MongoDb - Help for pipeline with $trunc - mongodb

Question

My data :
[
{ total: 7421356 },
{ total: 79421356 },
{ total: 105457854 },
{ total: 1054578540 },
{ total: 10545785400 },
]
I would like to have something like :
[
{ val: 7000000, count 1 },
{ val: 70000000, count 1 },
{ val: 100000000, count: 1 },
{ val: 1000000000, count 1 }
]
Actually i use this pipeline :
{
$addFields: {
length: {
$multiply: [
{
$add: [
{
$strLenCP: {
$toString: "$val",
}
},
-1
]
},
-1
]
},
},
},
{
$project: {
value: {
$trunc: ["$val", "$length"],
},
_id: 0,
}
},
{
$group: {
_id: "$value",
count: {
$sum: 1
}
}
},
{
$project: {
value: "$_id",
count: 1,
_id: 0,
}
},
{
$sort: {
value: 1
}
}
I have a problem when i have data like "10545785400".
It seems his length it too long and for my data "7421356" his result is now "0".
I thought the documents were going through the pipeline individually but it doesn't appear to be.
My first data seems to use the length of my last.
I hope someone can help me even if my explanations are not very clear.
EDIT : It seems to be a "type" problem. Data with greater than 1.000.000.000 are double not int32
EDIT 2 : It works with "24760000000" but not with "25661674539". I really don't understand why. They are stored in Double format.

Answer 1

tl;dr
add $toLong:"$total" (from here)
The doubles are being converted to scientific notation, and that's taking into account to measure the string length, which is 9 in most or all of the fields. As you said the type of those numbers is double.
See a working example here, converting the string to number before cutting off the data:
db.collection.aggregate({
$addFields: {
length: {
$multiply: [
{
$add: [
{
$strLenCP: {
$toString: {
$toLong: "$total"
},
}
},
-2
]
},
-1
]
},
}
},
{
$project: {
value: {
$toLong:{
$trunc: [
"$total",
"$length"
],
}
},
_id: 0,
}
})
Detail
You can't convert those big numbers to int using $toInt because the numbers are large. Large being larger than 10^10