RFR: JDK-8074003 java.time.zone.ZoneRules.getOffset(java.time.Instant) can be optimized

[Peter Levart]

Hi again,

Here's another optimization to be reviewed that has been discussed a 
while ago (just rebased from webrev.01) and approved by Stephen:

http://cr.openjdk.java.net/~plevart/jdk9-dev/ZoneOffsetTransition.epochSecond/webrev.02/


The discussion about it is intermingled with the ZoneId.systemDefault() 
discussion and starts about here:

http://mail.openjdk.java.net/pipermail/core-libs-dev/2015-February/031873.html


The rationale for the optimization is speeding-up the conversion from 
epoch time to LocalDateTime. This conversion uses 
ZoneRules.getOffset(Instant) where there is a loop over 
ZoneOffsetTransition[] array that searches for 1st transition that has 
its toEpochSecond value less than the Instant's epochSecond. This calls 
ZoneOffsetTransition.toEpochSecond repeatedly, converting 
ZoneOffsetTransition.transition which is a LocalDateTime to epochSecond. 
This repeated conversion is unnecessary, as ZoneOffsetTransition[] array 
is part of ZoneRules which is cached. Optimizing the 
ZoneOffsetTransition implementation (keeping both LocalDateTime variant 
and eposhSecond variant of transition time as the object's state) speeds 
up this conversion.


Regards, Peter