/**
 * Ground casting for street-level overlays on Google Photorealistic 3D.
 *
 * Draped lines and clamped billboards land on the top of the photoreal mesh,
 * so on a street lined with trees or under an overpass the coverage appears
 * to float at roof height. Casting places the overlays at the bare-earth
 * height from the terrain service (DEM) instead, plus a small lift, and lets
 * the mesh hide what is behind buildings.
 *
 * Heights are sampled on a coarse grid and interpolated, so one z14 coverage
 * tile costs at most a few hundred terrain lookups, and the grid matches the
 * ground-floor service (3 decimals) so cached cells are shared. Only real DEM
 * heights count; a geoid fallback (terrain proxy down) is treated as unknown
 * and the caller keeps its draped placement.
 */

/** Grid spacing in degrees (~111 m north-south): the ground-floor grid. */
export const GROUND_CAST_STEP_DEG = 0.001;
/** Longest line segment left undivided, in degrees, so lines follow the grid. */
export const GROUND_CAST_DENSIFY_DEG = 0.0005;
/** Metres above the bare earth, so lines sit on the road instead of in it. */
export const GROUND_CAST_LIFT_M = 2;
/** Grid corners one prepare call may request; more stays draped. */
export const GROUND_CAST_MAX_CORNERS = 1024;
/** Cached grid corners before the cache is dropped and refilled. */
const GROUND_CAST_CACHE_MAX = 50_000;

/**
 * Mesh refinement. The bare-earth grid cannot see a freeway in a trench, a
 * steep street between grid corners, or the road under a tree, so where the
 * rendered Google 3D surface has been sampled it refines the height:
 *   - mesh at most MESH_ROAD_ABOVE_M above bare earth (and not absurdly far
 *     below it) is the road itself: follow it, MESH_LIFT_M above;
 *   - mesh higher than that is something over the road (canopy, deck, roof):
 *     carry the road's offset from bare earth across from the nearest road
 *     samples on both sides, within MESH_BRIDGE_MAX_POINTS;
 *   - otherwise bare earth plus GROUND_CAST_LIFT_M, as without the mesh.
 */
export const MESH_ROAD_ABOVE_M = 3;
/** A sample this far below bare earth is a bad probe, not a road. */
export const MESH_ROAD_BELOW_M = 30;
/** Metres above the sampled road surface. */
export const MESH_LIFT_M = 1;
/** Longest run of covered points bridged from the road on either side. */
export const MESH_BRIDGE_MAX_POINTS = 8;
/** Line spacing, in degrees (~22 m), while the mesh refines heights. */
export const MESH_DENSIFY_DEG = 0.0002;

/**
 * Heights for a line's points from bare earth and, where sampled, the mesh.
 * Pure, so the rule above is testable without a scene.
 * @param {Array<{dem: number, mesh?: number|null}>} points
 * @param {{lift?: number}} [options]
 * @returns {Array<number>}
 */
export function refineHeights(points, { lift = GROUND_CAST_LIFT_M } = {}) {
  const road = points.map(({ dem, mesh }) =>
    Number.isFinite(mesh) &&
    mesh <= dem + MESH_ROAD_ABOVE_M &&
    mesh >= dem - MESH_ROAD_BELOW_M
      ? mesh - dem
      : null,
  );
  return points.map(({ dem, mesh }, i) => {
    if (road[i] !== null) return dem + road[i] + MESH_LIFT_M;
    if (Number.isFinite(mesh) && mesh > dem + MESH_ROAD_ABOVE_M) {
      let a = i - 1;
      while (a >= 0 && i - a <= MESH_BRIDGE_MAX_POINTS && road[a] === null) a--;
      let b = i + 1;
      while (
        b < points.length &&
        b - i <= MESH_BRIDGE_MAX_POINTS &&
        road[b] === null
      )
        b++;
      const left = a >= 0 && i - a <= MESH_BRIDGE_MAX_POINTS ? road[a] : null;
      const right =
        b < points.length && b - i <= MESH_BRIDGE_MAX_POINTS ? road[b] : null;
      if (left !== null && right !== null) {
        const t = (i - a) / (b - a);
        return dem + left + (right - left) * t + MESH_LIFT_M;
      }
    }
    return dem + lift;
  });
}

/** Camera height (m above ground) below which Google 3D switches to terrain mode. */
export const SURFACE_TERRAIN_ENTER_M = 1400;
/** Camera height above which terrain mode switches back to draped. */
export const SURFACE_TERRAIN_EXIT_M = 1800;

/** Surface modes: draped on whatever the globe shows, or cast to bare earth. */
export const SURFACE_MODES = Object.freeze(['draped', 'terrain']);

/**
 * The next surface mode for the camera. Terrain mode applies only on Google
 * 3D and only at street zoom; the gap between the enter and exit heights
 * stops the mode from flapping while the camera hovers at the boundary.
 * @param {'draped'|'terrain'} current
 * @param {{photoreal: boolean, heightM: number|null, available: boolean}} view
 * @returns {'draped'|'terrain'}
 */
export function nextSurfaceMode(current, { photoreal, heightM, available }) {
  if (!available || !photoreal || !Number.isFinite(heightM)) return 'draped';
  if (current === 'terrain')
    return heightM > SURFACE_TERRAIN_EXIT_M ? 'draped' : 'terrain';
  return heightM < SURFACE_TERRAIN_ENTER_M ? 'terrain' : 'draped';
}

/**
 * Split long segments so a line follows the terrain between its vertices.
 * @param {Array<[number, number]>} coords [lon, lat] pairs
 * @param {number} [maxStep] longest segment, in degrees
 * @returns {Array<[number, number]>}
 */
export function densifyLine(coords, maxStep = GROUND_CAST_DENSIFY_DEG) {
  if (!Array.isArray(coords) || coords.length < 2) return coords || [];
  const out = [coords[0]];
  for (let i = 1; i < coords.length; i++) {
    const [lon0, lat0] = coords[i - 1];
    const [lon1, lat1] = coords[i];
    const span = Math.max(Math.abs(lon1 - lon0), Math.abs(lat1 - lat0));
    const parts = Math.min(64, Math.ceil(span / maxStep - 1e-9));
    for (let k = 1; k < parts; k++) {
      const t = k / parts;
      out.push([lon0 + (lon1 - lon0) * t, lat0 + (lat1 - lat0) * t]);
    }
    out.push(coords[i]);
  }
  return out;
}

/**
 * A bare-earth height cache over the terrain service.
 * @param {{terrain: {resolveEllipsoidalGround: Function}, step?: number, lift?: number, maxCorners?: number}} options
 */
export function createGroundCaster({
  terrain,
  step = GROUND_CAST_STEP_DEG,
  lift = GROUND_CAST_LIFT_M,
  maxCorners = GROUND_CAST_MAX_CORNERS,
}) {
  if (typeof terrain?.resolveEllipsoidalGround !== 'function')
    throw new TypeError('Ground casting requires a terrain service');
  /** "i,j" grid corner → ellipsoidal ground height in metres. */
  const heights = new Map();
  /** Prepares run one at a time, so neighbouring tiles share their corners. */
  let queue = Promise.resolve();

  const cornerKey = (i, j) => `${i},${j}`;

  function missingCorners(points) {
    const missing = new Map();
    for (const point of points) {
      const [lon, lat] = point;
      if (!Number.isFinite(lon) || !Number.isFinite(lat)) continue;
      const i0 = Math.floor(lon / step);
      const j0 = Math.floor(lat / step);
      for (let di = 0; di <= 1; di++)
        for (let dj = 0; dj <= 1; dj++) {
          const key = cornerKey(i0 + di, j0 + dj);
          if (!heights.has(key) && !missing.has(key))
            missing.set(key, { i: i0 + di, j: j0 + dj });
        }
    }
    return [...missing.entries()];
  }

  async function resolve(points, signal) {
    if (signal?.aborted) return false;
    const missing = missingCorners(points);
    if (!missing.length) return true;
    if (missing.length > maxCorners) return false;
    const results = await terrain.resolveEllipsoidalGround(
      missing.map(([, { i, j }]) => ({
        lon: Number((i * step).toFixed(6)),
        lat: Number((j * step).toFixed(6)),
      })),
    );
    if (heights.size + missing.length > GROUND_CAST_CACHE_MAX) heights.clear();
    let complete = true;
    missing.forEach(([key], n) => {
      const result = results?.[n];
      if (result?.source === 'reearth' && Number.isFinite(result.ellipsoid))
        heights.set(key, result.ellipsoid);
      else complete = false;
    });
    return complete && !signal?.aborted;
  }

  /**
   * Fetch the grid corners around the given points. Resolves true when every
   * point can now be cast; false when some corners are unknown (terrain proxy
   * down, too many corners, or aborted). Never rejects.
   * @param {Array<[number, number]>} points [lon, lat] pairs
   * @param {{signal?: AbortSignal}} [options]
   */
  function prepare(points, { signal } = {}) {
    const run = queue.then(() => resolve(points, signal)).catch(() => false);
    queue = run;
    return run;
  }

  /** `prepare` for lines, including the points densifying adds. */
  function prepareLines(lines, options) {
    return prepare(
      lines.flatMap((coords) => densifyLine(coords)),
      options,
    );
  }

  /** Bare-earth ellipsoidal height at a point, from cached corners only; null when unknown. */
  function groundAt(lon, lat) {
    const x = lon / step;
    const y = lat / step;
    const i0 = Math.floor(x);
    const j0 = Math.floor(y);
    const h00 = heights.get(cornerKey(i0, j0));
    const h10 = heights.get(cornerKey(i0 + 1, j0));
    const h01 = heights.get(cornerKey(i0, j0 + 1));
    const h11 = heights.get(cornerKey(i0 + 1, j0 + 1));
    if (h00 === undefined || h10 === undefined) return null;
    if (h01 === undefined || h11 === undefined) return null;
    const fx = x - i0;
    const fy = y - j0;
    const south = h00 + (h10 - h00) * fx;
    const north = h01 + (h11 - h01) * fx;
    return south + (north - south) * fy;
  }

  /** Where an overlay goes: bare earth plus the lift; null when unknown. */
  function heightAt(lon, lat) {
    const ground = groundAt(lon, lat);
    return ground === null ? null : ground + lift;
  }

  /**
   * A line as flat [lon, lat, height, ...] degrees and metres, densified;
   * null when any corner it needs is not cached yet. With `meshAt` (sampled
   * mesh heights, undefined or null where unknown) the line is densified
   * finer and its heights refined by `refineHeights`.
   * @param {Array<[number, number]>} coords
   * @param {{meshAt?: (lon: number, lat: number) => number|null|undefined}} [options]
   */
  function castLine(coords, { meshAt = null } = {}) {
    const rows = [];
    const points = meshAt
      ? densifyLine(coords, MESH_DENSIFY_DEG)
      : densifyLine(coords);
    for (const [lon, lat] of points) {
      const dem = groundAt(lon, lat);
      if (dem === null) return null;
      rows.push({ lon, lat, dem, mesh: meshAt ? meshAt(lon, lat) : null });
    }
    const heights = refineHeights(rows, { lift });
    const flat = [];
    rows.forEach(({ lon, lat }, i) => flat.push(lon, lat, heights[i]));
    return flat;
  }

  return { prepare, prepareLines, groundAt, heightAt, castLine };
}