/**
 * @module olcs.core
 */
import {linear as linearEasing} from 'ol/easing.js';
import olLayerTile from 'ol/layer/Tile.js';
import olLayerImage from 'ol/layer/Image.js';
import {get as getProjection, transformExtent} from 'ol/proj.js';
import olSourceImageStatic from 'ol/source/ImageStatic.js';
import olSourceImageWMS from 'ol/source/ImageWMS.js';
import olSourceTileImage from 'ol/source/TileImage.js';
import olSourceTileWMS from 'ol/source/TileWMS.js';
import olSourceVectorTile from 'ol/source/VectorTile.js';
import {defaultImageLoadFunction} from 'ol/source/Image.js';
import olcsCoreOLImageryProvider from './core/OLImageryProvider.js';
import olcsUtil from './util.js';
import MVTImageryProvider from './MVTImageryProvider.js';
import VectorTileLayer from 'ol/layer/VectorTile.js';
import {getCenter as getExtentCenter} from 'ol/extent';

const exports = {};


/**
 * @typedef {Object} CesiumUrlDefinition
 * @property {string} url
 * @property {string} subdomains
 */


/**
 * Options for rotate around axis core function.
 * @typedef {Object} RotateAroundAxisOption
 * @property {number} [duration]
 * @property {function(number): number} [easing]
 * @property {function(): void} [callback]
 */



/**
 * @typedef {Object} LayerWithParents
 * @property {import('ol/layer/Base.js').default} layer
 * @property {Array<import('ol/layer/Group.js').default>} parents
 */


/**
 * Compute the pixel width and height of a point in meters using the
 * camera frustum.
 * @param {!Cesium.Scene} scene
 * @param {!Cesium.Cartesian3} target
 * @return {!Cesium.Cartesian2} the pixel size
 * @api
 */
exports.computePixelSizeAtCoordinate = function(scene, target) {
  const camera = scene.camera;
  const canvas = scene.canvas;
  const frustum = camera.frustum;
  const distance = Cesium.Cartesian3.magnitude(Cesium.Cartesian3.subtract(
      camera.position, target, new Cesium.Cartesian3()));
  return frustum.getPixelDimensions(canvas.clientWidth, canvas.clientHeight,
      distance, scene.pixelRatio, new Cesium.Cartesian2());
};


/**
 * Compute bounding box around a target point.
 * @param {!Cesium.Scene} scene
 * @param {!Cesium.Cartesian3} target
 * @param {number} amount Half the side of the box, in pixels.
 * @return {Array<Cesium.Cartographic>} bottom left and top right
 * coordinates of the box
 */
exports.computeBoundingBoxAtTarget = function(scene, target, amount) {
  const pixelSize = exports.computePixelSizeAtCoordinate(scene, target);
  const transform = Cesium.Transforms.eastNorthUpToFixedFrame(target);

  const bottomLeft = Cesium.Matrix4.multiplyByPoint(
      transform,
      new Cesium.Cartesian3(-pixelSize.x * amount, -pixelSize.y * amount, 0),
      new Cesium.Cartesian3());

  const topRight = Cesium.Matrix4.multiplyByPoint(
      transform,
      new Cesium.Cartesian3(pixelSize.x * amount, pixelSize.y * amount, 0),
      new Cesium.Cartesian3());

  return Cesium.Ellipsoid.WGS84.cartesianArrayToCartographicArray(
      [bottomLeft, topRight]);
};


/**
 *
 * @param {!ol.geom.Geometry} geometry
 * @param {number} height
 * @api
 */
exports.applyHeightOffsetToGeometry = function(geometry, height) {
  geometry.applyTransform((input, output, stride) => {
    console.assert(input === output);
    if (stride !== undefined && stride >= 3) {
      for (let i = 0; i < output.length; i += stride) {
        output[i + 2] = output[i + 2] + height;
      }
    }
    return output;
  });
};


/**
 * @param {ol.Coordinate} coordinates
 * @param {number=} rotation
 * @param {!Cesium.Cartesian3=} translation
 * @param {!Cesium.Cartesian3=} scale
 * @return {!Cesium.Matrix4}
 * @api
 */
exports.createMatrixAtCoordinates = function(coordinates, rotation = 0, translation = Cesium.Cartesian3.ZERO, scale = new Cesium.Cartesian3(1, 1, 1)) {
  const position = exports.ol4326CoordinateToCesiumCartesian(coordinates);
  const rawMatrix = Cesium.Transforms.eastNorthUpToFixedFrame(position);
  const quaternion = Cesium.Quaternion.fromAxisAngle(Cesium.Cartesian3.UNIT_Z, -rotation);
  const rotationMatrix = Cesium.Matrix4.fromTranslationQuaternionRotationScale(translation, quaternion, scale);
  return Cesium.Matrix4.multiply(rawMatrix, rotationMatrix, new Cesium.Matrix4());
};


/**
 * @param {!Cesium.Camera} camera
 * @param {number} angle
 * @param {!Cesium.Cartesian3} axis
 * @param {!Cesium.Matrix4} transform
 * @param {RotateAroundAxisOption=} opt_options
 * @api
 */
exports.rotateAroundAxis = function(camera, angle, axis, transform,
    opt_options) {
  const clamp = Cesium.Math.clamp;
  const defaultValue = Cesium.defaultValue;

  const options = opt_options || {};
  const duration = defaultValue(options.duration, 500); // ms
  const easing = defaultValue(options.easing, linearEasing);
  const callback = options.callback;

  let lastProgress = 0;
  const oldTransform = new Cesium.Matrix4();

  const start = Date.now();
  const step = function() {
    const timestamp = Date.now();
    const timeDifference = timestamp - start;
    const progress = easing(clamp(timeDifference / duration, 0, 1));
    console.assert(progress >= lastProgress);

    camera.transform.clone(oldTransform);
    const stepAngle = (progress - lastProgress) * angle;
    lastProgress = progress;
    camera.lookAtTransform(transform);
    camera.rotate(axis, stepAngle);
    camera.lookAtTransform(oldTransform);

    if (progress < 1) {
      window.requestAnimationFrame(step);
    } else {
      if (callback) {
        callback();
      }
    }
  };
  window.requestAnimationFrame(step);
};


/**
 * @param {!Cesium.Scene} scene
 * @param {number} heading
 * @param {!Cesium.Cartesian3} bottomCenter
 * @param {RotateAroundAxisOption=} opt_options
 * @api
 */
exports.setHeadingUsingBottomCenter = function(scene, heading,
    bottomCenter, opt_options) {
  const camera = scene.camera;
  // Compute the camera position to zenith quaternion
  const angleToZenith = exports.computeAngleToZenith(scene, bottomCenter);
  const axis = camera.right;
  const quaternion = Cesium.Quaternion.fromAxisAngle(axis, angleToZenith);
  const rotation = Cesium.Matrix3.fromQuaternion(quaternion);

  // Get the zenith point from the rotation of the position vector
  const vector = new Cesium.Cartesian3();
  Cesium.Cartesian3.subtract(camera.position, bottomCenter, vector);
  const zenith = new Cesium.Cartesian3();
  Cesium.Matrix3.multiplyByVector(rotation, vector, zenith);
  Cesium.Cartesian3.add(zenith, bottomCenter, zenith);

  // Actually rotate around the zenith normal
  const transform = Cesium.Matrix4.fromTranslation(zenith);
  const rotateAroundAxis = exports.rotateAroundAxis;
  rotateAroundAxis(camera, heading, zenith, transform, opt_options);
};


/**
 * Get the 3D position of the given pixel of the canvas.
 * @param {!Cesium.Scene} scene
 * @param {!Cesium.Cartesian2} pixel
 * @return {!Cesium.Cartesian3|undefined}
 * @api
 */
exports.pickOnTerrainOrEllipsoid = function(scene, pixel) {
  const ray = scene.camera.getPickRay(pixel);
  const target = scene.globe.pick(ray, scene);
  return target || scene.camera.pickEllipsoid(pixel);
};


/**
 * Get the 3D position of the point at the bottom-center of the screen.
 * @param {!Cesium.Scene} scene
 * @return {!Cesium.Cartesian3|undefined}
 * @api
 */
exports.pickBottomPoint = function(scene) {
  const canvas = scene.canvas;
  const bottom = new Cesium.Cartesian2(
      canvas.clientWidth / 2, canvas.clientHeight);
  return exports.pickOnTerrainOrEllipsoid(scene, bottom);
};


/**
 * Get the 3D position of the point at the center of the screen.
 * @param {!Cesium.Scene} scene
 * @return {!Cesium.Cartesian3|undefined}
 * @api
 */
exports.pickCenterPoint = function(scene) {
  const canvas = scene.canvas;
  const center = new Cesium.Cartesian2(
      canvas.clientWidth / 2,
      canvas.clientHeight / 2);
  return exports.pickOnTerrainOrEllipsoid(scene, center);
};


/**
 * Compute the signed tilt angle on globe, between the opposite of the
 * camera direction and the target normal. Return undefined if there is no
 * intersection of the camera direction with the globe.
 * @param {!Cesium.Scene} scene
 * @return {number|undefined}
 * @api
 */
exports.computeSignedTiltAngleOnGlobe = function(scene) {
  const camera = scene.camera;
  const ray = new Cesium.Ray(camera.position, camera.direction);
  let target = scene.globe.pick(ray, scene);

  if (!target) {
    // no tiles in the area were loaded?
    const ellipsoid = Cesium.Ellipsoid.WGS84;
    const obj = Cesium.IntersectionTests.rayEllipsoid(ray, ellipsoid);
    if (obj) {
      target = Cesium.Ray.getPoint(ray, obj.start);
    }
  }

  if (!target) {
    return undefined;
  }

  const normal = new Cesium.Cartesian3();
  Cesium.Ellipsoid.WGS84.geocentricSurfaceNormal(target, normal);

  const angleBetween = exports.signedAngleBetween;
  const angle = angleBetween(camera.direction, normal, camera.right) - Math.PI;
  return Cesium.Math.convertLongitudeRange(angle);
};


/**
 * Compute the ray from the camera to the bottom-center of the screen.
 * @param {!Cesium.Scene} scene
 * @return {!Cesium.Ray}
 */
exports.bottomFovRay = function(scene) {
  const camera = scene.camera;
  const fovy2 = camera.frustum.fovy / 2;
  const direction = camera.direction;
  const rotation = Cesium.Quaternion.fromAxisAngle(camera.right, fovy2);
  const matrix = Cesium.Matrix3.fromQuaternion(rotation);
  const vector = new Cesium.Cartesian3();
  Cesium.Matrix3.multiplyByVector(matrix, direction, vector);
  return new Cesium.Ray(camera.position, vector);
};


/**
 * Compute the angle between two Cartesian3.
 * @param {!Cesium.Cartesian3} first
 * @param {!Cesium.Cartesian3} second
 * @param {!Cesium.Cartesian3} normal Normal to test orientation against.
 * @return {number}
 */
exports.signedAngleBetween = function(first, second, normal) {
  // We are using the dot for the angle.
  // Then the cross and the dot for the sign.
  const a = new Cesium.Cartesian3();
  const b = new Cesium.Cartesian3();
  const c = new Cesium.Cartesian3();
  Cesium.Cartesian3.normalize(first, a);
  Cesium.Cartesian3.normalize(second, b);
  Cesium.Cartesian3.cross(a, b, c);

  const cosine = Cesium.Cartesian3.dot(a, b);
  const sine = Cesium.Cartesian3.magnitude(c);

  // Sign of the vector product and the orientation normal
  const sign = Cesium.Cartesian3.dot(normal, c);
  const angle = Math.atan2(sine, cosine);
  return sign >= 0 ? angle : -angle;
};


/**
 * Compute the rotation angle around a given point, needed to reach the
 * zenith position.
 * At a zenith position, the camera direction is going througth the earth
 * center and the frustrum bottom ray is going through the chosen pivot
 * point.
 * The bottom-center of the screen is a good candidate for the pivot point.
 * @param {!Cesium.Scene} scene
 * @param {!Cesium.Cartesian3} pivot Point around which the camera rotates.
 * @return {number}
 * @api
 */
exports.computeAngleToZenith = function(scene, pivot) {
  // This angle is the sum of the angles 'fy' and 'a', which are defined
  // using the pivot point and its surface normal.
  //        Zenith |    camera
  //           \   |   /
  //            \fy|  /
  //             \ |a/
  //              \|/pivot
  const camera = scene.camera;
  const fy = camera.frustum.fovy / 2;
  const ray = exports.bottomFovRay(scene);
  const direction = Cesium.Cartesian3.clone(ray.direction);
  Cesium.Cartesian3.negate(direction, direction);

  const normal = new Cesium.Cartesian3();
  Cesium.Ellipsoid.WGS84.geocentricSurfaceNormal(pivot, normal);

  const left = new Cesium.Cartesian3();
  Cesium.Cartesian3.negate(camera.right, left);

  const a = exports.signedAngleBetween(normal, direction, left);
  return a + fy;
};


/**
 * Convert an OpenLayers extent to a Cesium rectangle.
 * @param {ol.Extent} extent Extent.
 * @param {ol.ProjectionLike} projection Extent projection.
 * @return {Cesium.Rectangle} The corresponding Cesium rectangle.
 * @api
 */
exports.extentToRectangle = function(extent, projection) {
  if (extent && projection) {
    const ext = transformExtent(extent, projection, 'EPSG:4326');
    return Cesium.Rectangle.fromDegrees(ext[0], ext[1], ext[2], ext[3]);
  } else {
    return null;
  }
};


/**
 * @param {!ol.Map} olMap
 * @param {!ol.source.Source} source
 * @param {!ol.View} viewProj
 * @param {!ol.layer.Base} olLayer
 * @return {!Cesium.ImageryProvider}
 */
exports.sourceToImageryProvider = function(olMap, source, viewProj, olLayer) {
  const skip = source.get('olcs_skip');
  if (skip) {
    return null;
  }
  let provider = null;
  // Convert ImageWMS to TileWMS
  if (source instanceof olSourceImageWMS && source.getUrl() &&
    source.getImageLoadFunction() === defaultImageLoadFunction) {
    const sourceProps = {
      'olcs.proxy': source.get('olcs.proxy'),
      'olcs.extent': source.get('olcs.extent'),
      'olcs.projection': source.get('olcs.projection'),
      'olcs.imagesource': source
    };
    source = new olSourceTileWMS({
      url: source.getUrl(),
      attributions: source.getAttributions(),
      projection: source.getProjection(),
      params: source.getParams()
    });
    source.setProperties(sourceProps);
  }

  if (source instanceof olSourceTileImage) {
    let projection = olcsUtil.getSourceProjection(source);

    if (!projection) {
      // if not explicit, assume the same projection as view
      projection = viewProj;
    }

    if (exports.isCesiumProjection(projection)) {
      provider = new olcsCoreOLImageryProvider(olMap, source, viewProj);
    }
    // Projection not supported by Cesium
    else {
      return null;
    }
  } else if (source instanceof olSourceImageStatic) {
    let projection = olcsUtil.getSourceProjection(source);
    if (!projection) {
      projection = viewProj;
    }
    if (exports.isCesiumProjection(projection)) {
      provider = new Cesium.SingleTileImageryProvider({
        url: source.getUrl(),
        rectangle: new Cesium.Rectangle.fromDegrees(
            source.getImageExtent()[0],
            source.getImageExtent()[1],
            source.getImageExtent()[2],
            source.getImageExtent()[3]
        )
      });
    }
    // Projection not supported by Cesium
    else {
      return null;
    }
  } else if (source instanceof olSourceVectorTile) {
    let projection = olcsUtil.getSourceProjection(source);

    if (!projection) {
      projection = viewProj;
    }
    if (skip === false) {
      // MVT is experimental, it should be whitelisted to be synchronized
      const fromCode = projection.getCode().split(':')[1];
      const urls = source.urls.map(u => u.replace(fromCode, '3857'));
      const extent = olLayer.getExtent();
      const rectangle = exports.extentToRectangle(extent, projection);
      const minimumLevel = source.get('olcs_minimumLevel');
      const attributionsFunction = source.getAttributions();
      const styleFunction = olLayer.getStyleFunction();
      let credit;
      if (extent && attributionsFunction) {
        const center = getExtentCenter(extent);
        credit = attributionsFunctionToCredits(attributionsFunction, 0, center, extent)[0];
      }

      provider = new MVTImageryProvider({
        credit,
        rectangle,
        minimumLevel,
        styleFunction,
        urls
      });
      return provider;
    }
    return null; // FIXME: it is disabled by default right now
  } else {
    // sources other than TileImage|ImageStatic are currently not supported
    return null;
  }
  return provider;
};

/**
 * Creates Cesium.ImageryLayer best corresponding to the given ol.layer.Layer.
 * Only supports raster layers and static images
 * @param {!ol.Map} olMap
 * @param {!ol.layer.Base} olLayer
 * @param {!ol.proj.Projection} viewProj Projection of the view.
 * @return {?Cesium.ImageryLayer} null if not possible (or supported)
 * @api
 */
exports.tileLayerToImageryLayer = function(olMap, olLayer, viewProj) {

  if (!(olLayer instanceof olLayerTile) && !(olLayer instanceof olLayerImage) &&
    !(olLayer instanceof VectorTileLayer)) {
    return null;
  }

  const source = olLayer.getSource();
  if (!source) {
    return null;
  }
  let provider = source.get('olcs_provider');
  if (!provider) {
    provider = this.sourceToImageryProvider(olMap, source, viewProj, olLayer);
  }
  if (!provider) {
    return null;
  }

  const layerOptions = {};

  const forcedExtent = /** @type {ol.Extent} */ (olLayer.get('olcs.extent'));
  const ext = forcedExtent || olLayer.getExtent();
  if (ext) {
    layerOptions.rectangle = exports.extentToRectangle(ext, viewProj);
  }

  const cesiumLayer = new Cesium.ImageryLayer(provider, layerOptions);
  return cesiumLayer;
};


/**
 * Synchronizes the layer rendering properties (opacity, visible)
 * to the given Cesium ImageryLayer.
 * @param {olcsx.LayerWithParents} olLayerWithParents
 * @param {!Cesium.ImageryLayer} csLayer
 * @api
 */
exports.updateCesiumLayerProperties = function(olLayerWithParents, csLayer) {
  let opacity = 1;
  let visible = true;
  [olLayerWithParents.layer].concat(olLayerWithParents.parents).forEach((olLayer) => {
    const layerOpacity = olLayer.getOpacity();
    if (layerOpacity !== undefined) {
      opacity *= layerOpacity;
    }
    const layerVisible = olLayer.getVisible();
    if (layerVisible !== undefined) {
      visible &= layerVisible;
    }
  });
  csLayer.alpha = opacity;
  csLayer.show = visible;
};


/**
 * Convert a 2D or 3D OpenLayers coordinate to Cesium.
 * @param {ol.Coordinate} coordinate Ol3 coordinate.
 * @return {!Cesium.Cartesian3} Cesium cartesian coordinate
 * @api
 */
exports.ol4326CoordinateToCesiumCartesian = function(coordinate) {
  const coo = coordinate;
  return coo.length > 2 ?
    Cesium.Cartesian3.fromDegrees(coo[0], coo[1], coo[2]) :
    Cesium.Cartesian3.fromDegrees(coo[0], coo[1]);
};


/**
 * Convert an array of 2D or 3D OpenLayers coordinates to Cesium.
 * @param {Array.<!ol.Coordinate>} coordinates Ol3 coordinates.
 * @return {!Array.<Cesium.Cartesian3>} Cesium cartesian coordinates
 * @api
 */
exports.ol4326CoordinateArrayToCsCartesians = function(coordinates) {
  console.assert(coordinates !== null);
  const toCartesian = exports.ol4326CoordinateToCesiumCartesian;
  const cartesians = [];
  for (let i = 0; i < coordinates.length; ++i) {
    cartesians.push(toCartesian(coordinates[i]));
  }
  return cartesians;
};


/**
 * Reproject an OpenLayers geometry to EPSG:4326 if needed.
 * The geometry will be cloned only when original projection is not EPSG:4326
 * and the properties will be shallow copied.
 * @param {!T} geometry
 * @param {!ol.ProjectionLike} projection
 * @return {!T}
 * @template T
 * @api
 */
exports.olGeometryCloneTo4326 = function(geometry, projection) {
  console.assert(projection);

  const proj4326 = getProjection('EPSG:4326');
  const proj = getProjection(projection);
  if (proj !== proj4326) {
    const properties = geometry.getProperties();
    geometry = geometry.clone();
    geometry.transform(proj, proj4326);
    geometry.setProperties(properties);
  }
  return geometry;
};


/**
 * Convert an OpenLayers color to Cesium.
 * @param {ol.Color|CanvasGradient|CanvasPattern|string} olColor
 * @return {!Cesium.Color}
 * @api
 */
exports.convertColorToCesium = function(olColor) {
  olColor = olColor || 'black';
  if (Array.isArray(olColor)) {
    return new Cesium.Color(
        Cesium.Color.byteToFloat(olColor[0]),
        Cesium.Color.byteToFloat(olColor[1]),
        Cesium.Color.byteToFloat(olColor[2]),
        olColor[3]
    );
  } else if (typeof olColor == 'string') {
    return Cesium.Color.fromCssColorString(olColor);
  } else if (olColor instanceof CanvasPattern || olColor instanceof CanvasGradient) {
    // Render the CanvasPattern/CanvasGradient into a canvas that will be sent to Cesium as material
    const canvas = document.createElement('canvas');
    const ctx = canvas.getContext('2d');
    canvas.width = canvas.height = 256;
    ctx.fillStyle = olColor;
    ctx.fillRect(0, 0, canvas.width, canvas.height);
    return new Cesium.ImageMaterialProperty({
      image: canvas
    });
  }
  console.assert(false, 'impossible');
};


/**
 * Convert an OpenLayers url to Cesium.
 * @param {string} url
 * @return {!CesiumUrlDefinition}
 * @api
 */
exports.convertUrlToCesium = function(url) {
  let subdomains = '';
  const re = /\{(\d|[a-z])-(\d|[a-z])\}/;
  const match = re.exec(url);
  if (match) {
    url = url.replace(re, '{s}');
    const startCharCode = match[1].charCodeAt(0);
    const stopCharCode = match[2].charCodeAt(0);
    let charCode;
    for (charCode = startCharCode; charCode <= stopCharCode; ++charCode) {
      subdomains += String.fromCharCode(charCode);
    }
  }
  return {
    url,
    subdomains
  };
};


/**
 * Animate the return to a top-down view from the zenith.
 * The camera is rotated to orient to the North.
 * @param {!ol.Map} map
 * @param {!Cesium.Scene} scene
 * @return {Promise<undefined>}
 * @api
 */
exports.resetToNorthZenith = function(map, scene) {
  return new Promise((resolve, reject) => {
    const camera = scene.camera;
    const pivot = exports.pickBottomPoint(scene);
    if (!pivot) {
      reject('Could not get bottom pivot');
      return;
    }

    const currentHeading = map.getView().getRotation();
    if (currentHeading === undefined) {
      reject('The view is not initialized');
      return;
    }
    const angle = exports.computeAngleToZenith(scene, pivot);

    // Point to North
    exports.setHeadingUsingBottomCenter(scene, currentHeading, pivot);

    // Go to zenith
    const transform = Cesium.Matrix4.fromTranslation(pivot);
    const axis = camera.right;
    const options = {
      callback: () => {
        const view = map.getView();
        exports.normalizeView(view);
        resolve();
      }
    };
    exports.rotateAroundAxis(camera, -angle, axis, transform, options);
  });
};


/**
 * @param {!Cesium.Scene} scene
 * @param {number} angle in radian
 * @return {Promise<undefined>}
 * @api
 */
exports.rotateAroundBottomCenter = function(scene, angle) {
  return new Promise((resolve, reject) => {
    const camera = scene.camera;
    const pivot = exports.pickBottomPoint(scene);
    if (!pivot) {
      reject('could not get bottom pivot');
      return;
    }

    const options = {callback: resolve};
    const transform = Cesium.Matrix4.fromTranslation(pivot);
    const axis = camera.right;
    const rotateAroundAxis = exports.rotateAroundAxis;
    rotateAroundAxis(camera, -angle, axis, transform, options);
  });
};


/**
 * Set the OpenLayers view to a specific rotation and
 * the nearest resolution.
 * @param {ol.View} view
 * @param {number=} angle
 * @api
 */
exports.normalizeView = function(view, angle = 0) {
  const resolution = view.getResolution();
  view.setRotation(angle);
  if (view.constrainResolution) {
    view.setResolution(view.constrainResolution(resolution));
  } else {
    view.setResolution(view.getConstrainedResolution(resolution));
  }
};

/**
 * Check if the given projection is managed by Cesium (WGS84 or Mercator Spheric)
 *
 * @param {ol.proj.Projection} projection Projection to check.
 * @returns {boolean} Whether it's managed by Cesium.
 */
exports.isCesiumProjection = function(projection) {
  const is3857 = projection === getProjection('EPSG:3857');
  const is4326 = projection === getProjection('EPSG:4326');
  return is3857 || is4326;
};


export function attributionsFunctionToCredits(attributionsFunction, zoom, center, extent) {
  const frameState = {
    viewState: {zoom, center},
    extent,
  };

  if (!attributionsFunction) {
    return [];
  }
  let attributions = attributionsFunction(frameState);
  if (!Array.isArray(attributions)) {
    attributions = [attributions];
  }

  return attributions.map(html => new Cesium.Credit(html, true));
}

export default exports;


/**
 * calculate the distance between camera and centerpoint based on the resolution and latitude value
 * @param {number} resolution Number of map units per pixel.
 * @param {number} latitude Latitude in radians.
 * @param {import('cesium').Scene} scene.
 * @param {import('ol/proj/Projection').default} projection View projection.
 * @return {number} The calculated distance.
 * @api
 */
export function calcDistanceForResolution(resolution, latitude, scene, projection) {
  const canvas = scene.canvas;
  const camera = scene.camera;
  const fovy = camera.frustum.fovy; // vertical field of view
  console.assert(!isNaN(fovy));
  const metersPerUnit = projection.getMetersPerUnit();

  // number of "map units" visible in 2D (vertically)
  const visibleMapUnits = resolution * canvas.clientHeight;

  // The metersPerUnit does not take latitude into account, but it should
  // be lower with increasing latitude -- we have to compensate.
  // In 3D it is not possible to maintain the resolution at more than one point,
  // so it only makes sense to use the latitude of the "target" point.
  const relativeCircumference = Math.cos(Math.abs(latitude));

  // how many meters should be visible in 3D
  const visibleMeters = visibleMapUnits * metersPerUnit * relativeCircumference;

  // distance required to view the calculated length in meters
  //
  //  fovy/2
  //    |\
  //  x | \
  //    |--\
  // visibleMeters/2
  const requiredDistance = (visibleMeters / 2) / Math.tan(fovy / 2);

  // NOTE: This calculation is not absolutely precise, because metersPerUnit
  // is a great simplification. It does not take ellipsoid/terrain into account.

  return requiredDistance;
}

/**
 * calculate the resolution based on a distance(camera to position) and latitude value
 * @param {number} distance
 * @param {number} latitude
 * @param {import('cesium').Scene} scene.
 * @param {import('ol/proj/Projection').default} projection View projection.
 * @return {number} The calculated resolution.
 * @api
 */
export function calcResolutionForDistance(distance, latitude, scene, projection) {
  // See the reverse calculation (calcDistanceForResolution) for details
  const canvas = scene.canvas;
  const camera = scene.camera;
  const fovy = camera.frustum.fovy; // vertical field of view
  console.assert(!isNaN(fovy));
  const metersPerUnit = projection.getMetersPerUnit();

  const visibleMeters = 2 * distance * Math.tan(fovy / 2);
  const relativeCircumference = Math.cos(Math.abs(latitude));
  const visibleMapUnits = visibleMeters / metersPerUnit / relativeCircumference;
  const resolution = visibleMapUnits / canvas.clientHeight;

  return resolution;
}