[{"title":"test1",sort:1,parent:1},{"title":"test2",sort:7,parent:1},{"title":"test3",sort:5,parent:1},{"title":"test4",sort:1,parent:2},{"title":"test5",sort:7,parent:2},{"title":"test6",sort:5,parent:2}]

db.collection.aggregate([{$setWindowFields:{partitionBy:"$parent",sortBy:{sort:1},output:{sort:{$rank:{}}}}},{$set:{sort:{$add:[1,{$multiply:[{$subtract:["$sort",1]},10]}]}}},{$out:"collection"}])