import os from 'os';

/**Generic parallel runner with optional logging
 * Runs `targets` with `runFn` up to a maximum of `maxConcurrency` amount at a time
 * Shaped in a way that expects generally something that returns zx.ProcessOutput (or
 * something with .duration and .ok built-in to the return)
 * @param runFn Should NOT throw. Return { ok: false } instead
 */
export async function parallel<T, R extends { duration: number, ok: boolean }>(
  targets: T[],
  runFn: (t: T)=>Promise<R>,
  quiet: boolean = false,
  maxConcurrency: number = os.cpus().length
): Promise<R[]> {
  const resultMap = new Map<T, R>();

  const total = targets.length;
  let completed = 0;
  let running = 0;
  const completionTimes: number[] = [];
  const startTime = Date.now();

  const inFlight = new Set<Promise<void>>();

  function formatEta(): string {
    const left = total - completed;
    const avgSeconds = completionTimes.length > 0
      ? completionTimes.reduce((a, b) => a + b, 0) / completionTimes.length / 1000
      : 0;
    const etaSeconds = Math.round(left * avgSeconds);
    const pct = total > 0 ? Math.round((completed / total) * 100) : 100;
    const lastDuration = completionTimes.length > 0
      ? (completionTimes[completionTimes.length - 1] / 1000).toFixed(1)
      : '0.0';

    return `ETA: ${etaSeconds}s Left: ${left} AVG: ${avgSeconds.toFixed(2)}s  local:${running}/${completed}/${pct}%/${lastDuration}s`;
  }

  function printStatus(): void {
    if (quiet) {
      return;
    }
    process.stderr.write(`\r${formatEta()}`.padEnd(80));
  }

  async function runJob(t: T): Promise<void> {
    running++;
    printStatus();

    const result = await runFn(t);
    completionTimes.push(result.duration);

    resultMap.set(t, result);

    running--;
    completed++;
    printStatus();
  }

  const queue = targets.slice();
  // Process queue with concurrency limit
  while (queue.length > 0 || inFlight.size > 0) {
    // Fill up to max concurrency
    while (queue.length > 0 && inFlight.size < maxConcurrency) {
      const target = queue.shift()!;
      const promise = runJob(target).then(() => {
        inFlight.delete(promise);
      });
      inFlight.add(promise);
    }

    // Wait for at least one to complete if at capacity
    if (inFlight.size > 0) {
      await Promise.race(inFlight);
    }
  }

  // Final status line
  process.stderr.write('\n');
  const totalSeconds = ((Date.now() - startTime) / 1000).toFixed(1);
  const failed = Array.from(resultMap.values().filter(p => !p.ok));
  if (!quiet) {
    process.stderr.write(
      `\nCompleted ${total} jobs in ${totalSeconds}s (${failed.length} failed)\n`
    );
  }

  const output = targets
    .map(t => {
      const r = resultMap.get(t)!;
      return r;
    });

  return output;
}