feuerwerk/src/editor/polygonEdit.ts

/**
 * Editing an outline by its corners (#141, #151), the same in the Scene view (a polygon collider)
 * and in the Texture tab (a frame's collision outline): drag a corner, put one on an edge, take one
 * away. Pure, so the rules are testable without a view; which press does which is `pointGestures.ts`.
 *
 * The lower half of the file says the same about a ring of a vector art shape (#129, stage 3). It is
 * the same set of gestures on a richer point: one that may carry Bezier handles. They are kept here
 * rather than in a module of their own so the two cannot drift apart – a corner is grabbed by the
 * same rule in the Texture tab and in the Vector art tab.
 */
import type { VectorPoint, VectorPointMode, VectorRing } from '../core/model/Asset';
import type { Vec2 } from '../core/model/Properties';
import { closestOnSegment, cubicAt, splitCubic } from '../core/util/vec';
// Where a handle sits in canvas pixels; it is stored as an offset, and the model says so once.
import { cloneVectorPoint, cloneVectorRing, handleAt } from '../core/model/vectorart';

/** What lies under the pointer: a corner, or the edge from corner `index` to the next one. */
export type PolygonGrip = { kind: 'corner'; index: number } | { kind: 'edge'; index: number; at: Vec2 };

/** Fewer corners than this are no polygon; the last three cannot be taken away. */
export const MIN_CORNERS = 3;

/**
 * The corner or edge within `tolerance` of `p`, the nearest one; a corner wins over an edge, since
 * every corner lies on two edges. An outline of one or two corners has no edge to press on yet.
 */
export function gripAt(points: readonly Vec2[], p: Vec2, tolerance: number): PolygonGrip | null {
  let best: PolygonGrip | null = null;
  let nearest = tolerance;
  for (const [index, corner] of points.entries()) {
    const distance = Math.hypot(corner.x - p.x, corner.y - p.y);
    if (distance > nearest) continue;
    best = { kind: 'corner', index };
    nearest = distance;
  }
  if (best || points.length < MIN_CORNERS) return best;
  for (const [index, a] of points.entries()) {
    const at = closestOnSegment(p, a, points[(index + 1) % points.length]!);
    const distance = Math.hypot(at.x - p.x, at.y - p.y);
    if (distance > nearest) continue;
    best = { kind: 'edge', index, at };
    nearest = distance;
  }
  return best;
}

/** Whether `p` lies inside the outline (even-odd); fewer than three corners enclose nothing. */
export function insideOutline(points: readonly Vec2[], p: Vec2): boolean {
  if (points.length < MIN_CORNERS) return false;
  let inside = false;
  for (let i = 0, j = points.length - 1; i < points.length; j = i++) {
    const a = points[i]!;
    const b = points[j]!;
    if (a.y > p.y !== b.y > p.y && p.x < ((b.x - a.x) * (p.y - a.y)) / (b.y - a.y) + a.x) inside = !inside;
  }
  return inside;
}

/** A corner put at `at` on the edge after corner `index`; it becomes corner `index + 1`. */
export function withCornerAfter(points: readonly Vec2[], index: number, at: Vec2): Vec2[] {
  return [...points.slice(0, index + 1), { ...at }, ...points.slice(index + 1)];
}

export function withCornerMoved(points: readonly Vec2[], index: number, to: Vec2): Vec2[] {
  return points.map((p, i) => (i === index ? { ...to } : { ...p }));
}

/** The outline without corner `index`; null when that would leave it fewer than `min` corners. */
export function withoutCorner(points: readonly Vec2[], index: number, min = MIN_CORNERS): Vec2[] | null {
  if (points.length - 1 < min) return null;
  return points.filter((_, i) => i !== index).map((p) => ({ ...p }));
}

/** How finely a curved segment is walked when the pointer asks what it is near. */
const CURVE_STEPS = 16;

/**
 * What lies under the pointer on a ring: a point, one of its two handles, or the segment that
 * starts at point `index` – `at` is the place on it and `t` how far along it lies, which is where a
 * new point goes. For a closed ring the last segment is the way back to the first point.
 */
export type RingGrip =
  | { kind: 'point'; index: number }
  | { kind: 'handle'; index: number; side: 'in' | 'out' }
  | { kind: 'segment'; index: number; at: Vec2; t: number };

/**
 * The grip within `tolerance` of `p`, the nearest one. A handle wins over a point and a point over a
 * segment: a handle pulled onto its own point must stay grabbable, and every point lies on two
 * segments. Only the handles of `shown` are offered, because only those are drawn – a handle one
 * cannot see is one nobody means to grab.
 */
export function ringGripAt(
  ring: VectorRing,
  p: Vec2,
  tolerance: number,
  shown: readonly number[] = ring.points.map((_, i) => i),
): RingGrip | null {
  let best: RingGrip | null = null;
  let nearest = tolerance;
  for (const index of shown) {
    const point = ring.points[index];
    if (!point) continue;
    for (const side of ['in', 'out'] as const) {
      const at = handleAt(point, side);
      const distance = at ? Math.hypot(at.x - p.x, at.y - p.y) : Infinity;
      if (distance > nearest) continue;
      best = { kind: 'handle', index, side };
      nearest = distance;
    }
  }
  if (best) return best;
  for (const [index, point] of ring.points.entries()) {
    const distance = Math.hypot(point.x - p.x, point.y - p.y);
    if (distance > nearest) continue;
    best = { kind: 'point', index };
    nearest = distance;
  }
  if (best) return best;
  for (const [index, from] of ringSegments(ring)) {
    const to = ring.points[(index + 1) % ring.points.length]!;
    const hit = nearestOnSegment(from, to, p);
    if (hit.distance > nearest) continue;
    best = { kind: 'segment', index, at: hit.at, t: hit.t };
    nearest = hit.distance;
  }
  return best;
}

/** The segments of a ring as `[index, from]`; a closed ring has the way back from its last point. */
export function ringSegments(ring: VectorRing): [number, VectorPoint][] {
  const segments: [number, VectorPoint][] = [];
  const last = ring.points.length - (ring.closed ? 0 : 1);
  for (let i = 0; i < last; i++) segments.push([i, ring.points[i]!]);
  return segments;
}

/** The point on the segment nearest to `p`, straight or curved, and how far along it lies. */
export function nearestOnSegment(
  from: VectorPoint,
  to: VectorPoint,
  p: Vec2,
): { at: Vec2; t: number; distance: number } {
  if (!from.out && !to.in) {
    const at = closestOnSegment(p, from, to);
    const span = Math.hypot(to.x - from.x, to.y - from.y);
    const t = span > 0 ? Math.hypot(at.x - from.x, at.y - from.y) / span : 0;
    return { at, t, distance: Math.hypot(at.x - p.x, at.y - p.y) };
  }
  // A curve is walked rather than solved: a handful of samples is as near as a pointer can aim.
  let best = { at: { x: from.x, y: from.y }, t: 0, distance: Infinity };
  for (let step = 0; step <= CURVE_STEPS; step++) {
    const t = step / CURVE_STEPS;
    const at = pointOnSegment(from, to, t);
    const distance = Math.hypot(at.x - p.x, at.y - p.y);
    if (distance < best.distance) best = { at, t, distance };
  }
  return best;
}

/** Where the segment is at `t` (0 at `from`, 1 at `to`), curve or line. */
export function pointOnSegment(from: VectorPoint, to: VectorPoint, t: number): Vec2 {
  const c1 = handleAt(from, 'out') ?? { x: from.x, y: from.y };
  const c2 = handleAt(to, 'in') ?? { x: to.x, y: to.y };
  return cubicAt(from, c1, c2, to, t);
}

/** A ring walked as a polyline – for hit-testing its inside and for drawing it in the editor. */
export function ringOutline(ring: VectorRing): Vec2[] {
  const points: Vec2[] = ring.points.map((p) => ({ x: p.x, y: p.y }));
  if (ring.points.length < 2) return points;
  const walked: Vec2[] = [];
  for (const [index, from] of ringSegments(ring)) {
    const to = ring.points[(index + 1) % ring.points.length]!;
    walked.push({ x: from.x, y: from.y });
    if (!from.out && !to.in) continue;
    for (let step = 1; step < CURVE_STEPS; step++) walked.push(pointOnSegment(from, to, step / CURVE_STEPS));
  }
  if (!ring.closed)
    walked.push({ x: ring.points[ring.points.length - 1]!.x, y: ring.points[ring.points.length - 1]!.y });
  return walked;
}

/** Whether `p` lies inside the ring; an open ring encloses nothing one can click into. */
export function insideRing(ring: VectorRing, p: Vec2): boolean {
  return ring.closed && insideOutline(ringOutline(ring), p);
}

/** The point moved; its handles ride along, being offsets from it. */
export function withPointMoved(ring: VectorRing, index: number, to: Vec2): VectorRing {
  return mapPoints(ring, (point, i) => (i === index ? { ...point, x: to.x, y: to.y } : point));
}

/**
 * What a point is to the user (#195): straight has no handle at all, the other three are the mode
 * its handles move in. A point with handles and no mode is a corner.
 */
export type PointKind = 'straight' | VectorPointMode;

export const POINT_KINDS: readonly PointKind[] = ['straight', 'corner', 'smooth', 'symmetric'];

export function pointKind(point: VectorPoint): PointKind {
  return point.in || point.out ? (point.mode ?? 'corner') : 'straight';
}

/**
 * A handle pulled to `to`, in canvas pixels; the other handle follows the point's mode. A corner
 * leaves it where it is, a smooth point turns it to the opposite direction at its own length, a
 * symmetric point mirrors it – and grows it, if it was missing, which is how the pen draws a curve.
 * A handle dropped back onto its point is taken away; on a symmetric point its mirror goes with it.
 * `mode` overrides the point's own and is written onto it.
 */
export function withHandleMoved(
  ring: VectorRing,
  index: number,
  side: 'in' | 'out',
  to: Vec2,
  mode?: VectorPointMode,
): VectorRing {
  return mapPoints(ring, (point, i) => {
    if (i !== index) return point;
    const offset = { x: to.x - point.x, y: to.y - point.y };
    const length = Math.hypot(offset.x, offset.y);
    const next: VectorPoint = { ...point };
    const other = side === 'in' ? 'out' : 'in';
    const how = mode ?? point.mode ?? 'corner';
    if (mode) next.mode = mode;
    if (length < 1e-6) {
      delete next[side];
      if (how === 'symmetric') delete next[other];
      return next;
    }
    next[side] = offset;
    const across = next[other];
    if (how === 'symmetric') next[other] = { x: -offset.x, y: -offset.y };
    else if (how === 'smooth' && across) {
      const keep = Math.hypot(across.x, across.y);
      next[other] = { x: (-offset.x / length) * keep, y: (-offset.y / length) * keep };
    }
    return next;
  });
}

/**
 * The point turned into `kind` (#195). Straight takes both handles away. A point without handles
 * that becomes one of the other three grows them first, along the line between its neighbours at a
 * third of the way to each – the curve one expects, without pulling two handles out by hand. Smooth
 * and symmetric then line the handles up on the bisector of the two, so the curve keeps its general
 * direction: smooth keeps each length, symmetric gives both their mean. A missing second handle is
 * grown opposite the first at the same length.
 */
export function withPointKind(ring: VectorRing, index: number, kind: PointKind): VectorRing {
  const point = ring.points[index];
  if (!point) return cloneVectorRing(ring);
  if (kind === 'straight') return mapPoints(ring, (p, i) => (i === index ? { x: p.x, y: p.y } : p));
  const had = Boolean(point.in || point.out);
  // Grown handles are in line and of one length already; lining them up again would only add rounding.
  let handles = had ? { in: point.in, out: point.out } : grownHandles(ring, index);
  if (!handles) return cloneVectorRing(ring);
  if (had && kind !== 'corner') handles = linedUp(handles, kind);
  const next: VectorPoint = { x: point.x, y: point.y, mode: kind };
  if (handles.in) next.in = handles.in;
  if (handles.out) next.out = handles.out;
  return mapPoints(ring, (p, i) => (i === index ? next : p));
}

function grownHandles(ring: VectorRing, index: number): { in: Vec2; out: Vec2 } | null {
  const point = ring.points[index]!;
  const count = ring.points.length;
  // An open line's ends have one neighbour: the missing one stands on the point itself.
  const before = ring.closed || index > 0 ? ring.points[(index - 1 + count) % count]! : point;
  const after = ring.closed || index < count - 1 ? ring.points[(index + 1) % count]! : point;
  const dx = (after.x - before.x) / 3;
  const dy = (after.y - before.y) / 3;
  if (Math.hypot(dx, dy) < 1e-6) return null;
  return { in: { x: -dx, y: -dy }, out: { x: dx, y: dy } };
}

function linedUp(handles: { in?: Vec2; out?: Vec2 }, kind: 'smooth' | 'symmetric'): { in: Vec2; out: Vec2 } {
  const outLength = handles.out ? Math.hypot(handles.out.x, handles.out.y) : 0;
  const inLength = handles.in ? Math.hypot(handles.in.x, handles.in.y) : 0;
  const ahead = handles.out ? unit(handles.out) : { x: 0, y: 0 };
  const behind = handles.in ? unit(handles.in) : { x: 0, y: 0 };
  // The way out, averaged with the way in turned round; handles folded onto each other keep `out`'s.
  let direction = unit({ x: ahead.x - behind.x, y: ahead.y - behind.y });
  if (direction.x === 0 && direction.y === 0)
    direction = handles.out ? ahead : { x: -behind.x, y: -behind.y };
  let lengthIn = inLength || outLength;
  let lengthOut = outLength || inLength;
  if (kind === 'symmetric') lengthIn = lengthOut = (lengthIn + lengthOut) / 2;
  return {
    in: { x: -direction.x * lengthIn, y: -direction.y * lengthIn },
    out: { x: direction.x * lengthOut, y: direction.y * lengthOut },
  };
}

function unit(v: Vec2): Vec2 {
  const length = Math.hypot(v.x, v.y);
  return length < 1e-9 ? { x: 0, y: 0 } : { x: v.x / length, y: v.y / length };
}

/**
 * A point put on the segment after `index`, at `t` along it. A curved segment is split rather than
 * cut: the two halves together draw exactly the curve that was there (de Casteljau), so a point put
 * on a round side does not flatten it. A straight one gives a plain corner.
 */
export function withPointOnSegment(ring: VectorRing, index: number, t: number): VectorRing {
  const from = ring.points[index];
  const to = ring.points[(index + 1) % ring.points.length];
  if (!from || !to) return cloneVectorRing(ring);
  const points = ring.points.map(clonePoint);
  const at = pointOnSegment(from, to, t);
  let inserted: VectorPoint = { x: at.x, y: at.y };
  if (from.out || to.in) {
    const c1 = handleAt(from, 'out') ?? { x: from.x, y: from.y };
    const c2 = handleAt(to, 'in') ?? { x: to.x, y: to.y };
    const { left, right } = splitCubic([from, c1, c2, to], t);
    const next = points[(index + 1) % ring.points.length]!;
    setHandle(points[index]!, 'out', left[1]!);
    setHandle(next, 'in', right[2]!);
    // The split keeps the directions but shortens one handle of each end: no longer symmetric,
    // still smooth. The new point sits on a curve, so its handles are in line – smooth as well.
    for (const end of [points[index]!, next]) if (end.mode === 'symmetric') end.mode = 'smooth';
    inserted = { x: at.x, y: at.y, mode: 'smooth' };
    setHandle(inserted, 'in', left[2]!);
    setHandle(inserted, 'out', right[1]!);
    if (!inserted.in && !inserted.out) delete inserted.mode;
  }
  points.splice(index + 1, 0, inserted);
  return { closed: ring.closed, points };
}

/** The ring without point `index`; null when nothing would be left of it. */
export function withoutPoint(ring: VectorRing, index: number): VectorRing | null {
  if (ring.points.length <= 1) return null;
  return { closed: ring.closed, points: ring.points.filter((_, i) => i !== index).map(clonePoint) };
}

/** Alt-click: a straight point made round (symmetric), and one with handles made straight again. */
export function withSmoothToggled(ring: VectorRing, index: number): VectorRing {
  const point = ring.points[index];
  if (!point) return cloneVectorRing(ring);
  return withPointKind(ring, index, pointKind(point) === 'straight' ? 'symmetric' : 'straight');
}

/** The whole ring shifted – what dragging a shape does to each of its rings. */
export function ringMoved(ring: VectorRing, dx: number, dy: number): VectorRing {
  return mapPoints(ring, (point) => ({ ...point, x: point.x + dx, y: point.y + dy }));
}

function mapPoints(ring: VectorRing, fn: (point: VectorPoint, index: number) => VectorPoint): VectorRing {
  return { closed: ring.closed, points: ring.points.map((point, i) => clonePoint(fn(clonePoint(point), i))) };
}

/** A mode without a handle to steer is dropped, so a straight point reads the same however it got there. */
function clonePoint(point: VectorPoint): VectorPoint {
  const copy = cloneVectorPoint(point);
  if (!copy.in && !copy.out) delete copy.mode;
  return copy;
}

function setHandle(point: VectorPoint, side: 'in' | 'out', at: Vec2): void {
  const offset = { x: at.x - point.x, y: at.y - point.y };
  if (Math.hypot(offset.x, offset.y) < 1e-6) delete point[side];
  else point[side] = offset;
}