GH-35830: [C++] Fix fixed-size list scalar hashing with non-zero offsets

cpp/src/arrow/scalar.cc

@@ -179,17 +179,25 @@ struct ScalarHashImpl {
 
     // Hash the relevant child arrays for each type taking offset and length
     // from the parent array into account if necessary.
+    // - STRUCT: children share parent's offset/length
+    // - FIXED_SIZE_LIST: children use offset*list_size, length*list_size
+    // - Others (LIST, MAP, UNION, LIST_VIEW): have their own offset mechanisms
     switch (a.type->id()) {
       case Type::STRUCT:
         for (const auto& child : a.child_data) {
           RETURN_NOT_OK(ArrayHash(child, offset, length));
         }
         break;
-        // TODO(GH-35830): Investigate what should be the correct behavior for
-        // each nested type.
+      case Type::FIXED_SIZE_LIST: {
+        const auto& list_type = checked_cast<const FixedSizeListType&>(*a.type);
+        const int32_t list_size = list_type.list_size();
+        for (const auto& child : a.child_data) {
+          RETURN_NOT_OK(ArrayHash(child, offset * list_size, length * list_size));
+        }
+        break;
+      }
       default:
-        // By default, just hash the arrays without considering
-        // the offset and length of the parent.
+        // LIST, MAP, UNION, LIST_VIEW have their own offset mechanisms
         for (const auto& child : a.child_data) {
           RETURN_NOT_OK(ArrayHash(child));
         }

cpp/src/arrow/scalar_test.cc

@@ -1419,6 +1419,18 @@ TEST(TestFixedSizeListScalar, Cast) {
   ASSERT_EQ(casted_str->ToString(), scalar.ToString());
 }
 
+TEST(TestFixedSizeListScalar, Hashing) {
+  auto inner_type = fixed_size_list(int32(), 2);
+  auto outer_type = fixed_size_list(inner_type, 3);
+  auto g = ArrayFromJSON(outer_type,
+                         "[[[1, 2], [3, 4], [5, 6]], [[7, 8], [9, 10], [11, 12]]]");
+  auto h = ArrayFromJSON(outer_type, "[[[7, 8], [9, 10], [11, 12]]]");
+  ASSERT_OK_AND_ASSIGN(auto g1, g->GetScalar(1));
+  ASSERT_OK_AND_ASSIGN(auto h0, h->GetScalar(0));
+  ASSERT_EQ(*g1, *h0);
+  ASSERT_EQ(g1->hash(), h0->hash());
+}
+
 TEST(TestMapScalar, Basics) {
   auto value =
       ArrayFromJSON(struct_({field("key", utf8(), false), field("value", int8())}),

python/pyarrow/tests/test_scalars.py

@@ -193,6 +193,15 @@ def test_hashing_struct_scalar():
     assert hash1 == hash2
 
 
+def test_hashing_fixed_size_list_scalar():
+    # GH-35830
+    inner = pa.list_(pa.int32(), 2)
+    outer = pa.list_(inner, 3)
+    g = pa.array([[[1, 2], [3, 4], [5, 6]], [[7, 8], [9, 10], [11, 12]]], type=outer)
+    h = pa.array([[[7, 8], [9, 10], [11, 12]]], type=outer)
+    assert hash(g[1]) == hash(h[0])
+
+
 @pytest.mark.timezone_data
 def test_timestamp_scalar():
     a = repr(pa.scalar("0000-01-01").cast(pa.timestamp("s")))