add countDistinct()

Author: cherylEnkidu

Firestore/Swift/Source/ExpressionImplementation.swift

@@ -383,6 +383,10 @@ public extension Expression {
 
   // --- Added Aggregate Operations (on Expr) ---
 
+  func countDistinct() -> AggregateFunction {
+    return AggregateFunction("count_distinct", [self])
+  }
+
   func count() -> AggregateFunction {
     return AggregateFunction("count", [self])
   }

Firestore/Swift/Source/SwiftAPI/Pipeline/Expressions/Expression.swift

@@ -1054,6 +1054,16 @@ public protocol Expression: Sendable {
 
   // MARK: Aggregations
 
+  /// Creates an aggregation that counts the number of distinct values of this expression.
+  ///
+  /// ```swift
+  /// // Count the number of distinct categories.
+  /// Field("category").countDistinct().as("distinctCategories")
+  /// ```
+  ///
+  /// - Returns: A new `AggregateFunction` representing the "count_distinct" aggregation.
+  func countDistinct() -> AggregateFunction
+
   /// Creates an aggregation that counts the number of stage inputs where this expression evaluates
   /// to a valid, non-null value.
   ///

Firestore/Swift/Tests/Integration/PipelineTests.swift

@@ -824,6 +824,31 @@ class PipelineIntegrationTests: FSTIntegrationTestCase {
     }
   }
 
+  func testReturnsCountDistinctAccumulation() async throws {
+    let collRef = collectionRef(withDocuments: bookDocs)
+    let db = collRef.firestore
+
+    let pipeline = db.pipeline()
+      .collection(collRef.path)
+      .aggregate([
+        Field("genre").countDistinct().as("distinctGenres"),
+      ])
+
+    let snapshot = try await pipeline.execute()
+
+    XCTAssertEqual(snapshot.results.count, 1, "Aggregate should return a single document")
+
+    let expectedValues: [String: Sendable] = [
+      "distinctGenres": 8,
+    ]
+
+    if let result = snapshot.results.first {
+      TestHelper.compare(pipelineResult: result, expected: expectedValues)
+    } else {
+      XCTFail("No result for countDistinct aggregation")
+    }
+  }
+
   // Hide this test due to `.countIf()` design is incomplete.
 //  func testReturnsCountIfAccumulation() async throws {
 //    let collRef = collectionRef(withDocuments: bookDocs)