const h3 = require('h3-js');

const DEFAULT_RES = 9; // ~150m hex edge — good for foot-traffic granularity

function cellsForRoute(route, resolution = DEFAULT_RES) {
  if (!Array.isArray(route) || route.length === 0) return [];
  const set = new Set();
  for (const pt of route) {
    const lat = Number(pt?.latitude);
    const lng = Number(pt?.longitude);
    if (!isFinite(lat) || !isFinite(lng)) continue;
    set.add(h3.latLngToCell(lat, lng, resolution));
  }
  return [...set];
}

function cellsForBbox(sw, ne, resolution = DEFAULT_RES) {
  if (!sw || !ne) return [];
  const polygon = [[
    [sw.lat, sw.lng],
    [sw.lat, ne.lng],
    [ne.lat, ne.lng],
    [ne.lat, sw.lng],
    [sw.lat, sw.lng],
  ]];
  return h3.polygonToCells(polygon, resolution);
}

function boundary(cellId) {
  // returns array of [lat, lng] vertices
  return h3.cellToBoundary(cellId);
}

module.exports = { DEFAULT_RES, cellsForRoute, cellsForBbox, boundary };