ciaovolo/flight-comparator/frontend/src/hooks/useScanTimer.ts

import { useEffect, useRef, useState } from 'react';
import type { Scan } from '../api';

export interface ScanTimerResult {
  /** Seconds elapsed since the scan started processing. */
  elapsedSeconds: number;
  /**
   * Estimated seconds remaining, or null when not enough data yet
   * (fewer than 5 routes scanned or elapsed time is 0).
   */
  remainingSeconds: number | null;
  /** True while the estimate is still too early to be reliable. */
  isEstimating: boolean;
}

const MIN_ROUTES_FOR_ESTIMATE = 5;

function calcElapsed(startedAt: string): number {
  return Math.max(0, (Date.now() - new Date(startedAt).getTime()) / 1000);
}

function calcRemaining(
  elapsed: number,
  routesScanned: number,
  totalRoutes: number,
): number | null {
  if (elapsed <= 0 || routesScanned < MIN_ROUTES_FOR_ESTIMATE || totalRoutes <= 0) {
    return null;
  }
  const rate = routesScanned / elapsed; // routes per second
  const remaining = (totalRoutes - routesScanned) / rate;
  return Math.max(0, remaining);
}

export function useScanTimer(scan: Scan | null): ScanTimerResult {
  const [elapsedSeconds, setElapsedSeconds] = useState(0);
  const [remainingSeconds, setRemainingSeconds] = useState<number | null>(null);
  const intervalRef = useRef<ReturnType<typeof setInterval> | undefined>(undefined);

  useEffect(() => {
    if (!scan) return;

    // For completed / failed scans with both timestamps: compute static duration.
    if (
      (scan.status === 'completed' || scan.status === 'failed') &&
      scan.started_at &&
      scan.completed_at
    ) {
      const duration = Math.max(
        0,
        (new Date(scan.completed_at).getTime() - new Date(scan.started_at).getTime()) / 1000,
      );
      setElapsedSeconds(duration);
      setRemainingSeconds(0);
      return;
    }

    // For running scans with a start time: run a live 1-second timer.
    if (scan.status === 'running' && scan.started_at) {
      const tick = () => {
        const elapsed = calcElapsed(scan.started_at!);
        const remaining = calcRemaining(elapsed, scan.routes_scanned, scan.total_routes);
        setElapsedSeconds(elapsed);
        setRemainingSeconds(remaining);
      };

      tick(); // run immediately
      intervalRef.current = setInterval(tick, 1000);

      return () => {
        if (intervalRef.current !== undefined) {
          clearInterval(intervalRef.current);
          intervalRef.current = undefined;
        }
      };
    }

    // Pending or no started_at: reset
    setElapsedSeconds(0);
    setRemainingSeconds(null);
  }, [scan?.status, scan?.started_at, scan?.completed_at, scan?.routes_scanned, scan?.total_routes]);

  const isEstimating =
    scan?.status === 'running' &&
    (scan.routes_scanned < MIN_ROUTES_FOR_ESTIMATE || scan.total_routes <= 0);

  return { elapsedSeconds, remainingSeconds, isEstimating };
}