feuerwerk/src/editor/graph/edgeHit.ts

/** Which link a click or a rubber band means, kept out of the canvas so tests can reach it (#289). */

import type { Band } from '../framing';
import { curvePoint } from './beads';
import { type Loop, loopPoint, type Point, type Rect } from './transitions';

/** One link as the last draw laid it out, in graph units. */
export type EdgePath = { edgeId: string } & (
  | { kind: 'curve'; start: Point; end: Point }
  | { kind: 'line'; start: Point; end: Point }
  | { kind: 'loop'; loop: Loop }
);

/** A transition's name plate: clicking the name means its arrow. */
export type EdgePlate = Rect & { edgeId: string };

/** Straight pieces a curve is tested as; at 24 the error stays far below a pixel of tolerance. */
const CURVE_STEPS = 24;

export function pathPoints(path: EdgePath): Point[] {
  if (path.kind === 'line') return [path.start, path.end];
  const at = (t: number) =>
    path.kind === 'loop'
      ? loopPoint(path.loop, t)
      : curvePoint(path.start.x, path.start.y, path.end.x, path.end.y, t);
  return Array.from({ length: CURVE_STEPS + 1 }, (_, i) => at(i / CURVE_STEPS));
}

/** The point halfway along a link – where a check clicks it. */
export function pathMiddle(path: EdgePath): Point {
  if (path.kind === 'line') return { x: (path.start.x + path.end.x) / 2, y: (path.start.y + path.end.y) / 2 };
  return path.kind === 'loop'
    ? loopPoint(path.loop, 0.5)
    : curvePoint(path.start.x, path.start.y, path.end.x, path.end.y, 0.5);
}

function distanceToSegment(p: Point, a: Point, b: Point): number {
  const [dx, dy] = [b.x - a.x, b.y - a.y];
  const length = dx * dx + dy * dy;
  const t = length === 0 ? 0 : Math.min(1, Math.max(0, ((p.x - a.x) * dx + (p.y - a.y) * dy) / length));
  return Math.hypot(p.x - (a.x + dx * t), p.y - (a.y + dy * t));
}

export function distanceToPath(path: EdgePath, point: Point): number {
  const points = pathPoints(path);
  let nearest = Infinity;
  for (let i = 1; i < points.length; i++)
    nearest = Math.min(nearest, distanceToSegment(point, points[i - 1]!, points[i]!));
  return nearest;
}

/**
 * The link under a point, or null. A name plate wins over any line, and of several lines within
 * `tolerance` the one drawn last does – it is the one on top.
 */
export function edgeAt(
  paths: readonly EdgePath[],
  point: Point,
  tolerance: number,
  plates: readonly EdgePlate[] = [],
): string | null {
  for (let i = plates.length - 1; i >= 0; i--) {
    const p = plates[i]!;
    if (point.x >= p.x && point.x <= p.x + p.w && point.y >= p.y && point.y <= p.y + p.h) return p.edgeId;
  }
  for (let i = paths.length - 1; i >= 0; i--)
    if (distanceToPath(paths[i]!, point) <= tolerance) return paths[i]!.edgeId;
  return null;
}

/** Whether the segment a-b has any point inside the rectangle (Liang-Barsky). */
function segmentInRect(a: Point, b: Point, minX: number, minY: number, maxX: number, maxY: number): boolean {
  let [t0, t1] = [0, 1];
  const [dx, dy] = [b.x - a.x, b.y - a.y];
  for (const [p, q] of [
    [-dx, a.x - minX],
    [dx, maxX - a.x],
    [-dy, a.y - minY],
    [dy, maxY - a.y],
  ] as const) {
    if (p === 0) {
      if (q < 0) return false;
      continue;
    }
    const r = q / p;
    if (p < 0) t0 = Math.max(t0, r);
    else t1 = Math.min(t1, r);
    if (t0 > t1) return false;
  }
  return true;
}

/** Ids of every link the band touches, in drawing order: a line is thin, so crossing it counts. */
export function edgesInBand(paths: readonly EdgePath[], band: Band): string[] {
  const [minX, maxX] = [Math.min(band.x1, band.x2), Math.max(band.x1, band.x2)];
  const [minY, maxY] = [Math.min(band.y1, band.y2), Math.max(band.y1, band.y2)];
  return paths
    .filter((path) => {
      const points = pathPoints(path);
      return points.some((p, i) => i > 0 && segmentInRect(points[i - 1]!, p, minX, minY, maxX, maxY));
    })
    .map((path) => path.edgeId);
}