floats is when you start doing math, or the original data was a string. That's when you start losing precision and get funky results. I don't there are any actual issues with the infoset conversion itself. On 2023-09-25 08:57 AM, McGann, Mike wrote: > One thing to note is that by putting a value in a TDML document such as > "12.34e56" it is actually a string. Comparing that to a floating-point value > is going to require a conversion from string and that could invoke a rounding > step if it cannot be accurately represented by a float. If you really want to > compare two floating points exactly, using a binary representation is > probably the best such as putting in something like 0x1234p56. At least that > is how I think I understand it. Floating point math is a deep rabbit hole > that can be followed. That is probably overkill for TDML. > > // Mike > > -----Original Message----- > From: Steve Lawrence <slawre...@apache.org> > Sent: Monday, September 25, 2023 08:07 > To: dev@daffodil.apache.org > Subject: Re: Comparing Floating Point numbers > > +1 for type aware comparisons. It should be a very small change to > +this > function: > > https://github.com/apache/daffodil/blob/main/daffodil-lib/src/main/sca > la/org/apache/daffodil/lib/xml/XMLUtils.scala#L1098 > > And just need to add xsi:type to a few expected infosets that are > sensitive to the issue. > > Note that I *think* this might be the bug that caused the change: > > https://bugs.openjdk.org/browse/JDK-4511638 > > Based on that, it sounds like the issue is that Java wasn't creating > the shortest possible decimal representation, but the representation > it did create still parses back to the same floating point > representation. So we *probably* don't even really need epsilon > comparison, we just need type aware comparison, and can still expect > the floating point values to be exactly the same. > > Although epsilon comparison is the right way to compare floats, my > concern is that we might add some bug in Daffodil where we do math > wrong and end up with a very very very slightly wrong answer and it > would be hidden. But if our epsilon is small enough, maybe that amount > precision error is fine? > > Note that according to that JDK issue, the change was made in Java 19, > so if we add any conditional logic on java version, we should check if > it's at least 19. I guess if we do need epsilon comparisons we could > only do it for java 19 and newer. Older versions would expect exact > values and so would catch any off by very very small amount bugs. That > might be adding unnecessary complication though. > > > On 2023-09-24 12:09 PM, Mike Beckerle wrote: >> So Java 21 produces different floating point values in a few cases. >> Some of our tests (4) are sensitive to this. >> >> The "right way" to compare floating point numbers is like this: >> >> If(Math.abs(A - B) < epsilon) >> >> The TDML runner has outstanding bug >> https://issues.apache.org/jira/browse/DAFFODIL-2402 which is to add >> the ability to put xsi:type="double" for example on the expected >> infoset, and this instructructs the (schema unaware) TDML runner to >> do comparison using some sort of epsilon comparison like the above >> >> Does that seem like the right fix for this? >> >> The only alternative I can think of is some sort of conditional >> infoset construction, so that the expected values can vary for different >> JVMs. >> >> On Sat, Sep 23, 2023 at 2:13 PM Mike Beckerle <mbecke...@apache.org> wrote: >> >>> >>> JVM 21 LTS is now out. >>> >>> So I decided to try to building Daffodil using it. My WIP PR is >>> https://github.com/apache/daffodil/pull/1090 >>> >>> It looks pretty close. >>> >>> The --release 8 option for javac is now deprecated. So I >>> conditionalized that. >>> Fixed some deprecated calls. >>> >>> Remaining issues: >>> >>> 2 more deprecated calls (hence fatal warnings turned off for now) >>> >>> 5 tests fail. One each in these 3 test classes >>> >>> org.apache.daffodil.TresysTests.test_BG000 >>> >>> org.apache.daffodil.section13.text_number_props.TestTextNumberProps. >>> test_textNumberPattern_exponent01 >>> >>> >>> org.apache.daffodil.section05.simple_types.TestSimpleTypes.test_doub >>> le_binary_06 >>> >>> All 3 of those failures are floating point related like this: >>> (highlighted digit isn't output any more) >>> >>> [error] Expected (attributes stripped) >>> [error] <d_02>9.8765432109876544E16</d_02> >>> [error] Actual (attributes ignored for diff) [error] >>> <ex:d_02>9.876543210987654E16</ex:d_02> >>> >>> The Expected has one more digit 4 at the end. >>> >>> 1 other test failure is for reasons unknown. Possible change in >>> regex behavior? >>> >>> >>> org.apache.daffodil.io.layers.TestJavaIOStreams.testBase64ScanningFo >>> rDelimiter1 >>> >>> One CLI test failure: >>> >>> >>> org.apache.daffodil.cli.cliTest.TestEXIEncodeDecode.test_CLI_Encode_ >>> Decode_EXI >>> >>> >>> >> >
