import { SANDBOX_CODES } from "../error-codes/error-codes.js";
import { recordPoolAcquisition } from "../telemetry/telemetry.js";
import type {
    Clock,
    ExecutionContext,
    SandboxEngine,
    SandboxEngineConfig,
    SandboxOptions,
    SandboxResult,
} from "../types/sandbox.types.js";

/** Factory that builds engines on demand. */
export type EngineFactory = (config: SandboxEngineConfig) => SandboxEngine;

/** Pool configuration. */
export interface SandboxPoolConfig {
    readonly minSize: number;
    readonly maxSize: number;
    readonly engineConfig: SandboxEngineConfig;
    readonly factory: EngineFactory;
    readonly idleTimeoutMs: number;
    readonly maxExecutionsPerEngine: number;
    readonly clock?: Clock;
}

interface PooledEngine {
    engine: SandboxEngine;
    inUse: boolean;
    executionCount: number;
    lastUsed: number;
    createdAt: number;
}

const DEFAULTS: Omit<SandboxPoolConfig, "engineConfig" | "factory"> = {
    minSize: 2,
    maxSize: 10,
    idleTimeoutMs: 60_000,
    maxExecutionsPerEngine: 1000,
};

/**
 * Pool of pre-warmed sandbox engines. The pool grows lazily up to `maxSize`,
 * recycles engines past `maxExecutionsPerEngine`, and reclaims idle engines
 * above `minSize` whose `lastUsed` exceeds `idleTimeoutMs` (relative to the
 * injectable {@link Clock}, defaulting to `Date.now`).
 */
export class SandboxPool {
    private readonly config: SandboxPoolConfig;
    private readonly clock: Clock;
    private pool: PooledEngine[] = [];

    public constructor(
        config: Pick<SandboxPoolConfig, "engineConfig" | "factory"> &
            Partial<SandboxPoolConfig>
    ) {
        this.config = { ...DEFAULTS, ...config };
        this.clock = this.config.clock ?? (() => Date.now());
        this.warmUp();
    }

    /** Execute code on a pooled engine. */
    public async execute<T = unknown>(
        code: string,
        context?: ExecutionContext,
        options?: SandboxOptions
    ): Promise<SandboxResult<T>> {
        const pooled = this.acquire();
        if (pooled === null) {
            recordPoolAcquisition("exhausted");
            return {
                success: false,
                result: undefined,
                error: `Sandbox pool exhausted (${SANDBOX_CODES.POOL_EXHAUSTED})`,
                duration: 0,
                memoryUsed: 0,
                logs: [],
            };
        }
        try {
            const result = await pooled.engine.execute<T>(
                code,
                context,
                options
            );
            pooled.executionCount += 1;
            pooled.lastUsed = this.clock();
            return result;
        } finally {
            this.release(pooled);
        }
    }

    /** Current pool size. */
    public size(): number {
        return this.pool.length;
    }

    /** Idle engines. */
    public available(): number {
        return this.pool.filter((p) => !p.inUse).length;
    }

    /** In-use engines. */
    public inUse(): number {
        return this.pool.filter((p) => p.inUse).length;
    }

    /** Aggregate pool stats. */
    public stats(): {
        size: number;
        available: number;
        inUse: number;
        totalExecutions: number;
    } {
        let totalExecutions = 0;
        for (const p of this.pool) totalExecutions += p.executionCount;
        return {
            size: this.pool.length,
            available: this.available(),
            inUse: this.inUse(),
            totalExecutions,
        };
    }

    /** Bring the pool up to `minSize`. */
    public warmUp(): void {
        while (this.pool.length < this.config.minSize) {
            this.pool.push(this.createPooled());
        }
    }

    /** Single cleanup tick — reclaims idle engines above `minSize`. */
    public cleanupTick(): void {
        const now = this.clock();
        const survivors: PooledEngine[] = [];
        let removed = 0;
        for (const pooled of this.pool) {
            if (
                !pooled.inUse &&
                this.pool.length - removed > this.config.minSize &&
                now - pooled.lastUsed > this.config.idleTimeoutMs
            ) {
                pooled.engine.destroy();
                removed += 1;
                continue;
            }
            survivors.push(pooled);
        }
        this.pool = survivors;
    }

    /** Destroy every engine in the pool. */
    public destroy(): void {
        for (const pooled of this.pool) {
            pooled.engine.destroy();
        }
        this.pool = [];
    }

    private acquire(): PooledEngine | null {
        const idle = this.pool.find((p) => !p.inUse);
        if (idle !== undefined) {
            idle.inUse = true;
            recordPoolAcquisition("acquired");
            return idle;
        }
        if (this.pool.length < this.config.maxSize) {
            const pooled = this.createPooled();
            pooled.inUse = true;
            this.pool.push(pooled);
            recordPoolAcquisition("created");
            return pooled;
        }
        return null;
    }

    private release(pooled: PooledEngine): void {
        pooled.inUse = false;
        if (pooled.executionCount >= this.config.maxExecutionsPerEngine) {
            this.recycle(pooled);
        }
    }

    private recycle(pooled: PooledEngine): void {
        pooled.engine.destroy();
        // Replace in-place: copy a fresh engine onto the same PooledEngine
        // wrapper so the array reference stays valid for any other code
        // that holds it. We rotate every field so the recycled entry is
        // indistinguishable from a freshly created one.
        const fresh = this.createPooled();
        pooled.engine = fresh.engine;
        pooled.executionCount = 0;
        pooled.lastUsed = fresh.lastUsed;
        pooled.createdAt = fresh.createdAt;
    }

    private createPooled(): PooledEngine {
        const engine = this.config.factory(this.config.engineConfig);
        return {
            engine,
            inUse: false,
            executionCount: 0,
            lastUsed: this.clock(),
            createdAt: this.clock(),
        };
    }
}

export { DEFAULTS as DEFAULT_POOL_CONFIG };