Martin Desruisseaux created SIS-568:
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Summary: Simplify matrix internal and double-double arithmetic
Key: SIS-568
URL: https://issues.apache.org/jira/browse/SIS-568
Project: Spatial Information Systems
Issue Type: Task
Components: Referencing
Affects Versions: 1.3
Reporter: Martin Desruisseaux
Assignee: Martin Desruisseaux
Fix For: 1.4
Apache SIS uses double-double arithmetic internally for increasing the chances
to detect when concatenation of coordinate operations cancel each other or when
the operation can be simplified/optimized. For example with standard {{double}}
arithmetic, (A/B)*B is not always equal to A. The common approach is to use a
small tolerance factor, but the threshold is very difficult to choose. For
example datum shifts can be small enough for being confused with a threshold.
In addition to double-double arithmetic, Apache SIS uses its own matrix
implementation because it needs to handle NaN values in a different way than
what common packages do. SIS also handles non-square matrices in a different
way, knowing that they appear when the operation changes the number of CRS
dimensions. Finally Apache SIS matrix implementation is designed for working
with above-cited double-double arithmetic.
Performance concern has lead to a complicated implementation in all Apache SIS
versions up to 1.3. {{DoubleDouble}} is mutable for allowing codes to recycle
existing instances, which make expressions non-fluent. {{GeneralMatrix}} stores
({_}value{_} + {_}error{_}) components of {{DoubleDouble}} while {{MatrixN}} do
not, which forces SIS code to worry about whether a matrix uses double-double
arithmetic or not. In practice, we found (relatively minor) bugs in SIS 1.3
caused by double-double precision lost between matrix implementations.
We need to simplify the situation. It can be done with the following steps:
* Make {{DoubleDouble}} immutable. The original performance concern may be
obsolete thanks to compiler optimizations, and will probably become yet more
obsolete with _value object_ to be provided in a future Java version.
* Replace all {{DoubleDouble}} usages by fluent expressions. This is made
possible by the above point.
* Remove all explicit handling of double-double in {{{}GeneralMatrix{}}}.
Instead, stores the values as {{{}Number{}}}. The possibly increased memory
consumption is not a concern because this matrix size rarely exceed 5×5.
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