openlayers/src/ol/geom/flat/orient.js

/**
 * @module ol/geom/flat/orient
 */
import {coordinates as reverseCoordinates} from './reverse.js';

/**
 * Is the linear ring oriented clockwise in a coordinate system with a bottom-left
 * coordinate origin? For a coordinate system with a top-left coordinate origin,
 * the ring's orientation is clockwise when this function returns false.
 * @param {Array<number>} flatCoordinates Flat coordinates.
 * @param {number} offset Offset.
 * @param {number} end End.
 * @param {number} stride Stride.
 * @return {boolean} Is clockwise.
 */
export function linearRingIsClockwise(flatCoordinates, offset, end, stride) {
  // https://stackoverflow.com/q/1165647/clockwise-method#1165943
  // https://github.com/OSGeo/gdal/blob/master/gdal/ogr/ogrlinearring.cpp
  let edge = 0;
  let x1 = flatCoordinates[end - stride];
  let y1 = flatCoordinates[end - stride + 1];
  for (; offset < end; offset += stride) {
    const x2 = flatCoordinates[offset];
    const y2 = flatCoordinates[offset + 1];
    edge += (x2 - x1) * (y2 + y1);
    x1 = x2;
    y1 = y2;
  }
  return edge === 0 ? undefined : edge > 0;
}

/**
 * Determines if linear rings are oriented.  By default, left-hand orientation
 * is tested (first ring must be clockwise, remaining rings counter-clockwise).
 * To test for right-hand orientation, use the `opt_right` argument.
 *
 * @param {Array<number>} flatCoordinates Flat coordinates.
 * @param {number} offset Offset.
 * @param {Array<number>} ends Array of end indexes.
 * @param {number} stride Stride.
 * @param {boolean} [opt_right] Test for right-hand orientation
 *     (counter-clockwise exterior ring and clockwise interior rings).
 * @return {boolean} Rings are correctly oriented.
 */
export function linearRingsAreOriented(
  flatCoordinates,
  offset,
  ends,
  stride,
  opt_right
) {
  const right = opt_right !== undefined ? opt_right : false;
  for (let i = 0, ii = ends.length; i < ii; ++i) {
    const end = ends[i];
    const isClockwise = linearRingIsClockwise(
      flatCoordinates,
      offset,
      end,
      stride
    );
    if (i === 0) {
      if ((right && isClockwise) || (!right && !isClockwise)) {
        return false;
      }
    } else {
      if ((right && !isClockwise) || (!right && isClockwise)) {
        return false;
      }
    }
    offset = end;
  }
  return true;
}

/**
 * Determines if linear rings are oriented.  By default, left-hand orientation
 * is tested (first ring must be clockwise, remaining rings counter-clockwise).
 * To test for right-hand orientation, use the `opt_right` argument.
 *
 * @param {Array<number>} flatCoordinates Flat coordinates.
 * @param {number} offset Offset.
 * @param {Array<Array<number>>} endss Array of array of end indexes.
 * @param {number} stride Stride.
 * @param {boolean} [opt_right] Test for right-hand orientation
 *     (counter-clockwise exterior ring and clockwise interior rings).
 * @return {boolean} Rings are correctly oriented.
 */
export function linearRingssAreOriented(
  flatCoordinates,
  offset,
  endss,
  stride,
  opt_right
) {
  for (let i = 0, ii = endss.length; i < ii; ++i) {
    const ends = endss[i];
    if (
      !linearRingsAreOriented(flatCoordinates, offset, ends, stride, opt_right)
    ) {
      return false;
    }
    if (ends.length) {
      offset = ends[ends.length - 1];
    }
  }
  return true;
}

/**
 * Orient coordinates in a flat array of linear rings.  By default, rings
 * are oriented following the left-hand rule (clockwise for exterior and
 * counter-clockwise for interior rings).  To orient according to the
 * right-hand rule, use the `opt_right` argument.
 *
 * @param {Array<number>} flatCoordinates Flat coordinates.
 * @param {number} offset Offset.
 * @param {Array<number>} ends Ends.
 * @param {number} stride Stride.
 * @param {boolean} [opt_right] Follow the right-hand rule for orientation.
 * @return {number} End.
 */
export function orientLinearRings(
  flatCoordinates,
  offset,
  ends,
  stride,
  opt_right
) {
  const right = opt_right !== undefined ? opt_right : false;
  for (let i = 0, ii = ends.length; i < ii; ++i) {
    const end = ends[i];
    const isClockwise = linearRingIsClockwise(
      flatCoordinates,
      offset,
      end,
      stride
    );
    const reverse =
      i === 0
        ? (right && isClockwise) || (!right && !isClockwise)
        : (right && !isClockwise) || (!right && isClockwise);
    if (reverse) {
      reverseCoordinates(flatCoordinates, offset, end, stride);
    }
    offset = end;
  }
  return offset;
}

/**
 * Orient coordinates in a flat array of linear rings.  By default, rings
 * are oriented following the left-hand rule (clockwise for exterior and
 * counter-clockwise for interior rings).  To orient according to the
 * right-hand rule, use the `opt_right` argument.
 *
 * @param {Array<number>} flatCoordinates Flat coordinates.
 * @param {number} offset Offset.
 * @param {Array<Array<number>>} endss Array of array of end indexes.
 * @param {number} stride Stride.
 * @param {boolean} [opt_right] Follow the right-hand rule for orientation.
 * @return {number} End.
 */
export function orientLinearRingsArray(
  flatCoordinates,
  offset,
  endss,
  stride,
  opt_right
) {
  for (let i = 0, ii = endss.length; i < ii; ++i) {
    offset = orientLinearRings(
      flatCoordinates,
      offset,
      endss[i],
      stride,
      opt_right
    );
  }
  return offset;
}

/**
 * Return a two-dimensional endss
 * @param {Array<number>} flatCoordinates Flat coordinates
 * @param {Array<number>} ends Linear ring end indexes
 * @return {Array<Array<number>>} Two dimensional endss array that can
 * be used to contruct a MultiPolygon
 */
export function inflateEnds(flatCoordinates, ends) {
  const endss = [];
  let offset = 0;
  let prevEndIndex = 0;
  for (let i = 0, ii = ends.length; i < ii; ++i) {
    const end = ends[i];
    // classifies an array of rings into polygons with outer rings and holes
    if (!linearRingIsClockwise(flatCoordinates, offset, end, 2)) {
      endss.push(ends.slice(prevEndIndex, i + 1));
    } else {
      if (endss.length === 0) {
        continue;
      }
      endss[endss.length - 1].push(ends[prevEndIndex]);
    }
    prevEndIndex = i + 1;
    offset = end;
  }
  return endss;
}