SERVER-32873 Fix incorrect handling of NumberLong in $pow function

Signed-off-by: Charlie Swanson <charlie.swanson@mongodb.com>

Closes #1211

2 files changed, 28 insertions, 3 deletions

diff --git a/src/mongo/db/pipeline/expression.cpp b/src/mongo/db/pipeline/expression.cpp
index 3c4b5ef3dae..3ab0c300cd3 100644
--- a/src/mongo/db/pipeline/expression.cpp
+++ b/src/mongo/db/pipeline/expression.cpp
@@ -3154,14 +3154,23 @@ Value ExpressionPow::evaluate(const Document& root) const {
     long long baseLong = baseVal.getLong();
     long long expLong = expVal.getLong();
 
-    // If the result cannot be represented as a long, return a double. Otherwise if either number
-    // is a long, return a long. If both numbers are ints, then return an int if the result fits or
-    // a long if it is too big.
+    // If the result cannot be represented as a long, return a double. Otherwise if either number is
+    // a long, return a long. If both numbers are ints, then return an int if the result fits or a
+    // long if it is too big.
     if (!representableAsLong(baseLong, expLong)) {
         return Value(std::pow(baseLong, expLong));
     }
 
     long long result = 1;
+
+    // When 'baseLong' == -1 and 'expLong' is < 0 the following for loop will never run because
+    // 'expLong' will always be less than 0 so result will always be 1. This is not always correct
+    // because the result can potentially be -1. ex: 'baselong' = -1 'expLong' = -5 then result
+    // should be -1.
+    if (baseLong == -1 && expLong < 0) {
+        expLong = expLong % 2 == 0 ? 2 : 1;
+    }
+
     // Use repeated multiplication, since pow() casts args to doubles which could result in loss of
     // precision if arguments are very large.
     for (int i = 0; i < expLong; i++) {
diff --git a/src/mongo/db/pipeline/expression_test.cpp b/src/mongo/db/pipeline/expression_test.cpp
index b2610e97bdb..345c5042f09 100644
--- a/src/mongo/db/pipeline/expression_test.cpp
+++ b/src/mongo/db/pipeline/expression_test.cpp
@@ -2193,6 +2193,22 @@ TEST(ExpressionFromAccumulators, StdDevSamp) {
          {{}, Value(BSONNULL)}});
 }
 
+TEST(ExpressionPowTest, NegativeOneRaisedToNegativeOddExponentShouldOutPutNegativeOne) {
+    assertExpectedResults("$pow",
+                          {
+                              {{Value(-1), Value(-1)}, Value(-1)},
+                              {{Value(-1), Value(-2)}, Value(1)},
+                              {{Value(-1), Value(-3)}, Value(-1)},
+
+                              {{Value(-1LL), Value(0LL)}, Value(1LL)},
+                              {{Value(-1LL), Value(-1LL)}, Value(-1LL)},
+                              {{Value(-1LL), Value(-2LL)}, Value(1LL)},
+                              {{Value(-1LL), Value(-3LL)}, Value(-1LL)},
+                              {{Value(-1LL), Value(-4LL)}, Value(1LL)},
+                              {{Value(-1LL), Value(-5LL)}, Value(-1LL)},
+                          });
+}
+
 namespace FieldPath {
 
 /** The provided field path does not pass validation. */