// Elevation lookups against the same Terrarium-encoded terrain tiles
// overlays.js's HillshadeControl already renders — pulled into its own file
// since TODO.adoc lists two more near-future features (slope readout, solar
// calculator) that will need the same "elevation at a point" primitive,
// shadow.js's raymarch being the first real consumer.

import { ONLINE_TERRAIN_TILES } from "/js/overlays.js";

const DEM_ZOOM = 10; // fixed lookup zoom, independent of the map's own display zoom
const TILE_SIZE = 256;

const tileCache = new Map(); // "z/x/y" -> Promise<Int16Array|null>, so concurrent lookups on the same not-yet-fetched tile share one fetch
let offlinePmtiles = null;

function lngLatToTile(lng, lat, zoom) {
  const n = 2 ** zoom;
  const x = ((lng + 180) / 360) * n;
  const latRad = (lat * Math.PI) / 180;
  const y = ((1 - Math.log(Math.tan(latRad) + 1 / Math.cos(latRad)) / Math.PI) / 2) * n;
  const tileX = Math.floor(x);
  const tileY = Math.floor(y);
  return {
    tileX,
    tileY,
    px: Math.min(TILE_SIZE - 1, Math.floor((x - tileX) * TILE_SIZE)),
    py: Math.min(TILE_SIZE - 1, Math.floor((y - tileY) * TILE_SIZE)),
  };
}

async function decodeTerrarium(pngBytes) {
  const bitmap = await createImageBitmap(new Blob([pngBytes], { type: "image/png" }));
  const canvas = new OffscreenCanvas(bitmap.width, bitmap.height);
  const ctx = canvas.getContext("2d");
  ctx.drawImage(bitmap, 0, 0);
  const { data } = ctx.getImageData(0, 0, bitmap.width, bitmap.height);
  const elevations = new Int16Array(bitmap.width * bitmap.height);
  for (let i = 0; i < elevations.length; i++) {
    elevations[i] = data[i * 4] * 256 + data[i * 4 + 1] + data[i * 4 + 2] / 256 - 32768;
  }
  return elevations;
}

async function fetchTileBytes(z, x, y, offlineAvailable) {
  if (offlineAvailable) {
    if (!offlinePmtiles) offlinePmtiles = new window.pmtiles.PMTiles(`${window.location.origin}/tiles/terrain.pmtiles`);
    const result = await offlinePmtiles.getZxy(z, x, y);
    return result ? new Uint8Array(result.data) : null;
  }
  const url = ONLINE_TERRAIN_TILES[0].replace("{z}", z).replace("{x}", x).replace("{y}", y);
  const response = await fetch(url);
  return response.ok ? new Uint8Array(await response.arrayBuffer()) : null;
}

function fetchTile(z, x, y, offlineAvailable) {
  const key = `${z}/${x}/${y}`;
  let cached = tileCache.get(key);
  if (cached) return cached;

  const promise = fetchTileBytes(z, x, y, offlineAvailable)
    .then((bytes) => (bytes ? decodeTerrarium(bytes) : null))
    .catch(() => null);
  tileCache.set(key, promise);
  return promise;
}

// -> Promise<number|null> — null on a fetch/decode failure, or a point
// falling outside a partial offline extract's zoom range.
export async function elevationAt(lng, lat, offlineAvailable) {
  const { tileX, tileY, px, py } = lngLatToTile(lng, lat, DEM_ZOOM);
  const elevations = await fetchTile(DEM_ZOOM, tileX, tileY, offlineAvailable);
  return elevations ? elevations[py * TILE_SIZE + px] : null;
}