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/scala/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_double_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.testBase64ScanningForDelimiter1
One CLI test failure:
org.apache.daffodil.cli.cliTest.TestEXIEncodeDecode.test_CLI_Encode_Decode_EXI
