Mongodb aggregeringsforespørgsel til at trække fra og gruppere kumulativ værdi

Hvordan er det her? Den beregner ikke timen, men den gør alt andet.

[
    {
      $match: {
          $and: [
              {MachineID: {$in: [1001]}},
              {
                Timestamp: {
                    $gte: ISODate("2017-04-18T01:30:00.000Z"),
                    $lte: ISODate("2017-04-19T01:30:00.000Z")
                }
              }
          ]
      }
    },
    // Add all data to one array.
    {$group: {_id: "$MachineID", all: {$push: "$$ROOT"}}},
    // Create an array of (element, array index) pairs.
    {$addFields: {allWithIndex: {$zip: {inputs: ["$all", {$range: [0, {$size: "$all"}]}]}}}},
    // Create an array of {current: <element>, previous: <previous element>} pairs.
    {
      $project: {
          pairs: {
              $map: {
                  input: "$allWithIndex",
                  in : {
                      current: {$arrayElemAt: ["$$this", 0]},
                      prev: {
                          $arrayElemAt: [
                              "$all",
                              // Set prev == current for the first element.
                              {$max: [0, {$subtract: [{$arrayElemAt: ["$$this", 1]}, 1]}]}
                          ]
                      }
                  }
              }
          }
      }
    },
    // Compute the deltas.
    {$unwind: "$pairs"},
    {
      $group: {
          _id: {MachineID: "$_id", RunStatus: "$pairs.current.RunStatus"},
          ProductsCount:
              {$sum: {$subtract: ["$pairs.current.ProductsCount", "$pairs.prev.ProductsCount"]}},
          Utilization:
              {$sum: {$subtract: ["$pairs.current.Utilization", "$pairs.prev.Utilization"]}},
      }
    }
]