import * as os from 'os';
import { logInfo, logError } from '../utils';

/**
 * Default concurrency = max(2, cpus - 1). Leaves one core for the VS Code UI
 * thread and the extension host, scales up on bigger boxes. Static per-process
 * (no dynamic adjustment) — kept simple because the heavy work (LLM calls)
 * is gated by `missionId` locks elsewhere, not the action queue.
 */
function defaultConcurrencyLimit(): number {
    try {
        const cpus = os.cpus()?.length ?? 4;
        return Math.max(2, cpus - 1);
    } catch {
        return 3;
    }
}

/**
 * ActionQueueManager: Manages large-scale tasks by processing them
 * with a concurrency limit to prevent resource exhaustion and I/O bottlenecks
 * while maintaining high throughput under maximum load.
 */
export class ActionQueueManager {
    private queue: (() => Promise<void>)[] = [];
    private activeCount: number = 0;
    private readonly concurrencyLimit: number;

    constructor(concurrencyLimit: number = defaultConcurrencyLimit()) {
        this.concurrencyLimit = concurrencyLimit;
    }

    /**
     * Adds a task to the queue.
     */
    public async enqueue<T>(task: () => Promise<T>): Promise<T> {
        return new Promise<T>((resolve, reject) => {
            this.queue.push(async () => {
                try {
                    const result = await task();
                    resolve(result);
                } catch (error) {
                    reject(error);
                }
            });
            this.processNext();
        });
    }

    private async processNext() {
        if (this.activeCount >= this.concurrencyLimit || this.queue.length === 0) return;

        this.activeCount++;
        const task = this.queue.shift();

        if (task) {
            try {
                // Add a micro-delay to allow system breathing room between heavy I/O
                await new Promise(r => setTimeout(r, 10));
                await task();
            } catch (error) {
                logError('Task in queue failed:', error);
            } finally {
                this.activeCount--;
                this.processNext();
            }
        } else {
            this.activeCount--;
        }
    }

    public getPendingCount(): number {
        return this.queue.length;
    }

    public getActiveCount(): number {
        return this.activeCount;
    }
}

export const actionQueue = new ActionQueueManager();