orbit/js/tether/Tether.js

import { config } from '../config/Config.js';

const TAU = Math.PI * 2;
const EPS = 1e-9;

/**
 * A TETHER: the player's range. A tether anchors on a world or station and
 * defines a circular zone — radius = distance from the anchor's CENTER.
 *
 * Rules:
 *   - radius(level) = tether.level1Radius × tether.radiusGrowth^(level−1)
 *     (level 1 = 5120 px, per data/tether.json);
 *   - the ship may be anywhere inside the UNION of its tethers' zones —
 *     where two zones overlap there is no wall and no line;
 *   - outside all of them is a hard barrier: the ship (and its click /
 *     autopilot targets) are clamped to the union's boundary.
 *
 * This file is PURE (no Phaser): the record + the geometry the game is
 * built on — union membership, clamping to the union boundary (nearest
 * point + outward normal), and each tether's share of that boundary
 * (visibleArcs — the only place a barrier line is drawn). Node-testable:
 * dev/tether.test.mjs.
 */
export class Tether {
  /**
   * @param {string} id — unique within the field (e.g. 'home')
   * @param {number} x — world x of the anchor (a planet/station center)
   * @param {number} y — world y
   * @param {number} level — 1..tether.maxLevel (radius derives from this)
   */
  constructor(id, x, y, level) {
    this.id = id;
    this.x = x;
    this.y = y;
    this.level = Math.max(1, Math.round(level ?? 1));
    this.radius = Tether.radiusForLevel(this.level);
    this.label = ''; // anchor name for the HUD (set by the owner, optional)
  }

  /** radius(level) = level1Radius × radiusGrowth^(level−1) (data/tether.json). */
  static radiusForLevel(level) {
    const l1 = config.get('tether.level1Radius', 5120);
    const g = config.get('tether.radiusGrowth', 1.25);
    const n = Math.max(1, Math.round(level ?? 1));
    return l1 * Math.pow(g, n - 1);
  }

  /** Is the point inside this tether's zone (center distance ≤ radius)? */
  static contains(t, x, y) {
    const dx = x - t.x;
    const dy = y - t.y;
    return dx * dx + dy * dy <= t.radius * t.radius;
  }

  /** Union membership: the ship is allowed wherever ANY tether reaches. */
  static containsAny(x, y, tethers) {
    for (const t of tethers) {
      if (Tether.contains(t, x, y)) return true;
    }
    return false;
  }

  /**
   * The barrier, as a clamp: a point inside the union is returned
   * unchanged (clamped: false); a point outside is moved to the CLOSEST
   * point on the union's boundary, with the outward normal there
   * (the direction from the owning anchor — strip velocity along it and
   * the ship can never cross the line).
   *
   * Closest point on the union = min over tethers of the closest point
   * on that tether (the boundary point of the tether with the smallest
   * rim gap), so this is exact for any number of overlapping zones.
   *
   * @returns {{x:number, y:number, clamped:boolean, id:string|null, nx:number, ny:number}}
   */
  static clampPoint(x, y, tethers) {
    let best = null;
    for (const t of tethers) {
      const dx = x - t.x;
      const dy = y - t.y;
      const d = Math.hypot(dx, dy);
      if (d <= t.radius) {
        return { x, y, clamped: false, id: t.id, nx: 0, ny: 0 };
      }
      const gap = d - t.radius; // distance from the point to this tether's rim
      if (!best || gap < best.gap - EPS) {
        const nx = dx / d;
        const ny = dy / d;
        best = { gap, id: t.id, x: t.x + nx * t.radius, y: t.y + ny * t.radius, nx, ny };
      }
    }
    if (best) return { x: best.x, y: best.y, clamped: true, id: best.id, nx: best.nx, ny: best.ny };
    return { x, y, clamped: false, id: null, nx: 0, ny: 0 }; // no tethers → free space
  }

  /**
   * This tether's share of the union boundary: the rim arcs NOT covered
   * by any other tether's zone. The barrier line is drawn ONLY on these
   * arcs — where two zones overlap, the boundary simply ends where one
   * zone begins (no line, no wall, in the overlap).
   *
   * Math: a point P(θ) = center + r·(cosθ, sinθ) on this rim is inside
   * another zone o ⟺ |P − o| ≤ o.radius ⟺ cos(θ − β) ≥ k, where
   *   β = atan2(o.y − y, o.x − x),  k = (r² + d² − o.radius²) / (2·r·d),
   * d = |center → o|. So each other tether covers the angular interval
   * [β − arccos k, β + arccos k] (when −1 < k < 1); k < −1 means o
   * contains this whole rim; k ≥ 1 means no overlap. Union the covered
   * intervals on the circle; the complement is the visible arcs.
   *
   * @returns {Array<{a0:number, a1:number}>} arcs in [0, TAU), a1 > a0, sorted
   */
  static visibleArcs(t, tethers) {
    const r = t.radius;
    const covered = [];
    for (const o of tethers) {
      if (o === t) continue;
      const dx = o.x - t.x;
      const dy = o.y - t.y;
      const d = Math.hypot(dx, dy);
      if (d === 0) {
        // Concentric: only a larger (or equal) radius can cover this rim.
        if (o.radius >= r) return [];
        continue;
      }
      const k = (r * r + d * d - o.radius * o.radius) / (2 * r * d);
      if (k < -1) return []; // o contains this disc → the whole rim is covered
      if (k <= -1 || k >= 1) continue; // tangent or disjoint → no covered arc
      const beta = Math.atan2(dy, dx);
      const half = Math.acos(k);
      covered.push([beta - half, beta + half]);
    }
    return complementOf(unionOnCircle(covered));
  }
}

/** Wrap an angle into [0, TAU). */
function wrapAngle(a) {
  const w = a % TAU;
  return w < 0 ? w + TAU : w;
}

/**
 * Union of angular intervals on the circle. Inputs may be unwrapped
 * (a1 may exceed a0 + TAU); outputs are merged intervals in [0, TAU],
 * sorted, possibly covering the full circle.
 */
function unionOnCircle(intervals) {
  const pieces = [];
  for (const [a0, a1] of intervals) {
    const span = a1 - a0;
    if (span <= EPS) continue;
    const s = wrapAngle(a0);
    if (s + span >= TAU - EPS) {
      pieces.push([s, TAU]);
      const tail = s + span - TAU;
      if (tail > EPS) pieces.push([0, tail]);
    } else {
      pieces.push([s, s + span]);
    }
  }
  pieces.sort((p, q) => p[0] - q[0]);
  const merged = [];
  for (const p of pieces) {
    const last = merged[merged.length - 1];
    if (last && p[0] <= last[1] + EPS) last[1] = Math.max(last[1], p[1]);
    else merged.push([p[0], p[1]]);
  }
  // NOTE: no seam merge. Intervals that touch across 0/TAU (e.g. [5.9, TAU]
  // and [0, 0.9]) stay two flat intervals: on the circle they are one arc,
  // but "wrapping" cannot be represented as a single [a0, a1] ⊂ [0, TAU].
  // The complement (complementOf) handles the seam correctly — merging them
  // would wrongly report the whole circle as covered.
  return merged;
}

/** Complement of merged [0, TAU] intervals → the uncovered arcs. */
function complementOf(covered) {
  if (covered.length === 0) return [{ a0: 0, a1: TAU }];
  const out = [];
  let cursor = 0;
  for (const [a0, a1] of covered) {
    if (a0 > cursor + EPS) out.push({ a0: cursor, a1: a0 });
    if (a1 > cursor) cursor = a1;
    if (cursor >= TAU - EPS) break;
  }
  if (cursor < TAU - EPS) out.push({ a0: cursor, a1: TAU });
  return out;
}