Fix Glancer to return correct FileWindow reflecting actual content read (#20)

The Glancer.glance() method was returning the requested FileWindow
instead of the actual window that was read from the file. For example,
requesting 100 lines from an empty file would return window with
line_count=100 instead of line_count=0.

Changes:
- Add actual_window field to FileReadResult to track what was actually read
- Update StreamingFileReader to return actual window information
- Modify Glancer.glance() to use actual_window instead of requested window
- Allow FileWindow.line_count=0 for reporting empty reads
- Add comprehensive tests covering various edge cases

Author: rbharvs

src/glob_grep_glance/_defaults.py

@@ -73,6 +73,7 @@ def read_window(
 
         contents = ""
         truncated = False
+        lines_read = 0
 
         try:
             # Open file with UTF-8 encoding and ignore errors for binary files
@@ -87,7 +88,6 @@ def read_window(
                     current_line += 1
 
                 # Read the requested number of lines
-                lines_read = 0
                 while lines_read < window.line_count:
                     line = f.readline()
                     if not line:  # EOF reached
@@ -103,7 +103,14 @@ def read_window(
         except BudgetExceeded:
             truncated = True
 
-        return FileReadResult(contents=contents, truncated=truncated)
+        # Create actual window reflecting what was actually read
+        actual_window = FileWindow(
+            line_offset=window.line_offset, line_count=lines_read
+        )
+
+        return FileReadResult(
+            contents=contents, truncated=truncated, actual_window=actual_window
+        )
 
 
 class StreamingRegexSearcher(BaseModel):

src/glob_grep_glance/common.py

@@ -24,7 +24,7 @@ class FileWindow(BaseModel):
     """Defines bounds for viewing a portion of a file."""
 
     line_offset: int = Field(ge=0, description="Starting line number (0-based)")
-    line_count: int = Field(ge=1, description="Number of lines to read")
+    line_count: int = Field(ge=0, description="Number of lines to read")
 
 
 class FileContent(BaseModel):
@@ -40,3 +40,4 @@ class FileReadResult(BaseModel):
 
     contents: str
     truncated: bool
+    actual_window: FileWindow

src/glob_grep_glance/glance.py

@@ -36,7 +36,9 @@ def glance(
         # Read the file window using the file reader
         result = self.file_reader.read_window(file_path, window, budget)
 
-        # Create FileContent from the result
-        view = FileContent(path=file_path, contents=result.contents, window=window)
+        # Create FileContent from the result using the actual window that was read
+        view = FileContent(
+            path=file_path, contents=result.contents, window=result.actual_window
+        )
 
         return GlanceOutput(view=view, truncated=result.truncated)

tests/test_file_reader.py

@@ -306,14 +306,14 @@ def test_nonexistent_file(
     def test_zero_line_count_window(
         self, temp_sandbox: tuple[Path, Sandbox], reader: StreamingFileReader
     ) -> None:
-        """Test window with zero line count (should be invalid)."""
+        """Test window with zero line count (now valid for actual windows)."""
         sandbox_dir, _ = temp_sandbox
         test_file = sandbox_dir / "test.txt"
         test_file.write_text("line 1\n", encoding="utf-8")
 
-        # FileWindow should reject line_count=0 during validation
-        with pytest.raises(ValueError):
-            FileWindow(line_offset=0, line_count=0)
+        # FileWindow should now accept line_count=0 (for actual window reporting)
+        window = FileWindow(line_offset=0, line_count=0)
+        assert window.line_count == 0
 
     def test_negative_line_offset_window(
         self, temp_sandbox: tuple[Path, Sandbox], reader: StreamingFileReader

tests/test_glancer_window_behavior.py

@@ -0,0 +1,113 @@
+"""Test cases for Glancer FileWindow behavior to ensure returned window reflects actual content."""
+
+import tempfile
+from pathlib import Path
+from typing import Generator
+
+import pytest
+
+from glob_grep_glance import FileWindow, Glancer, OutputBudget, Sandbox
+
+
+class TestGlancerWindowBehavior:
+    """Test that Glancer returns accurate FileWindow information."""
+
+    @pytest.fixture
+    def temp_sandbox(self) -> Generator[Path, None, None]:
+        """Create a temporary directory for sandboxed operations."""
+        with tempfile.TemporaryDirectory() as temp_dir:
+            sandbox_path = Path(temp_dir)
+
+            # Create test files
+            (sandbox_path / "empty.txt").write_text("")
+            (sandbox_path / "one_line.txt").write_text("single line\n")
+            (sandbox_path / "three_lines.txt").write_text("line 1\nline 2\nline 3\n")
+
+            yield sandbox_path
+
+    @pytest.fixture
+    def sandbox(self, temp_sandbox: Path) -> Sandbox:
+        """Create a sandbox configuration."""
+        return Sandbox(sandbox_dir=temp_sandbox, blocked_files=[], allow_hidden=False)
+
+    @pytest.fixture
+    def budget(self) -> OutputBudget:
+        """Create an output budget for testing."""
+        return OutputBudget(limit=1000)
+
+    def test_empty_file_returns_correct_window(
+        self, sandbox: Sandbox, budget: OutputBudget
+    ) -> None:
+        """Test that empty file returns window with line_count=0."""
+        glancer = Glancer.from_sandbox(sandbox)
+        empty_file = sandbox.sandbox_dir / "empty.txt"
+        window = FileWindow(line_offset=0, line_count=100)
+
+        result = glancer.glance(empty_file, window, budget)
+
+        # Should return window reflecting what was actually read (0 lines)
+        assert result.view.window.line_offset == 0
+        assert result.view.window.line_count == 0
+        assert result.view.contents == ""
+
+    def test_request_more_lines_than_available(
+        self, sandbox: Sandbox, budget: OutputBudget
+    ) -> None:
+        """Test requesting more lines than file contains."""
+        glancer = Glancer.from_sandbox(sandbox)
+        one_line_file = sandbox.sandbox_dir / "one_line.txt"
+        window = FileWindow(line_offset=0, line_count=5)
+
+        result = glancer.glance(one_line_file, window, budget)
+
+        # Should return window reflecting what was actually read (1 line)
+        assert result.view.window.line_offset == 0
+        assert result.view.window.line_count == 1
+        assert "single line" in result.view.contents
+
+    def test_offset_beyond_file_end(
+        self, sandbox: Sandbox, budget: OutputBudget
+    ) -> None:
+        """Test offset beyond end of file."""
+        glancer = Glancer.from_sandbox(sandbox)
+        three_line_file = sandbox.sandbox_dir / "three_lines.txt"
+        window = FileWindow(line_offset=10, line_count=5)
+
+        result = glancer.glance(three_line_file, window, budget)
+
+        # Should return window with line_count=0 since no lines were read
+        assert result.view.window.line_offset == 10
+        assert result.view.window.line_count == 0
+        assert result.view.contents == ""
+
+    def test_partial_read_from_offset(
+        self, sandbox: Sandbox, budget: OutputBudget
+    ) -> None:
+        """Test reading from offset with more lines requested than available."""
+        glancer = Glancer.from_sandbox(sandbox)
+        three_line_file = sandbox.sandbox_dir / "three_lines.txt"
+        window = FileWindow(line_offset=2, line_count=5)  # Start at line 2, request 5
+
+        result = glancer.glance(three_line_file, window, budget)
+
+        # Should return window reflecting what was actually read (1 line from offset 2)
+        assert result.view.window.line_offset == 2
+        assert result.view.window.line_count == 1
+        assert "line 3" in result.view.contents
+
+    def test_exact_line_count_match(
+        self, sandbox: Sandbox, budget: OutputBudget
+    ) -> None:
+        """Test when requested lines exactly matches available lines."""
+        glancer = Glancer.from_sandbox(sandbox)
+        three_line_file = sandbox.sandbox_dir / "three_lines.txt"
+        window = FileWindow(line_offset=0, line_count=3)
+
+        result = glancer.glance(three_line_file, window, budget)
+
+        # Should return window reflecting what was actually read (3 lines)
+        assert result.view.window.line_offset == 0
+        assert result.view.window.line_count == 3
+        assert "line 1" in result.view.contents
+        assert "line 2" in result.view.contents
+        assert "line 3" in result.view.contents

tests/test_protocols.py

@@ -61,7 +61,11 @@ class TestFileReader:
     def test_protocol_compliance(self) -> None:
         """Test that a mock implementation satisfies the FileReader protocol."""
         mock_reader = Mock(spec=FileReader)
-        expected_result = FileReadResult(contents="test content", truncated=False)
+        expected_result = FileReadResult(
+            contents="test content",
+            truncated=False,
+            actual_window=FileWindow(line_offset=0, line_count=1),
+        )
         mock_reader.read_window = Mock(return_value=expected_result)
 
         file_path = Path("test.txt")
@@ -76,7 +80,11 @@ def test_protocol_compliance(self) -> None:
     def test_budget_constraint_handling(self) -> None:
         """Test FileReader handles budget constraints properly."""
         mock_reader = Mock(spec=FileReader)
-        expected_result = FileReadResult(contents="truncated", truncated=True)
+        expected_result = FileReadResult(
+            contents="truncated",
+            truncated=True,
+            actual_window=FileWindow(line_offset=0, line_count=1),
+        )
         mock_reader.read_window = Mock(return_value=expected_result)
 
         file_path = Path("large_file.txt")
@@ -91,7 +99,11 @@ def test_budget_constraint_handling(self) -> None:
     def test_window_parameters(self) -> None:
         """Test FileReader respects window parameters."""
         mock_reader = Mock(spec=FileReader)
-        expected_result = FileReadResult(contents="lines 5-15", truncated=False)
+        expected_result = FileReadResult(
+            contents="lines 5-15",
+            truncated=False,
+            actual_window=FileWindow(line_offset=5, line_count=10),
+        )
         mock_reader.read_window = Mock(return_value=expected_result)
 
         file_path = Path("data.txt")
@@ -179,7 +191,11 @@ def test_budget_consistency_across_protocols(self) -> None:
         # FileReader uses OutputBudget
         mock_reader = Mock(spec=FileReader)
         mock_reader.read_window = Mock(
-            return_value=FileReadResult(contents="", truncated=False)
+            return_value=FileReadResult(
+                contents="",
+                truncated=False,
+                actual_window=FileWindow(line_offset=0, line_count=0),
+            )
         )
         mock_reader.read_window(
             Path("test.txt"), FileWindow(line_offset=0, line_count=1), budget