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

// fracX/fracY (not pixel coordinates): the tile hasn't been fetched yet at
// this point, so its actual pixel size isn't known — online tiles are
// 256px PNG, offline ones (Mapterhorn, see lib/maptool/pmtiles_builder.rb)
// are 512px WebP. elevationAt scales these once the real size is known.
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, fracX: x - tileX, fracY: y - tileY };
}

async function decodeTerrarium(bytes, mime) {
  const bitmap = await createImageBitmap(new Blob([bytes], { type: mime }));
  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, size: bitmap.width };
}

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 ? { bytes: new Uint8Array(result.data), mime: "image/webp" } : null;
  }
  const url = ONLINE_TERRAIN_TILES[0].replace("{z}", z).replace("{x}", x).replace("{y}", y);
  const response = await fetch(url);
  return response.ok ? { bytes: new Uint8Array(await response.arrayBuffer()), mime: "image/png" } : 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((result) => (result ? decodeTerrarium(result.bytes, result.mime) : 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, fracX, fracY } = lngLatToTile(lng, lat, DEM_ZOOM);
  const tile = await fetchTile(DEM_ZOOM, tileX, tileY, offlineAvailable);
  if (!tile) return null;

  const px = Math.min(tile.size - 1, Math.floor(fracX * tile.size));
  const py = Math.min(tile.size - 1, Math.floor(fracY * tile.size));
  return tile.elevations[py * tile.size + px];
}