sum value and remove duplicates in List - flutter

Question

I have this list and want to sum value and remove duplicates in List
1 - check of productName
2 - sum NumberOfItems if productName equals
For Example :
"Orders":[
{
"productName":"Apple",
"NumberOfItems":"5"
},
{
"productName":"Orange",
"NumberOfItems":"2"
},
{
"productName":"Egg",
"NumberOfItems":"5"
},
{
"productName":"Apple",
"NumberOfItems":"3"
},
{
"productName":"Orange",
"NumberOfItems":"4"
},
{
"productName":"Egg",
"NumberOfItems":"9"
},
]
The result I need look like this result : (Sum Depend on productName)
"Orders":[
{
"productName":"Apple",
"NumberOfItems":"8"
},
{
"productName":"Orange",
"NumberOfItems":"6"
},
{
"productName":"Egg",
"NumberOfItems":"14"
},
]

Answer 1

final orders = data["Orders"] as List;
final mapped = orders.fold<Map<String, Map<String, dynamic>>>({}, (p, v) {
final name = v["productName"];
if (p.containsKey(name)) {
p[name]["NumberOfItems"] += int.parse(v["NumberOfItems"]);
} else {
p[name] = {
...v,
"NumberOfItems": int.parse(v["NumberOfItems"])
};
}
return p;
});
final newData = {
...data,
"Orders": mapped.values,
};
print(newData);
Result is:
{Orders: ({productName: Apple, NumberOfItems: 8}, {productName: Orange, NumberOfItems: 6}, {productName: Egg, NumberOfItems: 14})}

Answer 2

Notice: This code has 2 loop which means slower.
Igor Kharakhordin answered smarter one, but may be difficult for those who ask this question.(since he is doing two things at once.) Basically I am doing same thing.
String string = await rootBundle.loadString("asset/data/Orders.json");
Map orders = jsonDecode(string);
List orderList = orders["Orders"];
Map<String,int> sums = {};
for(int i = 0 ; i < orderList.length; i++){
dynamic item = orderList[i];
if(sums.containsKey(item["productName"])){
sums[item["productName"]] += int.parse(item["NumberOfItems"]);
}
else{
sums[item["productName"]] = int.parse(item["NumberOfItems"]);
}
}
List sumList = [];
sums.forEach((key,value)=>
sumList.add({
"productName":key,
"NumberOfItems":value.toString()
})
);
Map result = {
"Orders":sumList
};
print(jsonEncode(result));
Result
{
"Orders": [
{
"productName": "Apple",
"NumberOfItems": "8"
},
{
"productName": "Orange",
"NumberOfItems": "6"
},
{
"productName": "Egg",
"NumberOfItems": "14"
}
]
}