chartjs/chartjs-plugin-datalabels

import HitBox from './hitbox';

function coordinates(el, model, geometry) {
  var point = model.positioner(el, model);
  var vx = point.vx;
  var vy = point.vy;

  if (!vx && !vy) {
    // if aligned center, we don't want to offset the center point
    return {x: point.x, y: point.y};
  }

  var w = geometry.w;
  var h = geometry.h;

  // take in account the label rotation
  var rotation = model.rotation;
  var dx = Math.abs(w / 2 * Math.cos(rotation)) + Math.abs(h / 2 * Math.sin(rotation));
  var dy = Math.abs(w / 2 * Math.sin(rotation)) + Math.abs(h / 2 * Math.cos(rotation));

  // scale the unit vector (vx, vy) to get at least dx or dy equal to
  // w or h respectively (else we would calculate the distance to the
  // ellipse inscribed in the bounding rect)
  var vs = 1 / Math.max(Math.abs(vx), Math.abs(vy));
  dx *= vx * vs;
  dy *= vy * vs;

  // finally, include the explicit offset
  dx += model.offset * vx;
  dy += model.offset * vy;

  return {
    x: point.x + dx,
    y: point.y + dy
  };
}

function collide(labels, collider) {
  var i, j, s0, s1;

  // IMPORTANT Iterate in the reverse order since items at the end of the
  // list have an higher weight/priority and thus should be less impacted
  // by the overlapping strategy.

  for (i = labels.length - 1; i >= 0; --i) {
    s0 = labels[i].$layout;

    for (j = i - 1; j >= 0 && s0._visible; --j) {
      s1 = labels[j].$layout;

      if (s1._visible && s0._box.intersects(s1._box)) {
        collider(s0, s1);
      }
    }
  }

  return labels;
}

function compute(labels) {
  var i, ilen, label, state, geometry, center, proxy;

  // Initialize labels for overlap detection
  for (i = 0, ilen = labels.length; i < ilen; ++i) {
    label = labels[i];
    state = label.$layout;

    if (state._visible) {
      // Chart.js 3 removed el._model in favor of getProps(), making harder to
      // abstract reading values in positioners. Also, using string arrays to
      // read values (i.e. var {a,b,c} = el.getProps(["a","b","c"])) would make
      // positioners inefficient in the normal case (i.e. not the final values)
      // and the code a bit ugly, so let's use a Proxy instead.
      proxy = new Proxy(label._el, {get: (el, p) => el.getProps([p], true)[p]});

      geometry = label.geometry();
      center = coordinates(proxy, label.model(), geometry);
      state._box.update(center, geometry, label.rotation());
    }
  }

  // Auto hide overlapping labels
  return collide(labels, function(s0, s1) {
    var h0 = s0._hidable;
    var h1 = s1._hidable;

    if ((h0 && h1) || h1) {
      s1._visible = false;
    } else if (h0) {
      s0._visible = false;
    }
  });
}

export default {
  prepare: function(datasets) {
    var labels = [];
    var i, j, ilen, jlen, label;

    for (i = 0, ilen = datasets.length; i < ilen; ++i) {
      for (j = 0, jlen = datasets[i].length; j < jlen; ++j) {
        label = datasets[i][j];
        labels.push(label);
        label.$layout = {
          _box: new HitBox(),
          _hidable: false,
          _visible: true,
          _set: i,
          _idx: label._index
        };
      }
    }

    // TODO New `z` option: labels with a higher z-index are drawn
    // of top of the ones with a lower index. Lowest z-index labels
    // are also discarded first when hiding overlapping labels.
    labels.sort(function(a, b) {
      var sa = a.$layout;
      var sb = b.$layout;

      return sa._idx === sb._idx
        ? sb._set - sa._set
        : sb._idx - sa._idx;
    });

    this.update(labels);

    return labels;
  },

  update: function(labels) {
    var dirty = false;
    var i, ilen, label, model, state;

    for (i = 0, ilen = labels.length; i < ilen; ++i) {
      label = labels[i];
      model = label.model();
      state = label.$layout;
      state._hidable = model && model.display === 'auto';
      state._visible = label.visible();
      dirty |= state._hidable;
    }

    if (dirty) {
      compute(labels);
    }
  },

  lookup: function(labels, point) {
    var i, state;

    // IMPORTANT Iterate in the reverse order since items at the end of
    // the list have an higher z-index, thus should be picked first.

    for (i = labels.length - 1; i >= 0; --i) {
      state = labels[i].$layout;

      if (state && state._visible && state._box.contains(point)) {
        return labels[i];
      }
    }

    return null;
  },

  draw: function(chart, labels) {
    var i, ilen, label, state, geometry, center;

    for (i = 0, ilen = labels.length; i < ilen; ++i) {
      label = labels[i];
      state = label.$layout;

      if (state._visible) {
        geometry = label.geometry();
        center = coordinates(label._el, label.model(), geometry);
        state._box.update(center, geometry, label.rotation());
        label.draw(chart, center);
      }
    }
  }
};