arrow.js

import {create} from "../context.js";
import {radians} from "../math.js";
import {constant} from "../options.js";
import {Mark} from "../plot.js";
import {applyChannelStyles, applyDirectStyles, applyIndirectStyles, applyTransform} from "../style.js";
import {maybeSameValue} from "./link.js";

const defaults = {
  ariaLabel: "arrow",
  fill: "none",
  stroke: "currentColor",
  strokeLinecap: "round",
  strokeMiterlimit: 1,
  strokeWidth: 1.5
};

export class Arrow extends Mark {
  constructor(data, options = {}) {
    const {
      x1,
      y1,
      x2,
      y2,
      bend = 0,
      headAngle = 60,
      headLength = 8, // Disable the arrow with headLength = 0; or, use Plot.link.
      inset = 0,
      insetStart = inset,
      insetEnd = inset
    } = options;
    super(
      data,
      {
        x1: {value: x1, scale: "x"},
        y1: {value: y1, scale: "y"},
        x2: {value: x2, scale: "x", optional: true},
        y2: {value: y2, scale: "y", optional: true}
      },
      options,
      defaults
    );
    this.bend = bend === true ? 22.5 : Math.max(-90, Math.min(90, bend));
    this.headAngle = +headAngle;
    this.headLength = +headLength;
    this.insetStart = +insetStart;
    this.insetEnd = +insetEnd;
  }
  render(index, scales, channels, dimensions, context) {
    const {x1: X1, y1: Y1, x2: X2 = X1, y2: Y2 = Y1, SW} = channels;
    const {strokeWidth, bend, headAngle, headLength, insetStart, insetEnd} = this;
    const sw = SW ? (i) => SW[i] : constant(strokeWidth === undefined ? 1 : strokeWidth);

    // When bending, the offset between the straight line between the two points
    // and the outgoing tangent from the start point. (Also the negative
    // incoming tangent to the end point.) This must be within ±π/2. A positive
    // angle will produce a clockwise curve; a negative angle will produce a
    // counterclockwise curve; zero will produce a straight line.
    const bendAngle = bend * radians;

    // The angle between the arrow’s shaft and one of the wings; the “head”
    // angle between the wings is twice this value.
    const wingAngle = (headAngle * radians) / 2;

    // The length of the arrowhead’s “wings” (the line segments that extend from
    // the end point) relative to the stroke width.
    const wingScale = headLength / 1.5;

    return create("svg:g", context)
      .call(applyIndirectStyles, this, scales, dimensions)
      .call(applyTransform, this, scales)
      .call((g) =>
        g
          .selectAll()
          .data(index)
          .enter()
          .append("path")
          .call(applyDirectStyles, this)
          .attr("d", (i) => {
            // The start ⟨x1,y1⟩ and end ⟨x2,y2⟩ points may be inset, and the
            // ending line angle may be altered for inset swoopy arrows.
            let x1 = X1[i],
              y1 = Y1[i],
              x2 = X2[i],
              y2 = Y2[i];
            const lineLength = Math.hypot(x2 - x1, y2 - y1);
            if (lineLength <= insetStart + insetEnd) return null;
            let lineAngle = Math.atan2(y2 - y1, x2 - x1);

            // We don’t allow the wing length to be too large relative to the
            // length of the arrow. (Plot.vector allows arbitrarily large
            // wings, but that’s okay since vectors are usually small.)
            const headLength = Math.min(wingScale * sw(i), lineLength / 3);

            // The radius of the circle that intersects with the two endpoints
            // and has the specified bend angle.
            const r = Math.hypot(lineLength / Math.tan(bendAngle), lineLength) / 2;

            // Apply insets.
            if (insetStart || insetEnd) {
              if (r < 1e5) {
                // For inset swoopy arrows, compute the circle-circle
                // intersection between a circle centered around the
                // respective arrow endpoint and the center of the circle
                // segment that forms the shaft of the arrow.
                const sign = Math.sign(bendAngle);
                const [cx, cy] = pointPointCenter([x1, y1], [x2, y2], r, sign);
                if (insetStart) {
                  [x1, y1] = circleCircleIntersect([cx, cy, r], [x1, y1, insetStart], -sign * Math.sign(insetStart));
                }
                // For the end inset, rotate the arrowhead so that it aligns
                // with the truncated end of the arrow. Since the arrow is a
                // segment of the circle centered at ⟨cx,cy⟩, we can compute
                // the angular difference to the new endpoint.
                if (insetEnd) {
                  const [x, y] = circleCircleIntersect([cx, cy, r], [x2, y2, insetEnd], sign * Math.sign(insetEnd));
                  lineAngle += Math.atan2(y - cy, x - cx) - Math.atan2(y2 - cy, x2 - cx);
                  (x2 = x), (y2 = y);
                }
              } else {
                // For inset straight arrows, offset along the straight line.
                const dx = x2 - x1,
                  dy = y2 - y1,
                  d = Math.hypot(dx, dy);
                if (insetStart) (x1 += (dx / d) * insetStart), (y1 += (dy / d) * insetStart);
                if (insetEnd) (x2 -= (dx / d) * insetEnd), (y2 -= (dy / d) * insetEnd);
              }
            }

            // The angle of the arrow as it approaches the endpoint, and the
            // angles of the adjacent wings. Here “left” refers to if the
            // arrow is pointing up.
            const endAngle = lineAngle + bendAngle;
            const leftAngle = endAngle + wingAngle;
            const rightAngle = endAngle - wingAngle;

            // The endpoints of the two wings.
            const x3 = x2 - headLength * Math.cos(leftAngle);
            const y3 = y2 - headLength * Math.sin(leftAngle);
            const x4 = x2 - headLength * Math.cos(rightAngle);
            const y4 = y2 - headLength * Math.sin(rightAngle);

            // If the radius is very large (or even infinite, as when the bend
            // angle is zero), then render a straight line.
            return `M${x1},${y1}${
              r < 1e5 ? `A${r},${r} 0,0,${bendAngle > 0 ? 1 : 0} ` : `L`
            }${x2},${y2}M${x3},${y3}L${x2},${y2}L${x4},${y4}`;
          })
          .call(applyChannelStyles, this, channels)
      )
      .node();
  }
}

// Returns the center of a circle that goes through the two given points ⟨ax,ay⟩
// and ⟨bx,by⟩ and has radius r. There are two such points; use the sign +1 or
// -1 to chose between them. Returns [NaN, NaN] if r is too small.
function pointPointCenter([ax, ay], [bx, by], r, sign) {
  const dx = bx - ax,
    dy = by - ay,
    d = Math.hypot(dx, dy);
  const k = (sign * Math.sqrt(r * r - (d * d) / 4)) / d;
  return [(ax + bx) / 2 - dy * k, (ay + by) / 2 + dx * k];
}

// Given two circles, one centered at ⟨ax,ay⟩ with radius ar, and the other
// centered at ⟨bx,by⟩ with radius br, returns a point at which the two circles
// intersect. There are typically two such points; use the sign +1 or -1 to
// chose between them. Returns [NaN, NaN] if there is no intersection.
// https://mathworld.wolfram.com/Circle-CircleIntersection.html
function circleCircleIntersect([ax, ay, ar], [bx, by, br], sign) {
  const dx = bx - ax,
    dy = by - ay,
    d = Math.hypot(dx, dy);
  const x = (dx * dx + dy * dy - br * br + ar * ar) / (2 * d);
  const y = sign * Math.sqrt(ar * ar - x * x);
  return [ax + (dx * x + dy * y) / d, ay + (dy * x - dx * y) / d];
}

/**
 * ```js
 * Plot.arrow(inequality, {x1: "POP_1980", y1: "R90_10_1980", x2: "POP_2015", y2: "R90_10_2015", bend: true})
 * ```
 *
 * Returns a new arrow with the given *data* and *options*.
 */
export function arrow(data, options = {}) {
  let {x, x1, x2, y, y1, y2, ...remainingOptions} = options;
  [x1, x2] = maybeSameValue(x, x1, x2);
  [y1, y2] = maybeSameValue(y, y1, y2);
  return new Arrow(data, {...remainingOptions, x1, x2, y1, y2});
}