Fix fulltext search pagination by correcting offset default value

Co-authored-by: mekarpeles <978325+mekarpeles@users.noreply.github.com>

Author: Copilot

openlibrary/core/fulltext.py

@@ -46,7 +46,7 @@ async def fulltext_search_api(params):
         return {'error': 'Error converting search engine data to JSON'}
 
 
-async def fulltext_search_async(q, page=1, offset=0, limit=100, js=False, facets=False):
+async def fulltext_search_async(q, page=1, offset=None, limit=100, js=False, facets=False):
     if offset is None:
         offset = (page - 1) * limit
     params = {

openlibrary/tests/core/test_fulltext.py

@@ -39,3 +39,37 @@ async def test_bad_json(self, httpx_mock, monkeypatch):
 
         response = await fulltext.fulltext_search_api({"q": "hello"})
         assert response == {"error": "Error converting search engine data to JSON"}
+
+    @pytest.mark.asyncio
+    async def test_pagination_offset_calculation(self, httpx_mock, monkeypatch):
+        """Test that page parameter correctly calculates offset for pagination."""
+        url = "http://mock"
+        monkeypatch.setattr(
+            config, "plugin_inside", {"search_endpoint": url}, raising=False
+        )
+        
+        # Mock a successful response
+        httpx_mock.add_response(json={"hits": {"hits": []}})
+        
+        # Test page 1 should have offset 0
+        await fulltext.fulltext_search_async("test", page=1, limit=20)
+        request = httpx_mock.get_request()
+        assert "from=0" in request.url.query.decode()
+        
+        # Reset mock
+        httpx_mock.reset(False)
+        httpx_mock.add_response(json={"hits": {"hits": []}})
+        
+        # Test page 10 should have offset 180 (9 * 20)
+        await fulltext.fulltext_search_async("test", page=10, limit=20)
+        request = httpx_mock.get_request()
+        assert "from=180" in request.url.query.decode()
+        
+        # Reset mock
+        httpx_mock.reset(False)
+        httpx_mock.add_response(json={"hits": {"hits": []}})
+        
+        # Test explicit offset overrides page
+        await fulltext.fulltext_search_async("test", page=5, offset=100, limit=20)
+        request = httpx_mock.get_request()
+        assert "from=100" in request.url.query.decode()