/* =============================================================================
 * b_polygon_clip.ts
 * 선분 · 점열을 폴리곤 경계에서 자르는 공용 조각 — B07 횡단 오버레이(안쪽 조각)와
 * B06 설계선 고름(벽 · 본체 밖 조각 · 137-2)이 같이 쓴다. 좌표계는 부르는 쪽 것 그대로.
 * ========================================================================== */

export type XY = [number, number];

/** 점이 폴리곤 안인가 — 오목한 벽 단면도 되도록 광선 교차 홀짝으로 본다. */
export function pointInPolygon(point: XY, polygon: XY[]): boolean {
  let inside = false;
  for (let i = 0, j = polygon.length - 1; i < polygon.length; j = i, i += 1) {
    const [xi, yi] = polygon[i];
    const [xj, yj] = polygon[j];
    const straddles = yi > point[1] !== yj > point[1];
    if (straddles && point[0] < ((xj - xi) * (point[1] - yi)) / (yj - yi) + xi) inside = !inside;
  }
  return inside;
}

/** 선분을 폴리곤 경계에서 잘라 **안쪽 조각들**(`keep = "outside"` 면 바깥 조각들)만 돌려준다. */
export function clipSegment(
  start: XY,
  end: XY,
  polygon: XY[],
  keep: "inside" | "outside" = "inside",
): XY[][] {
  const [x0, y0] = start;
  const [x1, y1] = end;
  const dx = x1 - x0;
  const dy = y1 - y0;
  const cuts = [0, 1];
  for (let i = 0, j = polygon.length - 1; i < polygon.length; j = i, i += 1) {
    const [ex, ey] = polygon[j];
    const [fx, fy] = polygon[i];
    const denominator = dx * (fy - ey) - dy * (fx - ex);
    if (Math.abs(denominator) < 1e-12) continue;
    const t = ((ex - x0) * (fy - ey) - (ey - y0) * (fx - ex)) / denominator;
    const u = ((ex - x0) * dy - (ey - y0) * dx) / denominator;
    if (t > 0 && t < 1 && u >= 0 && u <= 1) cuts.push(t);
  }
  cuts.sort((a, b) => a - b);
  const runs: XY[][] = [];
  for (let index = 0; index + 1 < cuts.length; index += 1) {
    const from = cuts[index];
    const to = cuts[index + 1];
    if (to - from < 1e-9) continue;
    const mid = (from + to) / 2;
    if (pointInPolygon([x0 + dx * mid, y0 + dy * mid], polygon) !== (keep === "inside")) continue;
    runs.push([
      [x0 + dx * from, y0 + dy * from],
      [x0 + dx * to, y0 + dy * to],
    ]);
  }
  return runs;
}

/** 점열을 폴리곤 안으로 자른다 — 조각마다 한 줄. */
export function clipPointsToPolygon(points: XY[], polygon: XY[]): XY[][] {
  const runs: XY[][] = [];
  for (let index = 0; index + 1 < points.length; index += 1) {
    runs.push(...clipSegment(points[index], points[index + 1], polygon));
  }
  return runs;
}