Circe decoder only array size - json

Question

I have an Api that I need to do multiple concurrent call with cursor, but I'm interested in only knowing the size of an array in the Json only, I wonder if is it possible to make a custom decoder with Circe or Jsoniter for it?
The Json look simple only like this:
{
"myArray":[
{
"multipleJsonObjectsHere"
}
],
"cursor":"abc"
}
I just want to retrieve only the size of myArray to know how many items are there in it

Answer 1

Using jsoniter-scala you can write a custom codec that will parse/serialize JSON array to/from Int value:
case class Message(myArray: Int, cursor: String)
implicit val messageCodec: JsonValueCodec[Message] = {
implicit val intCodec: JsonValueCodec[Int] = new JsonValueCodec[Int] {
override def decodeValue(in: JsonReader, default: Int): Int =
if (in.isNextToken('[')) {
var i = 0
if (!in.isNextToken(']')) {
in.rollbackToken()
while ({
in.skip()
i += 1
in.isNextToken(',')
}) ()
if (!in.isCurrentToken(']')) in.arrayEndOrCommaError()
}
i
} else in.readNullOrTokenError(default, '[')
override def encodeValue(x: Int, out: JsonWriter): _root_.scala.Unit = {
out.writeArrayStart()
var i = x
while (i > 0) {
out.writeNull()
i -= 1
}
out.writeArrayEnd()
}
override def nullValue: Int = 0
}
JsonCodecMaker.make[Message]
}
val json1 =
"""{
| "myArray":[
| {
| "multipleJsonObjects": "Here"
| }
| ],
| "cursor":"abc"
|}""".stripMargin.getBytes(StandardCharsets.UTF_8)
assert(readFromArray[Message](json1) == Message(1, "abc"))
val json2 = """{"myArray":[null,null,null],"cursor":"www"}""".getBytes(StandardCharsets.UTF_8)
assert(readFromArray[Message](json2) == Message(3, "www"))