waseel/lib/dispatch.ts

// Uber-style auto-match dispatch. A requested ride has no driver; this engine
// offers it to the nearest eligible driver of the matching service. Drivers
// accept/decline; a decline (or a 15s offer expiry) triggers the next-nearest
// match. There is no background worker — matchNextDriver is called lazily from
// the rider status poll and the driver poll, so matching progresses on every
// request cycle.

import { transaction } from "@/lib/db";
import { haversine } from "@/lib/utils";

// A driver has this long to respond to an offer before it expires and the next
// driver is offered. Tuned short so a rider searching for a driver isn't left
// hanging on a phone that's face-down on a seat.
const OFFER_TTL_SECONDS = 15;
// A driver whose last location ping is older than this is treated as offline
// even if their `online` flag is still true (they closed the app without
// toggling off).
const DRIVER_STALE_SECONDS = 60;

type EligibleDriver = {
  id: number;
  latitude: number;
  longitude: number;
};

// Offer `rideId` to the nearest eligible driver, if no offer is already in
// flight for it. Idempotent: safe to call on every poll. Returns the driver id
// that was offered, or null if no driver was available.
export const matchNextDriver = async (
  rideId: number,
): Promise<number | null> => {
  try {
    return await transaction(async (tx) => {
      // Lock the ride row so concurrent matchers serialize on it.
      const rides = await tx<{ status: string; service: string }>`
        SELECT status, service FROM rides WHERE ride_id = ${rideId} FOR UPDATE
      `;
      const ride = rides[0];
      if (!ride || ride.status !== "requested") return null;

      // Expire any offers that have been sitting past their TTL.
      await tx`
        UPDATE ride_offers
        SET status = 'expired', responded_at = CURRENT_TIMESTAMP
        WHERE ride_id = ${rideId}
          AND status = 'offered'
          AND offered_at < CURRENT_TIMESTAMP - make_interval(secs => ${OFFER_TTL_SECONDS})
      `;

      // If there is still an active (unexpired) offer in flight, leave it —
      // don't stack a second offer on top.
      const inFlight = await tx<{ n: number }>`
        SELECT COUNT(*)::int AS n FROM ride_offers
        WHERE ride_id = ${rideId} AND status = 'offered'
      `;
      if ((inFlight[0]?.n ?? 0) > 0) return null;

      const rideOrigin = await tx<{ lat: number; lng: number }>`
        SELECT origin_latitude AS lat, origin_longitude AS lng
        FROM rides WHERE ride_id = ${rideId}
      `;
      const origin = rideOrigin[0];
      if (!origin) return null;

      // Eligible: right service, online, fresh, a real account, not on an
      // active ride, and not already offered/declined for THIS ride.
      const candidates = await tx<EligibleDriver>`
        SELECT d.id, d.latitude, d.longitude
        FROM drivers d
        WHERE d.service = ${ride.service}
          AND d.online = TRUE
          AND d.user_id IS NOT NULL
          AND d.latitude IS NOT NULL
          AND d.longitude IS NOT NULL
          AND d.last_seen > CURRENT_TIMESTAMP - make_interval(secs => ${DRIVER_STALE_SECONDS})
          AND NOT EXISTS (
            SELECT 1 FROM rides r
            WHERE r.driver_id = d.id AND r.status IN ('accepted', 'en_route')
          )
          AND NOT EXISTS (
            SELECT 1 FROM ride_offers ro
            WHERE ro.ride_id = ${rideId} AND ro.driver_id = d.id
          )
      `;

      if (candidates.length === 0) return null;

      // Nearest by great-circle distance to the pickup point.
      candidates.sort((a, b) => {
        const da = haversine(origin.lat, origin.lng, a.latitude, a.longitude);
        const db = haversine(origin.lat, origin.lng, b.latitude, b.longitude);
        return da - db;
      });
      const nearest = candidates[0];

      await tx`
        INSERT INTO ride_offers (ride_id, driver_id, status)
        VALUES (${rideId}, ${nearest.id}, 'offered')
      `;

      return nearest.id;
    });
  } catch (error) {
    console.error("[MATCH_NEXT_DRIVER]: ", error);
    return null;
  }
};