[{_id:1,"hash":[1,2,7]},{_id:2,"hash":[3,2,1]}]

db.collection.aggregate([{$addFields:{score:{$reduce:{input:{$range:[0,{$size:[[1,2,3]]}]},initialValue:0,in:{$cond:{if:{$eq:[{$arrayElemAt:["$hash","$$this"]},{$arrayElemAt:[[1,2,3],"$$this"]}]},then:{$add:["$$value",1]},else:"$$value"}}}}}}])