langchain[patch]: inconsistent results with `RecursiveCharacterTextSplitter`'s `add_start_index=True` (#16583)

This PR fixes issue #16579

libs/langchain/langchain/text_splitter.py

@@ -141,12 +141,15 @@ class TextSplitter(BaseDocumentTransformer, ABC):
         _metadatas = metadatas or [{}] * len(texts)
         documents = []
         for i, text in enumerate(texts):
-            index = -1
+            index = 0
+            previous_chunk_len = 0
             for chunk in self.split_text(text):
                 metadata = copy.deepcopy(_metadatas[i])
                 if self._add_start_index:
-                    index = text.find(chunk, index + 1)
+                    offset = index + previous_chunk_len - self._chunk_overlap
+                    index = text.find(chunk, max(0, offset))
                     metadata["start_index"] = index
+                    previous_chunk_len = len(chunk)
                 new_doc = Document(page_content=chunk, metadata=metadata)
                 documents.append(new_doc)
         return documents

libs/langchain/tests/unit_tests/test_text_splitter.py

@@ -13,6 +13,7 @@ from langchain.text_splitter import (
     MarkdownHeaderTextSplitter,
     PythonCodeTextSplitter,
     RecursiveCharacterTextSplitter,
+    TextSplitter,
     Tokenizer,
     split_text_on_tokens,
 )
@@ -169,19 +170,47 @@ def test_create_documents_with_metadata() -> None:
     assert docs == expected_docs
 
 
-def test_create_documents_with_start_index() -> None:
+@pytest.mark.parametrize(
+    "splitter, text, expected_docs",
+    [
+        (
+            CharacterTextSplitter(
+                separator=" ", chunk_size=7, chunk_overlap=3, add_start_index=True
+            ),
+            "foo bar baz 123",
+            [
+                Document(page_content="foo bar", metadata={"start_index": 0}),
+                Document(page_content="bar baz", metadata={"start_index": 4}),
+                Document(page_content="baz 123", metadata={"start_index": 8}),
+            ],
+        ),
+        (
+            RecursiveCharacterTextSplitter(
+                chunk_size=6,
+                chunk_overlap=0,
+                separators=["\n\n", "\n", " ", ""],
+                add_start_index=True,
+            ),
+            "w1 w1 w1 w1 w1 w1 w1 w1 w1",
+            [
+                Document(page_content="w1 w1", metadata={"start_index": 0}),
+                Document(page_content="w1 w1", metadata={"start_index": 6}),
+                Document(page_content="w1 w1", metadata={"start_index": 12}),
+                Document(page_content="w1 w1", metadata={"start_index": 18}),
+                Document(page_content="w1", metadata={"start_index": 24}),
+            ],
+        ),
+    ],
+)
+def test_create_documents_with_start_index(
+    splitter: TextSplitter, text: str, expected_docs: List[Document]
+) -> None:
     """Test create documents method."""
-    texts = ["foo bar baz 123"]
-    splitter = CharacterTextSplitter(
-        separator=" ", chunk_size=7, chunk_overlap=3, add_start_index=True
-    )
-    docs = splitter.create_documents(texts)
-    expected_docs = [
-        Document(page_content="foo bar", metadata={"start_index": 0}),
-        Document(page_content="bar baz", metadata={"start_index": 4}),
-        Document(page_content="baz 123", metadata={"start_index": 8}),
-    ]
+    docs = splitter.create_documents([text])
     assert docs == expected_docs
+    for doc in docs:
+        s_i = doc.metadata["start_index"]
+        assert text[s_i : s_i + len(doc.page_content)] == doc.page_content
 
 
 def test_metadata_not_shallow() -> None: