feat(layout): follow cached lines beside a square object on the paper

CHANGELOG.md

@@ -6,6 +6,9 @@
 
 ### 고침
 
+- 쪽 수 추정(실험, `estimate_pages`)이 곁에 자리를 남기는 종이·쪽 기준 어울림(SQUARE) 개체의 띠에 닿는 줄을, 그 줄이
+  줄 캐시를 가질 때 따른다. 한/글은 그런 줄을 같은 높이·간격으로 개체 곁에 좁혀 둔다(양옆에 자리가 있으면 한 높이에
+  두 조각). 캐시 없는 줄·표·개체가 닿으면 전처럼 지원 밖이다.
 - 쪽 수 추정(실험, `estimate_pages`)이 같은 폭의 여러 단 쪽에서 종이 기준으로 둔 위아래 배치 개체가 본문 너비를
   다 덮지 않으면 지원 밖(`objects placed on the paper`)으로 두던 것을 고친다. 한/글은 개체가 가로로 걸치는
   단에서만 줄을 개체 아래로 보내고, 걸치지 않는 단의 줄은 개체가 없을 때 자리에 둔다. 이제 종이의 왼쪽·가운데·

src/hwpx/layout/pages.py

@@ -40,7 +40,8 @@
   reaches over from the paper's left, centre or right, the others running past it), and so does one
   wrapped square there that leaves less than a line's room
   (1440) on either side of it across the text; one that leaves more (in one column) changes nothing
-  when no line, table or object of its page reaches its band, and is not followed otherwise. An
+  when no line, table or object of its page reaches its band, and a line keeping its layout cache
+  that reaches it stays where it is (beside it); anything else reaching it is not followed. An
   object anchored at the top of a paragraph's first line that such a band pushes down stands right
   below the band, the line below the object; a table flowing with the text alone in its paragraph
   stands there too, its paragraph's line below it. A table or picture set as a character is one
@@ -930,6 +931,8 @@ class _Para:
     reflow: _Reflow | None = None
     #: holding nothing, in a section hiding a page's first empty lines (see :meth:`_Paginator._paragraph`)
     hides: bool = False
+    #: its lines are Hancom's own, from the paragraph's valid layout cache
+    kept: bool = False
 
     def height(self, line: int) -> int:
         return self.cached[line][0] if self.cached else self.size
@@ -2005,8 +2008,9 @@ def __init__(self, body: int, columns: int, notes: _NoteShape,
         self.widths = widths  # each column's width when they differ
         #: frame -> (top, bottom) of the objects placed on the paper there: no line in them
         self.bands = bands or {}
-        #: frame -> (top, bottom) of the ones leaving a line's room beside them, and whether a line, a table
-        #: or an object reaches one of them (set beside it by Hancom, which is not followed)
+        #: frame -> (top, bottom) of the ones leaving a line's room beside them, and whether a line without
+        #: a layout cache, a table or an object reaches one of them (set beside it by Hancom, which is not
+        #: followed: a line keeping its cache stays where it is)
         self.sides = sides or {}
         self.beside = False
         self.columns = columns
@@ -2081,7 +2085,8 @@ def _flow(self, para: _Para, table: _FlowTable, start: int) -> None:
             self._flow_below_band(para, table, self._below_bands(start))
             return
         self.out.append((self.frame, start))  # the anchor paragraph's line, under the table's top
-        self._reach(self.frame, start, start + para.height(0))
+        if not para.kept:
+            self._reach(self.frame, start, start + para.height(0))
         before = self.frame
         top = start - para.prev + table.offset + table.above  # from the paragraph's top, above its spacing
         frame, end = _flow_table(table, self.frame, top, self.body)
@@ -2108,7 +2113,8 @@ def _flow_below_band(self, para: _Para, table: _FlowTable, top: int) -> None:
         self.page_notes = [0, 0] if frame != self.frame else self.page_notes
         self.frame, self.table_end = frame, max(self.table_end, frame)
         self.out.append((self.frame, line))
-        self._reach(self.frame, line, line + para.height(0))
+        if not para.kept:
+            self._reach(self.frame, line, line + para.height(0))
         self.last_vp, self.last_pitch, self.pending_next = line, para.advance(0), para.next
 
     def _anchored(self, index: int, paras: list[_Para], para: _Para, start: int) -> None:
@@ -2422,7 +2428,8 @@ def _place(self, para: _Para, first: int, count: int) -> None:
             note_height, notes, _ = para.notes.get(line, (0, 0, 0))
             self.page_notes[0] += note_height
             self.page_notes[1] += notes
-            self._reach(frame, top, top + para.height(line))
+            if not para.kept:
+                self._reach(frame, top, top + para.height(line))
 
 
 @dataclass(frozen=True)
@@ -2442,6 +2449,7 @@ def _lay_section(measure: _Measure, section: Any) -> _SectionLayout:
     for paragraph in section.findall(f"{HP}p"):
         para, wrap = _wrapped_paragraph(measure, page, paragraph, wrap)
         band = _paper_band(measure, page, paragraph)
+        para = replace(para, kept=bool(_cached_metrics(paragraph)))
         paras.append(para if band is None else replace(para, paper=band))
     paper = {index: para.paper for index, para in enumerate(paras) if para.paper is not None}
     bands: dict[int, list[tuple[int, int]]] = {}

tests/test_layout_page_estimate.py

@@ -461,12 +461,34 @@ def test_a_header_wrapped_square_with_room_beside_it_is_not_followed() -> None:
     assert estimate_pages(out.getvalue()).unsupported == ("section 0: text beside an object placed on the paper",)
 
 
-def test_a_line_reaching_a_picture_on_the_paper_with_room_beside_it_is_not_followed() -> None:
-    # The picture's top 100 above the fifth line's foot: Hancom sets that line beside it, narrower.
+def test_a_line_reaching_a_picture_on_the_paper_with_room_beside_it_keeps_its_cached_place() -> None:
+    # The picture's top 100 above the fifth line's foot: Hancom sets that line beside it, narrower, at the
+    # same height. Keeping its cache, it is followed; without it, where it breaks is not.
     data = (FIXTURES / "pages_square_picture_on_paper_reaching_the_last_line.hwpx").read_bytes()
 
-    for source in (data, _without_caches(data)):
-        assert estimate_pages(source).unsupported == ("section 0: text beside an object placed on the paper",)
+    _assert_like_hancom(estimate_pages(data), data, 1)
+    assert estimate_pages(_without_caches(data)).unsupported == (
+        "section 0: text beside an object placed on the paper",)
+
+
+@pytest.mark.parametrize(
+    ("name", "pages"),
+    [
+        ("pages_square_picture_on_paper_lines_beside_it", 2),  # at the text's left, 800 below its top
+        ("pages_square_picture_on_paper_lines_on_both_sides", 2),  # at its middle: two pieces a line
+        ("pages_square_picture_on_paper_lines_beside_it_on_page_2", 5),  # anchored after a page break
+    ],
+)
+def test_lines_keeping_their_caches_beside_a_picture_on_the_paper_stay_where_hancom_put_them(name: str,
+                                                                                            pages: int) -> None:
+    # A picture wrapped square from the paper's top with room beside it, long paragraphs flowing beside
+    # it (the line of the paragraph before its anchor too): each line keeps its height and spacing there,
+    # narrower. Without the caches, where the lines beside it break is not followed.
+    data = (FIXTURES / f"{name}.hwpx").read_bytes()
+
+    _assert_like_hancom(estimate_pages(data), data, pages)
+    assert estimate_pages(_without_caches(data)).unsupported == (
+        "section 0: text beside an object placed on the paper",)
 
 
 def _with_empty_runs_in_cells(data: bytes) -> bytes: