From 5e478bc00048e76ed28bff4a4f9ea3fd7296cdcf Mon Sep 17 00:00:00 2001
From: Benedikt Waldvogel <mail@bwaldvogel.de>
Date: Wed, 31 May 2023 10:22:26 +0200
Subject: [PATCH] Fix return type of $split aggregation #214

This is relevant if the values are processed in other expressions.
---
 .../mongo/backend/aggregation/Expression.java |  3 ++-
 .../mongo/wire/bson/BsonEncoder.java          |  6 ++---
 .../backend/aggregation/ExpressionTest.java   |  4 ++--
 .../backend/AbstractAggregationTest.java      | 22 +++++++++++++++++++
 4 files changed, 28 insertions(+), 7 deletions(-)

diff --git a/core/src/main/java/de/bwaldvogel/mongo/backend/aggregation/Expression.java b/core/src/main/java/de/bwaldvogel/mongo/backend/aggregation/Expression.java
index 46e24ea2..47a116ed 100644
--- a/core/src/main/java/de/bwaldvogel/mongo/backend/aggregation/Expression.java
+++ b/core/src/main/java/de/bwaldvogel/mongo/backend/aggregation/Expression.java
@@ -1379,7 +1379,8 @@ Object apply(List<?> expressionValue, Document document) {
                     name() + " requires an expression that evaluates to a string as a second argument, found: " + describeType(delimiter));
             }
 
-            return ((String) string).split(Pattern.quote((String) delimiter));
+            String[] values = ((String) string).split(Pattern.quote((String) delimiter), -1);
+            return List.of(values);
         }
     },
 
diff --git a/core/src/main/java/de/bwaldvogel/mongo/wire/bson/BsonEncoder.java b/core/src/main/java/de/bwaldvogel/mongo/wire/bson/BsonEncoder.java
index c3107cde..2f4dea3a 100644
--- a/core/src/main/java/de/bwaldvogel/mongo/wire/bson/BsonEncoder.java
+++ b/core/src/main/java/de/bwaldvogel/mongo/wire/bson/BsonEncoder.java
@@ -184,7 +184,7 @@ public static byte determineType(Object value) {
             return BsonConstants.TYPE_BOOLEAN;
         } else if (value instanceof BinData || value instanceof UUID || value instanceof LegacyUUID) {
             return BsonConstants.TYPE_DATA;
-        } else if (value instanceof Collection<?> || value instanceof String[]) {
+        } else if (value instanceof Collection<?>) {
             return BsonConstants.TYPE_ARRAY;
         } else if (value instanceof Instant) {
             return BsonConstants.TYPE_UTC_DATETIME;
@@ -206,9 +206,7 @@ public static byte determineType(Object value) {
     }
 
     private static List<?> collectionToList(Object value) {
-        if (value instanceof String[]) {
-            return List.of((String[]) value);
-        } else if (value instanceof List<?>) {
+        if (value instanceof List<?>) {
             return (List<?>) value;
         } else {
             return new ArrayList<>((Collection<?>) value);
diff --git a/core/src/test/java/de/bwaldvogel/mongo/backend/aggregation/ExpressionTest.java b/core/src/test/java/de/bwaldvogel/mongo/backend/aggregation/ExpressionTest.java
index 87085f10..0a9b5415 100644
--- a/core/src/test/java/de/bwaldvogel/mongo/backend/aggregation/ExpressionTest.java
+++ b/core/src/test/java/de/bwaldvogel/mongo/backend/aggregation/ExpressionTest.java
@@ -1087,8 +1087,8 @@ void testEvaluateSlice() throws Exception {
 
     @Test
     void testEvaluateSplit() throws Exception {
-        assertThat((String[]) Expression.evaluate(json("$split: ['June-15-2013', '-']"), json(""))).containsExactly("June", "15", "2013");
-        assertThat((String[]) Expression.evaluate(json("$split: ['$a', '$b']"), json("a: 'foo bar', b: ' '"))).containsExactly("foo", "bar");
+        assertThat((List<String>) Expression.evaluate(json("$split: ['June-15-2013', '-']"), json(""))).containsExactly("June", "15", "2013");
+        assertThat((List<String>) Expression.evaluate(json("$split: ['$a', '$b']"), json("a: 'foo bar', b: ' '"))).containsExactly("foo", "bar");
         assertThat(Expression.evaluate(json("$split: [null, ' ']"), json(""))).isNull();
         assertThat(Expression.evaluate(json("$split: ['$doesNotExist', ' ']"), json(""))).isNull();
 
diff --git a/test-common/src/main/java/de/bwaldvogel/mongo/backend/AbstractAggregationTest.java b/test-common/src/main/java/de/bwaldvogel/mongo/backend/AbstractAggregationTest.java
index 6150a820..cb252614 100644
--- a/test-common/src/main/java/de/bwaldvogel/mongo/backend/AbstractAggregationTest.java
+++ b/test-common/src/main/java/de/bwaldvogel/mongo/backend/AbstractAggregationTest.java
@@ -1177,6 +1177,28 @@ void testAggregateWithSplit() throws Exception {
             );
     }
 
+    // https://github.com/bwaldvogel/mongo-java-server/issues/214
+    @Test
+    void testAggregateWithSplitAndArrayElementAt() throws Exception {
+        List<Document> pipeline = jsonList("$addFields: { pathSegments: { $split: ['$path', '/'] }}",
+            "$project: { pathSegments: 1, firstSegment: { $arrayElemAt: ['$pathSegments', 0] }}");
+
+        assertThat(collection.aggregate(pipeline)).isEmpty();
+
+        collection.insertOne(json("_id: 1, path: 'path/to/file'"));
+        collection.insertOne(json("_id: 2, path: '/path/to/file'"));
+        collection.insertOne(json("_id: 3, path: '/path/to/file/'"));
+        collection.insertOne(json("_id: 4"));
+
+        assertThat(collection.aggregate(pipeline))
+            .containsExactly(
+                json("_id: 1, pathSegments: ['path', 'to', 'file'], firstSegment: 'path'"),
+                json("_id: 2, pathSegments: ['', 'path', 'to', 'file'], firstSegment: ''"),
+                json("_id: 3, pathSegments: ['', 'path', 'to', 'file', ''], firstSegment: ''"),
+                json("_id: 4, pathSegments: null, firstSegment: null")
+            );
+    }
+
     @Test
     void testAggregateWithUnwind() throws Exception {
         testAggregateWithUnwind(json("$unwind: '$sizes'"));