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https://issues.apache.org/jira/browse/TINKERPOP3-670?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14544036#comment-14544036
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ASF GitHub Bot commented on TINKERPOP3-670:
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Github user mhfrantz commented on the pull request:
https://github.com/apache/incubator-tinkerpop/pull/60#issuecomment-102105037
I'm looking in more detail at how `RangeGlobalStep` works. It seems there
are a few places where this exact step is a special case (via `instanceof`
checks, e.g. in `TraverserMapReduce` as well as in several strategies).
The way that `Bypassing` is used also has to be considered.
I also see that `ComputerVerificationStrategy` looks for specific types of
steps, and `TailGlobalStep` might belong there.
It feels like some refactoring is warranted. `RangeGlobalStep` and
`CollectingBarrierStep` each represent some class of steps to which
`TailGlobalStep` may or may not belong. Perhaps `Bypassing` is one such
aspect, but are there others?
Maybe `TailGlobalStep` is really a new animal, but it feels like it ought
to be composed from pieces that mean "it is like a barrier, but not exactly
CollectingBarrierStep" and "it is like a range, but not exactly
RangeGlobalStep".
I'm still coming up to speed on the design, and I'm not yet totally lost,
but any pointers would be appreciated. I can appreciate the tension between a
fully a generalized "step aspects" API and one that assumes that the number of
fundamental steps is small and so will scale with special case logic.
> tail step
> ---------
>
> Key: TINKERPOP3-670
> URL: https://issues.apache.org/jira/browse/TINKERPOP3-670
> Project: TinkerPop 3
> Issue Type: Improvement
> Components: process
> Reporter: Matt Frantz
>
> The tail step has two scopes:
> * global: emits the last N traversers that pass through it.
> * local: emits a {{List}} containing the last N elements of the inbound
> {{List}}
> Additionally, for {{tail(local, 1)}}, it emits a single element rather than a
> {{List}} containing a single element.
> TINKERPOP3-639 provides motivation.
> Scenario: Global scope
> {noformat}
> gremlin> g.V().order().by(T.id, incr).tail(global, 2)
> ==>v[5]
> ==>v[6]
> {noformat}
> Scenario: Default scope is global
> {noformat}
> gremlin> g.V().order().by(T.id, incr).tail(2)
> ==>v[5]
> ==>v[6]
> {noformat}
> Scenario: Local scope, {{List}} input, N>1
> {noformat}
> gremlin> g.V().as('a').out().as('a').out().as('a').select('a').tail(local, 2)
> ==>[v[4], v[5]]
> ==>[v[4], v[3]]
> {noformat}
> Scenario: Local scope, {{List}} input, N=1
> {noformat}
> gremlin> g.V().as('a').out().as('a').out().as('a').select('a').tail(local, 1)
> ==>v[5]
> ==>v[3]
> {noformat}
> Scenario: Local scope, {{Map}} input, N>1
> {noformat}
> gremlin> g.V().as('a').out().as('b').out().as('c').select().tail(local, 2)
> ==>[b:v[4], c:v[5]]
> ==>[b:v[4], c:v[3]]
> {noformat}
> Scenario: Local scope, {{Map}} input, N=1
> {noformat}
> gremlin> g.V().as('a').out().as('b').out().as('c').select().tail(local, 1)
> ==>[c:v[5]]
> ==>[c:v[3]]
> {noformat}
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