Gremlin arithmetic on where predicate?

In this question Gremlin graph traversal that uses previous edge property value to filter later edges we can use where to compare property of edge. I don't want just using simple neq or eq or gt. Can gremlin support on arithmetic on this two edge? suck like gtv('firstEdge', 0.2) or g.V(1).outE().has('weight',1.0).as('firstEdge').inV().outE().as('secondEdge').filter((secondEdge-firstEdge) > 0.2)

I seem don't find such thing in document.

Solution

There are a couple of ways to approach a situation like this. For simple queries where you just want a+b < some value, using sack works well. For example, using the air-routes data set:

g.withSack(0).
  V('44').
  repeat(outE('route').sack(sum).by('dist').inV().simplePath()).
  times(2).
  where(sack().is(lt(500))).
  path().
    by('code').
    by('dist').
  limit(2)

which yields:

1   path[SAF, 369, PHX, 110, TUS]
2   path[SAF, 369, PHX, 119, FLG]

To use the math step requires just a little more work:

Let's first just see how the math step works in such a case via a query to take the difference between some route distances:

g.V('44').
  outE('route').as('a').inV().
  outE('route').as('b').inV().
  project('b','a','diff').
    by(select('b').values('dist')).
    by(select('a').values('dist')).
    by(math('b - a').by('dist')).
  limit(3)

which yields:

1   {'b': 1185, 'a': 549, 'diff': 636.0}
2   {'b': 6257, 'a': 549, 'diff': 5708.0}
3   {'b': 8053, 'a': 549, 'diff': 7504.0}

we can now refine the query to find routes where the difference is less than 100.

g.V('44').
  outE('route').as('a').inV().
  outE('route').as('b').inV().
  where(math('b - a').by('dist').is(lt(100))).
  path().
    by('code').
    by('dist').
  limit(3)

which gives us:

1   path[SAF, 549, DFW, 430, MEM]
2   path[SAF, 549, DFW, 461, MCI]
3   path[SAF, 549, DFW, 550, STL]

you can also use the absolute value in the calculation if preferred:

g.V('44').
  outE('route').as('a').inV().
  outE('route').as('b').inV().
  where(math('abs(b - a)').by('dist').is(lt(100))).
  path().
    by('code').
    by('dist').
  limit(3)