SkateSpots/src/lib/routeCache.ts

import type { RouteResult, TranslateFn } from '@/lib/routing';

/**
 * Tiny localStorage cache of the last few routes.
 *
 * Why: routing needs the network. If you lose signal mid-skate (or the routing
 * server is briefly down), Directions would show nothing at all. Keeping the
 * last route per destination+mode means we can still show the line, the time and
 * the steps — clearly labelled as cached — instead of an error.
 *
 * The full OSRM geometry can be thousands of points, so the line is
 * DOWNSAMPLED before storing (a route line at this zoom doesn't need them all).
 * Steps are kept verbatim — they're what you actually read.
 */
const KEY = 'skatespots:route-cache:v1';
const MAX_ENTRIES = 8;
const MAX_POINTS = 250;

interface StoredRoute {
  at: number;
  distanceM: number;
  durationS: number;
  steps?: RouteResult['steps'];
  coordinates: Array<[number, number]>;
}

type Store = Record<string, StoredRoute>;

function keyFor(from: { lat: number; lng: number }, to: { lat: number; lng: number }, mode: string): string {
  // Origin is part of the key: a route saved from a different start point is not
  // a valid fallback for the one the user chose (the "Saved route" label would lie).
  return `${from.lat.toFixed(4)},${from.lng.toFixed(4)}>${to.lat.toFixed(4)},${to.lng.toFixed(4)}:${mode}`;
}

function read(): Store {
  try {
    const raw = localStorage.getItem(KEY);
    return raw ? (JSON.parse(raw) as Store) : {};
  } catch {
    return {};
  }
}

function write(store: Store): void {
  try {
    localStorage.setItem(KEY, JSON.stringify(store));
  } catch {
    // quota / private mode — a cache is a nicety, never an error
  }
}

/** Keep every Nth point so the stored line stays small. */
function downsample(coords: Array<[number, number]>, max = MAX_POINTS): Array<[number, number]> {
  if (coords.length <= max) return coords;
  const step = Math.ceil(coords.length / max);
  const out: Array<[number, number]> = [];
  for (let i = 0; i < coords.length; i += step) out.push(coords[i]);
  const last = coords[coords.length - 1];
  if (out[out.length - 1] !== last) out.push(last);
  return out;
}

/** Remember a freshly-fetched route (best effort). */
export function saveRoute(
  from: { lat: number; lng: number },
  to: { lat: number; lng: number },
  mode: string,
  route: RouteResult,
): void {
  if (!route.coordinates?.length) return;
  const store = read();
  store[keyFor(from, to, mode)] = {
    at: Date.now(),
    distanceM: route.distanceM,
    durationS: route.durationS,
    steps: route.steps,
    coordinates: downsample(route.coordinates),
  };
  // Keep only the newest few destinations so this can never grow unbounded.
  const newest = Object.entries(store)
    .sort((a, b) => b[1].at - a[1].at)
    .slice(0, MAX_ENTRIES);
  write(Object.fromEntries(newest));
}

/** Read a cached route for this destination+mode, with how old it is. */
export function loadRoute(
  from: { lat: number; lng: number },
  to: { lat: number; lng: number },
  mode: string,
): { route: RouteResult; ageMs: number } | null {
  const stored = read()[keyFor(from, to, mode)];
  if (!stored || !Array.isArray(stored.coordinates) || stored.coordinates.length < 2) return null;
  return {
    route: {
      coordinates: stored.coordinates,
      distanceM: stored.distanceM,
      durationS: stored.durationS,
      steps: stored.steps,
      mode: mode as RouteResult['mode'],
      approximate: false,
    },
    ageMs: Date.now() - stored.at,
  };
}

/** "just now" | "12 min ago" | "2 h ago" — for the cached-route badge. */
export function formatAge(ageMs: number, t: TranslateFn): string {
  const mins = Math.round(ageMs / 60000);
  if (mins < 1) return t('routeUi.ageJustNow');
  if (mins < 60) return t('routeUi.ageMinAgo', { count: mins });
  const hours = Math.round(mins / 60);
  if (hours < 24) return t('routeUi.ageHourAgo', { count: hours });
  return t('routeUi.ageDayAgo', { count: Math.round(hours / 24) });
}