tile_calculations.js

export const validateMinMaxValues = (minX, minY, maxX, maxY) => {
  if (isNaN(minX) || isNaN(maxX) || isNaN(minY) || isNaN(maxY)) {
    throw new Error("One or more tile coordinates are NaN");
  }

  if (minX > maxX) {
    throw new Error("minX cannot be greater than maxX");
  }
  if (minY > maxY) {
    throw new Error("minY cannot be greater than maxY");
  }
};

// Converts lat/long value to a tile coordinate at a given zoom level, so as to
// determine X and Y position (column) of a tile in a grid based on geographic longitude
// in the Web Mercator projection.
// See: https://wiki.openstreetmap.org/wiki/Slippy_map_tilenames#Lon./lat._to_tile_numbers
export const convertCoordinatesToTiles = (lon, lat, zoom) => {
  const n = Math.pow(2, zoom);
  const x = Math.floor(((lon + 180) / 360) * n);
  const y = Math.floor(
    ((1 -
      Math.log(
        Math.tan((lat * Math.PI) / 180) + 1 / Math.cos((lat * Math.PI) / 180),
      ) /
        Math.PI) /
      2) *
      n,
  );
  return { x, y };
};

// Calculates the range of tile coordinates that cover a given geographic bounding box
// at a specific zoom level.
export const calculateTileRangeForBounds = (bounds, zoom) => {
  const [minLon, minLat, maxLon, maxLat] = bounds;

  const { x: minX, y: maxY } = convertCoordinatesToTiles(minLon, minLat, zoom);
  const { x: maxX, y: minY } = convertCoordinatesToTiles(maxLon, maxLat, zoom);

  return { minX, minY, maxX, maxY };
};

// Converts tile X and Y coordinates at a given zoom level back to geographic coordinates
// (longitude and latitude).
// See: https://wiki.openstreetmap.org/wiki/Slippy_map_tilenames#Tile_numbers_to_lon..2Flat._2
export const convertTilesToCoordinates = (x, y, zoom) => {
  const lon = (x / Math.pow(2, zoom)) * 360 - 180;
  const latRad = Math.atan(
    Math.sinh(Math.PI * (1 - (2 * y) / Math.pow(2, zoom))),
  );
  const lat = (latRad * 180.0) / Math.PI;
  return { lon, lat };
};

// Calculates mercator-normalized center coordinates of a tile at a given zoom level.
// More about mercator tile normalization: https://maplibre.org/maplibre-native/docs/book/design/coordinate-system.html
export const calculateNormalizedCenterCoords = (x, y, zoom) => {
  // Calculate longitude and latitude from tile x, y, and zoom
  const nw = convertTilesToCoordinates(x, y, zoom);
  const se = convertTilesToCoordinates(x + 1, y + 1, zoom);

  // Normalize latitude to the Mercator projection
  const mercatorNwY = Math.log(
    Math.tan(Math.PI / 4 + (nw.lat * Math.PI) / 360),
  );
  const mercatorSeY = Math.log(
    Math.tan(Math.PI / 4 + (se.lat * Math.PI) / 360),
  );
  const avgMercatorY = (mercatorNwY + mercatorSeY) / 2;
  const centerLat = (Math.atan(Math.exp(avgMercatorY)) * 360) / Math.PI - 90;

  // Longitude remains a simple average
  const centerLon = (nw.lon + se.lon) / 2;

  return [centerLon, centerLat];
};