Count unchanged lines in Text Comparison

docs/tool-guide.md

@@ -49,8 +49,10 @@ Unknown algorithms raise `ValueError`.
 
 ## compare_text(before, after)
 
-Returns `added_lines`, `removed_lines`, and a unified `diff` string. A replacement
-counts as both removed and added lines. Identical inputs return an empty diff.
+Returns `added_lines`, `removed_lines`, `unchanged_lines`, and a unified `diff`
+string. A replacement counts as both removed and added lines, and `unchanged_lines`
+counts the matching lines in the same alignment, so `added_lines + unchanged_lines`
+equals the number of lines in `after`. Identical inputs return an empty diff.
 Comparison ignores CRLF versus LF and the presence of a final newline;
 other whitespace is significant. Empty input is supported.
 

src/mcp_tools_lab/tools/diff.py

@@ -4,14 +4,19 @@
 
 
 def compare_text(before: str, after: str) -> dict:
-    """Return a unified diff and counts; ignore line-ending style and final newline."""
+    """Return a unified diff and counts; ignore line-ending style and final newline.
+
+    unchanged_lines counts the lines the alignment marks as equal.
+    """
     old_lines, new_lines = before.splitlines(), after.splitlines()
-    added = removed = 0
+    added = removed = unchanged = 0
     matcher = difflib.SequenceMatcher(None, old_lines, new_lines, autojunk=False)
     for tag, old_start, old_end, new_start, new_end in matcher.get_opcodes():
         if tag in {"replace", "delete"}:
             removed += old_end - old_start
         if tag in {"replace", "insert"}:
             added += new_end - new_start
+        if tag == "equal":
+            unchanged += old_end - old_start
     diff = "\n".join(difflib.unified_diff(old_lines, new_lines, fromfile="before", tofile="after", lineterm=""))
-    return {"added_lines": added, "removed_lines": removed, "diff": diff}
+    return {"added_lines": added, "removed_lines": removed, "unchanged_lines": unchanged, "diff": diff}

tests/test_diff.py

@@ -14,3 +14,30 @@ def test_equal_empty_and_newlines():
     assert compare_text("same\r\n", "same")["diff"] == ""
     assert compare_text("", "new")["added_lines"] == 1
     assert compare_text("old", "")["removed_lines"] == 1
+
+
+def test_unchanged_lines_for_replacement():
+    result = compare_text("hello\nold", "hello\nnew")
+    assert result["unchanged_lines"] == 1
+    assert result["added_lines"] == 1
+    assert result["removed_lines"] == 1
+
+
+def test_unchanged_lines_identical_and_empty():
+    assert compare_text("one\ntwo", "one\ntwo")["unchanged_lines"] == 2
+    assert compare_text("", "")["unchanged_lines"] == 0
+
+
+def test_unchanged_lines_entirely_different():
+    assert compare_text("old", "new")["unchanged_lines"] == 0
+
+
+def test_unchanged_lines_ignores_line_endings_and_final_newline():
+    assert compare_text("same\r\n", "same")["unchanged_lines"] == 1
+
+
+def test_unchanged_lines_with_repeated_lines():
+    result = compare_text("a\nb\nb\nc", "a\nb\nc\nc")
+    assert result["unchanged_lines"] + result["added_lines"] == 4
+    assert result["unchanged_lines"] + result["removed_lines"] == 4
+    assert result["unchanged_lines"] == 3