feat: support FM-Index scalar index for substring search (#3532)

Adds an FM-Index — a scalar index over string and binary columns that
accelerates substring search (`contains(col, 'needle')`), distinct from
the tokenized `FTS` index — across the Rust core and the Python and
TypeScript bindings.

## Rust

- `Index::Fm(FmIndexBuilder)` and `IndexType::Fm`.
- `make_index_params` maps `Index::Fm` to Lance's
`ScalarIndexParams::for_builtin(BuiltinIndexType::Fm)`.
- `supported_fm_data_type` validates
`Utf8`/`LargeUtf8`/`Binary`/`LargeBinary` columns.
- `list_indices` round-trips the type (`"Fm"` → `IndexType::Fm`); the
remote wire type is `"FM"`.

## Python

Adds `lancedb.index.Fm`, accepted by `create_index`:

```python
from lancedb.index import Fm

await tbl.create_index("text", config=Fm())
```

## TypeScript

Adds the `Index.fm()` factory:

```ts
await tbl.createIndex("text", { config: Index.fm() });
```

python/python/lancedb/_lancedb.pyi

@@ -10,6 +10,7 @@ from .index import (
     IvfSq,
     Bitmap,
     LabelList,
+    Fm,
     HnswPq,
     HnswSq,
     HnswFlat,
@@ -186,6 +187,7 @@ class Table:
             BTree,
             Bitmap,
             LabelList,
+            Fm,
             FTS,
         ],
         replace: Optional[bool],

python/python/lancedb/index.py

@@ -93,6 +93,20 @@ class LabelList:
     pass
 
 
+@dataclass
+class Fm:
+    """Describe an FM-Index configuration.
+
+    `Fm` is a scalar index on string or binary columns that accelerates
+    substring search, i.e. `contains(col, 'needle')`. Unlike the tokenized
+    `FTS` index, it matches arbitrary substrings of the raw bytes.
+
+    For example, it works with `url`, `path`, `content`, etc.
+    """
+
+    pass
+
+
 @dataclass
 class FTS:
     """Describe a FTS index configuration.
@@ -828,4 +842,5 @@ __all__ = [
     "FTS",
     "Bitmap",
     "LabelList",
+    "Fm",
 ]

python/python/lancedb/table.py

@@ -55,6 +55,7 @@ from .index import (
     Bitmap,
     IvfRq,
     LabelList,
+    Fm,
     HnswPq,
     HnswSq,
     HnswFlat,
@@ -213,6 +214,7 @@ IndexConfigType = Union[
     BTree,
     Bitmap,
     LabelList,
+    Fm,
     FTS,
 ]
 
@@ -938,7 +940,7 @@ class Table(ABC):
         config : IndexConfigType, optional
             The index configuration object. If provided, uses the new unified API.
             Can be one of: IvfFlat, IvfPq, IvfSq, IvfRq, HnswPq, HnswSq,
-            BTree, Bitmap, LabelList, FTS.
+            BTree, Bitmap, LabelList, Fm, FTS.
         replace : bool, default True
             Whether to replace an existing index on this column.
         wait_timeout : timedelta, optional
@@ -2487,7 +2489,7 @@ class LanceTable(Table):
         config : IndexConfigType, optional
             The index configuration object. If provided, uses the new unified API.
             Can be one of: IvfFlat, IvfPq, IvfSq, IvfRq, HnswPq, HnswSq,
-            BTree, Bitmap, LabelList, FTS.
+            BTree, Bitmap, LabelList, Fm, FTS.
         replace : bool, default True
             Whether to replace an existing index on this column.
         wait_timeout : timedelta, optional
@@ -4487,6 +4489,7 @@ class AsyncTable:
                 BTree,
                 Bitmap,
                 LabelList,
+                Fm,
                 FTS,
             ]
         ] = None,
@@ -4539,12 +4542,14 @@ class AsyncTable:
                     BTree,
                     Bitmap,
                     LabelList,
+                    Fm,
                     FTS,
                 ),
             ):
                 raise TypeError(
                     "config must be an instance of IvfSq, IvfPq, IvfRq, HnswPq, HnswSq,"
-                    " BTree, Bitmap, LabelList, or FTS, but got " + str(type(config))
+                    " BTree, Bitmap, LabelList, Fm, or FTS, but got "
+                    + str(type(config))
                 )
         try:
             await self._inner.create_index(

python/python/tests/test_index.py

@@ -20,6 +20,7 @@ from lancedb.index import (
     IvfRq,
     Bitmap,
     LabelList,
+    Fm,
     HnswPq,
     HnswSq,
     HnswFlat,
@@ -203,6 +204,16 @@ async def test_create_fixed_size_binary_index(some_table: AsyncTable):
     assert indices[0].columns == ["fsb"]
 
 
+@pytest.mark.asyncio
+async def test_create_fm_index(some_table: AsyncTable):
+    # FM-Index accelerates substring search on string/binary columns.
+    await some_table.create_index("data", config=Fm())
+    indices = await some_table.list_indices()
+    assert len(indices) == 1
+    assert indices[0].index_type == "Fm"
+    assert indices[0].columns == ["data"]
+
+
 @pytest.mark.asyncio
 async def test_create_bitmap_index(some_table: AsyncTable):
     await some_table.create_index("id", config=Bitmap())