Fix: do not increment field index for nested lists (#1303)

* Fix: do not increment field index for nested lists

* Test: check that fix works!

* Refactor: use simpler form keys

* Test: check case for multiple lists

* Test: make test more complex

* Fix: correctly reset index when wrapping

* Revert "Fix: correctly reset index when wrapping"

This reverts commit b27e480.

* Fix: correctly wrap index

src/libawkward/layoutbuilder/RecordArrayBuilder.cpp

@@ -92,8 +92,15 @@ namespace awkward {
   template <typename T, typename I>
   int64_t
   RecordArrayBuilder<T, I>::field_index() {
-    return (field_index_ < contents_size_ - 1) ?
-      field_index_++ : (field_index_ = 0);
+    if (!list_field_index_.empty())
+    {
+      return field_index_;
+    }
+    auto index = field_index_;
+    field_index_ = (++field_index_ < contents_size_)
+                       ? field_index_
+                       : field_index_ % contents_size_;
+    return index;
   }
 
   template <typename T, typename I>

tests/test_1302-layout-builder-nested-list.py

@@ -0,0 +1,89 @@
+# BSD 3-Clause License; see https://github.com/scikit-hep/awkward-1.0/blob/main/LICENSE
+
+import pytest  # noqa: F401
+import awkward as ak  # noqa: F401
+
+
+def test():
+    builder = ak.layout.LayoutBuilder64(
+        """
+    {
+        "form_key": "root",
+        "class": "RecordArray",
+        "contents": {
+            "u": {
+                "form_key": "u",
+                "class": "ListOffsetArray64",
+                "offsets": "i64",
+                "content": {
+                    "form_key": "u-content",
+                    "class": "RecordArray",
+                    "contents": {
+                        "i": {
+                            "class": "NumpyArray",
+                            "primitive": "int64",
+                            "form_key": "i"
+                        },
+                        "j": {
+                            "class": "ListOffsetArray64",
+                            "offsets": "i64",
+                            "content": "int64",
+                            "form_key": "j"
+                        }
+                    }
+                }
+            },
+            "v": {
+                "class": "NumpyArray",
+                "primitive": "int64",
+                "form_key": "v"
+            },
+            "w": {
+                "class": "NumpyArray",
+                "primitive": "int64",
+                "form_key": "w"
+            },
+            "x": {
+                "class": "NumpyArray",
+                "primitive": "int64",
+                "form_key": "x"
+            }
+        }
+    }
+    """
+    )
+
+    builder.begin_list()  # u
+
+    builder.int64(1)  # i
+    builder.begin_list()  # j
+    builder.int64(9)
+    builder.int64(8)
+    builder.int64(7)
+    builder.end_list()  # j
+
+    builder.end_list()  # u
+
+    builder.int64(2)  # v
+    builder.int64(3)  # w
+    builder.int64(4)  # x
+
+    builder.begin_list()  # u
+
+    builder.int64(1)  # i
+    builder.begin_list()  # j
+    builder.int64(9)
+    builder.int64(8)
+    builder.int64(7)
+    builder.end_list()  # j
+
+    builder.end_list()  # u
+
+    builder.int64(2)  # v
+    builder.int64(3)  # w
+    builder.int64(4)  # x
+
+    assert ak.to_list(builder.snapshot()) == [
+        {"u": [{"i": 1, "j": [9, 8, 7]}], "v": 2, "w": 3, "x": 4},
+        {"u": [{"i": 1, "j": [9, 8, 7]}], "v": 2, "w": 3, "x": 4},
+    ]