DOCSP-30246: kotlin aggregation expression operations (#86)

# Pull Request Info

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Guidelines](https://github.com/mongodb/docs-java/blob/master/REVIEWING.md)

JIRA - [DOCSP-30246](https://jira.mongodb.org/browse/DOCSP-30246)

[**Staging**](https://docs-mongodbcom-staging.corp.mongodb.com/kotlin/docsworker-xlarge/DOCSP-30246-kotlin-agg-exp/fundamentals/aggregation-expression-operations/)

## Self-Review Checklist

- [x] Is this free of any warnings or errors in the RST?
- [x] Did you run a spell-check?
- [ ] Did you run a grammar-check?
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Author: rustagir

examples/src/test/kotlin/AggExpressionsTest.kt

@@ -0,0 +1,8 @@
+val month = current().getDate("date").month(of("UTC"))
+val precip = current().getInteger("precipitation")
+
+listOf(
+    Aggregates.group(
+        month,
+        Accumulators.avg("avgPrecipMM", precip.multiply(25.4))
+))

source/examples/generated/AggExpressionsTest.snippet.arithmetic-aggregation.kt

@@ -0,0 +1,14 @@
+val showtimes = current().getArray<MqlDocument>("showtimes")
+
+listOf(
+    Aggregates.project(
+        Projections.fields(
+            Projections.computed("availableShowtimes", showtimes
+                .filter { showtime ->
+                    val seats = showtime.getArray<MqlInteger>("seats")
+                    val totalSeats = seats.sum { n -> n }
+                    val ticketsBought = showtime.getInteger("ticketsBought")
+                    val isAvailable = ticketsBought.lt(totalSeats)
+                    isAvailable
+                })
+)))

source/examples/generated/AggExpressionsTest.snippet.array-aggregation.kt

@@ -0,0 +1,9 @@
+val temperature = current().getInteger("temperature")
+
+listOf(
+    Aggregates.project(
+        Projections.fields(
+            Projections.computed("extremeTemp", temperature
+                .lt(of(10))
+                .or(temperature.gt(of(95))))
+)))

source/examples/generated/AggExpressionsTest.snippet.boolean-aggregation.kt

@@ -0,0 +1,6 @@
+val location = current().getString("location")
+
+listOf(
+    Aggregates.match(
+        Filters.expr(location.eq(of("California")))
+))

source/examples/generated/AggExpressionsTest.snippet.comparison-aggregation.kt

@@ -0,0 +1,12 @@
+val member = current().getField("member")
+
+listOf(
+    Aggregates.project(
+        Projections.fields(
+            Projections.computed("membershipLevel",
+                member.switchOn{field -> field
+                    .isString{s-> s}
+                    .isBoolean{b -> b.cond(of("Gold"), of("Guest"))}
+                    .isArray { a -> a.last()}
+                    .defaults{ d -> of("Guest")}})
+)))

source/examples/generated/AggExpressionsTest.snippet.conditional-aggregation.kt

@@ -0,0 +1,13 @@
+fun gradeAverage(students: MqlArray<MqlDocument>, fieldName: String): MqlNumber {
+    val sum = students.sum{ student -> student.getInteger(fieldName) }
+    val avg = sum.divide(students.size())
+    return avg
+}
+
+fun evaluate(grade: MqlNumber, cutoff1: MqlNumber, cutoff2: MqlNumber): MqlString {
+    val message = grade.switchOn{ on -> on
+        .lte(cutoff1) { g -> of("Needs improvement") }
+        .lte(cutoff2) { g -> of("Meets expectations") }
+        .defaults{g -> of("Exceeds expectations")}}
+    return message
+}

source/examples/generated/AggExpressionsTest.snippet.convenience-aggregation-methods.kt

@@ -0,0 +1,9 @@
+val students = current().getArray<MqlDocument>("students")
+
+listOf(
+    Aggregates.project(
+        Projections.fields(
+            Projections.computed("evaluation", students
+                .passArrayTo { s -> gradeAverage(s, "finalGrade") }
+                .passNumberTo { grade -> evaluate(grade, of(70), of(85)) })
+)))

source/examples/generated/AggExpressionsTest.snippet.convenience-aggregation.kt

@@ -0,0 +1,9 @@
+val graduationYear = current().getString("graduationYear")
+
+listOf(
+    Aggregates.addFields(
+        Field("reunionYear",
+            graduationYear
+                .parseInteger()
+                .add(5))
+))

source/examples/generated/AggExpressionsTest.snippet.conversion-aggregation.kt

@@ -0,0 +1,9 @@
+val deliveryDate = current().getString("deliveryDate")
+
+listOf(
+    Aggregates.match(
+        Filters.expr(deliveryDate
+            .parseDate()
+            .dayOfWeek(of("America/New_York"))
+            .eq(of(2))
+)))