'use strict';

var invariant = require('@turf/invariant');

/**
 * Takes a triangular plane as a {@link Polygon}
 * and a {@link Point} within that triangle and returns the z-value
 * at that point. The Polygon should have properties `a`, `b`, and `c`
 * that define the values at its three corners. Alternatively, the z-values
 * of each triangle point can be provided by their respective 3rd coordinate
 * if their values are not provided as properties.
 *
 * @name planepoint
 * @param {Coord} point the Point for which a z-value will be calculated
 * @param {Feature<Polygon>} triangle a Polygon feature with three vertices
 * @returns {number} the z-value for `interpolatedPoint`
 * @example
 * var point = turf.point([-75.3221, 39.529]);
 * // "a", "b", and "c" values represent the values of the coordinates in order.
 * var triangle = turf.polygon([[
 *   [-75.1221, 39.57],
 *   [-75.58, 39.18],
 *   [-75.97, 39.86],
 *   [-75.1221, 39.57]
 * ]], {
 *   "a": 11,
 *   "b": 122,
 *   "c": 44
 * });
 *
 * var zValue = turf.planepoint(point, triangle);
 * point.properties.zValue = zValue;
 *
 * //addToMap
 * var addToMap = [triangle, point];
 */
function planepoint(point, triangle) {
  // Normalize input
  var coord = invariant.getCoord(point);
  var geom = invariant.getGeom(triangle);
  var coords = geom.coordinates;
  var outer = coords[0];
  if (outer.length < 4)
    throw new Error("OuterRing of a Polygon must have 4 or more Positions.");
  var properties = triangle.properties || {};
  var a = properties.a;
  var b = properties.b;
  var c = properties.c;

  // Planepoint
  var x = coord[0];
  var y = coord[1];
  var x1 = outer[0][0];
  var y1 = outer[0][1];
  var z1 = a !== undefined ? a : outer[0][2];
  var x2 = outer[1][0];
  var y2 = outer[1][1];
  var z2 = b !== undefined ? b : outer[1][2];
  var x3 = outer[2][0];
  var y3 = outer[2][1];
  var z3 = c !== undefined ? c : outer[2][2];
  var z =
    (z3 * (x - x1) * (y - y2) +
      z1 * (x - x2) * (y - y3) +
      z2 * (x - x3) * (y - y1) -
      z2 * (x - x1) * (y - y3) -
      z3 * (x - x2) * (y - y1) -
      z1 * (x - x3) * (y - y2)) /
    ((x - x1) * (y - y2) +
      (x - x2) * (y - y3) +
      (x - x3) * (y - y1) -
      (x - x1) * (y - y3) -
      (x - x2) * (y - y1) -
      (x - x3) * (y - y2));

  return z;
}

module.exports = planepoint;
module.exports.default = planepoint;