From 789922cedbd7deabba8db5e40d124905d7176e6d Mon Sep 17 00:00:00 2001
From: umsangdon <umsangdon@gmail.com>
Date: Fri, 16 Oct 2026 20:30:02 +0900
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- reach 너머 설계선 점 거름 · 꼬리가 reach 에서 수직으로 옵셋선까지 닫음
- 시험 test_288 · helper_288 에 양측절토 + reach 형상 더함

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01TteK8mfYZetk5P3bEZkyVZ
---
 B06_Section/B06_Section_UI_Cross_Underdrain.ts |  8 +++++++-
 resources/tester/helper_288_underdrain.cjs     | 12 ++++++++++++
 resources/tester/test_288_underdrain_cross.py  | 16 ++++++++++++++++
 3 files changed, 35 insertions(+), 1 deletion(-)

diff --git a/B06_Section/B06_Section_UI_Cross_Underdrain.ts b/B06_Section/B06_Section_UI_Cross_Underdrain.ts
index 743c564ef..cb41c0033 100644
--- a/B06_Section/B06_Section_UI_Cross_Underdrain.ts
+++ b/B06_Section/B06_Section_UI_Cross_Underdrain.ts
@@ -5,6 +5,7 @@
  *   · 노면 = 설계 노면 — 서버 · TS 짝이 암거 경사로 지대 낮은 쪽 한 경사로 기울여 냄 · 측구 없음.
  *   · 옵셋선 = 노면을 암거 높이만큼 아래로 같은 각도.
  *   · 성토 쪽 = 옵셋선을 성토 사면과 만날 때까지 · 절토 쪽 = 절토 사면을 아래로 늘여 옵셋선과 만남.
+ *   · 양측절토(낮은 쪽 늘인 노면 `tilt_reach`) = 늘인 노면이 원지반과 만나는 점에서 수직으로 끊음(356-1).
  *   · 노면 · 양쪽 선 · 옵셋선이 둘러싼 안 = 돌 무늬(306-1) — 켜마다 반 칸 엇갈리고 이웃과 겹친 돌 ·
  *     기울기를 번갈아 서로 교차 · 그림 전용(누름 안 받음 · CSS `.b06-chart__underdrain`).
  * 높이가 비면 그리지 않음(수치를 지어내지 않음).
@@ -93,7 +94,12 @@ export function underdrainOutline(
   const span = left.offset_m - right.offset_m;
   if (!(Math.abs(span) > 1e-9)) return null;
   const grade = (left.elevation_m - right.elevation_m) / span;
-  const line = design.design_line ?? [];
+  // 양측절토(356-1) — 낮은 쪽 늘인 노면이 원지반과 만나는 점(tilt_reach)에서 끊음 · 너머 점 거름 → 꼬리가 수직으로 닫음
+  const reach = design.tilt_reach;
+  const outward = reach?.side === "left" ? 1 : -1;
+  const line = (design.design_line ?? []).filter(
+    (p) => !reach || (p.offset_m - reach.offset_m) * outward <= 1e-6,
+  );
   const leftPath = sidePath(left, 1, line, grade, heightM);
   const rightPath = sidePath(right, -1, line, grade, heightM);
   return [
diff --git a/resources/tester/helper_288_underdrain.cjs b/resources/tester/helper_288_underdrain.cjs
index 5a81ab179..eaf9c50ed 100644
--- a/resources/tester/helper_288_underdrain.cjs
+++ b/resources/tester/helper_288_underdrain.cjs
@@ -65,6 +65,17 @@ const design = {
     { offset_m: -7, elevation_m: 97.2 },
   ],
 };
+// 356-1 양측절토 — 우(낮은 쪽) 노면 끝도 절토 → 노면을 늘여 원지반과 만난 점(tilt_reach) · 그 너머 원지반 오름
+const bothCut = {
+  ...design,
+  design_line: [
+    ...design.design_line.slice(0, 3),
+    { offset_m: -4, elevation_m: 99.8 },
+    { offset_m: -4.5, elevation_m: 99.775 },
+    { offset_m: -6, elevation_m: 100.5 },
+  ],
+  tilt_reach: { side: "right", offset_m: -4.5, elevation_m: 99.775 },
+};
 const svg = new Node("g");
 const group = draw.appendUnderdrainOverlay(
   svg,
@@ -96,6 +107,7 @@ process.stdout.write(
     attached,
     sections: sections.map((s) => s.underdrain ?? null),
     outline: draw.underdrainOutline(design, 1),
+    bothCut: draw.underdrainOutline(bothCut, 1),
     noHeight: draw.underdrainOutline(design, null),
     drew: !!group,
     bodyId: body?.attrs["data-structure-id"] ?? null,
diff --git a/resources/tester/test_288_underdrain_cross.py b/resources/tester/test_288_underdrain_cross.py
index 51107f608..50c5526c8 100644
--- a/resources/tester/test_288_underdrain_cross.py
+++ b/resources/tester/test_288_underdrain_cross.py
@@ -131,6 +131,22 @@ def test_형상_옵셋선_성토_절토와_만남(ts):
     assert ts["drew"] is True and ts["stones"] > 0
 
 
+def test_양측절토_늘인_노면_원지반_교점에서_끊음(ts):
+    """356-1 — 낮은 쪽 노면 끝도 절토(tilt_reach) = reach 에서 수직으로 끊음 · 너머 원지반 따라가지 않음."""
+    outline = [(p["offset"], p["elevation"]) for p in ts["bothCut"]]
+    expected = [
+        (3, 100.15),
+        (-3, 99.85),
+        (-4, 99.8),
+        (-4.5, 99.775),  # 늘인 노면 ∩ 원지반(tilt_reach)
+        (-4.5, 98.775),  # reach 아래 옵셋선
+        (1.846154, 99.092308),
+    ]
+    assert len(outline) == len(expected)
+    for got, want in zip(outline, expected):
+        assert got == pytest.approx(want, abs=1e-5)
+
+
 def test_횡단도_B07_이_맹암거를_그림():
     view = (ROOT / "B06_Section" / "B06_Section_UI_Cross_View.ts").read_text(encoding="utf-8")
     assert (