MongoDB aggregation. Trying to use switch in map

Question

Hi im trying to use a switch statement inside of map to increment a value depending on the case but im getting

MongoServerError: $switch found an unknown argument: $$favcast

is this viable?

     db.movies.aggregate([
    {$match: {cast: { $elemMatch: { $exists: true } }}},
    {$match:{ "tomatoes.viewer.rating": {$gte: 3}}},
    {$match:{ countries: {$in: ["USA"]}}},
    {$addFields: {num_favs: 0}},
    {$project: {
      _id: 0,
      cast: {
        $map: {
          input: "$cast",
          as: "favcast",
          in: { $switch: { 
            $$favcast:[
              { case: "Sandra Bullock", then: { $add: [ "$num_favs", 1] } },
              { case: "Tom Hanks", then: { $add: [ "$num_favs", 1] } },
              { case: "Julia Roberts", then: { $add: [ "$num_favs", 1] } },
              { case: "Kevin Spacey", then: { $add: [ "$num_favs", 1] } },
              { case: "George Clooney", then: { $add: [ "$num_favs", 1] } },
            ],
            default: { $add: [ "$num_favs", 0 ]}
          }
        }
      }
    }
  }
},
{$match:{ "num_favs": {$gt: 0}}},
{$project: { num_favs: 1, title: 1 }}  
])

Answer 1

I didn't end up making the switch work but this did the job for my query. $filter and $reduce

     db.movies.aggregate([
    {$match: {cast: { $elemMatch: { $exists: true } }}},
    {$match:{ countries: {$in: ["USA"]}}},
    {$match:{ "tomatoes.viewer.rating": {$gte: 3}}},
    {$addFields: {favcast: {$filter:{
        input: "$cast",
        as: "fave",
        cond: {$or: [
          {$eq: ["$$fave", "Sandra Bullock"] },
          {$eq: ["$$fave", "Tom Hanks"] },
          {$eq: ["$$fave", "Julia Roberts"] },
          {$eq: ["$$fave", "Kevin Spacey"] },
          {$eq: ["$$fave", "George Clooney"] },
        ]}
    } } } },
    {$addFields: { num_favs: 
        {$reduce: { 
            input: "$favcast",
            initialValue: {sum: 0},
            in: {
              sum: { $add : ["$$value.sum", 1] }
            }
          }}
        }},
    {$project: {title: 1, favcast: 1, num_favs: 1, _id: 0 }},
    {$sort: { num_favs: -1, "tomatoes.viewer.rating": -1 , title: -1}},
    {$skip : 20}
])