import { Injectable, Logger, Inject, forwardRef } from "@nestjs/common";
import { ValkeyService } from "../valkey/valkey.service";
import { AgentSpawnerService } from "./agent-spawner.service";
import type { AgentState, AgentStatus, AgentEvent } from "../valkey/types";
import { isValidAgentTransition } from "../valkey/types/state.types";

/**
 * Service responsible for managing agent lifecycle state transitions
 *
 * Manages state transitions through the agent lifecycle:
 * spawning → running → completed/failed/killed
 *
 * - Enforces valid state transitions using state machine
 * - Persists agent state changes to Valkey
 * - Emits pub/sub events on state changes
 * - Tracks agent metadata (startedAt, completedAt, error)
 * - Uses per-agent mutex to prevent TOCTOU race conditions (CQ-ORCH-5)
 */
@Injectable()
export class AgentLifecycleService {
  private readonly logger = new Logger(AgentLifecycleService.name);

  /**
   * Per-agent mutex map to serialize state transitions.
   * Uses promise chaining so concurrent transitions to the same agent
   * are queued and executed sequentially, preventing TOCTOU races
   * where two concurrent requests could both read the same state,
   * both validate it as valid, and both write, causing lost updates.
   */
  private readonly agentLocks = new Map<string, Promise<void>>();

  constructor(
    private readonly valkeyService: ValkeyService,
    @Inject(forwardRef(() => AgentSpawnerService))
    private readonly spawnerService: AgentSpawnerService
  ) {
    this.logger.log("AgentLifecycleService initialized");
  }

  /**
   * Register a newly spawned agent in persistent state and emit spawned event.
   */
  async registerSpawnedAgent(agentId: string, taskId: string): Promise<AgentState> {
    await this.valkeyService.createAgent(agentId, taskId);
    const createdState = await this.getAgentState(agentId);

    const event: AgentEvent = {
      type: "agent.spawned",
      agentId,
      taskId,
      timestamp: new Date().toISOString(),
    };
    await this.valkeyService.publishEvent(event);

    return createdState;
  }

  /**
   * Acquire a per-agent mutex to serialize state transitions.
   * Uses promise chaining: each caller chains onto the previous lock,
   * ensuring transitions for the same agent are strictly sequential.
   * Different agents can transition concurrently without contention.
   *
   * @param agentId Agent to acquire lock for
   * @param fn Critical section to execute while holding the lock
   * @returns Result of the critical section
   */
  private async withAgentLock<T>(agentId: string, fn: () => Promise<T>): Promise<T> {
    const previousLock = this.agentLocks.get(agentId) ?? Promise.resolve();

    let releaseLock!: () => void;
    const currentLock = new Promise<void>((resolve) => {
      releaseLock = resolve;
    });
    this.agentLocks.set(agentId, currentLock);

    try {
      await previousLock;
      return await fn();
    } finally {
      releaseLock();
      // Clean up the map entry if we are the last in the chain
      if (this.agentLocks.get(agentId) === currentLock) {
        this.agentLocks.delete(agentId);
      }
    }
  }

  /**
   * Transition agent from spawning to running state
   * @param agentId Unique agent identifier
   * @returns Updated agent state
   * @throws Error if agent not found or invalid transition
   */
  async transitionToRunning(agentId: string): Promise<AgentState> {
    return this.withAgentLock(agentId, async () => {
      this.logger.log(`Transitioning agent ${agentId} to running`);

      const currentState = await this.getAgentState(agentId);
      this.validateTransition(currentState.status, "running");

      // Set startedAt timestamp if not already set
      const startedAt = currentState.startedAt ?? new Date().toISOString();

      // Update state in Valkey
      const updatedState = await this.valkeyService.updateAgentStatus(
        agentId,
        "running",
        undefined
      );

      // Ensure startedAt is set
      if (!updatedState.startedAt) {
        updatedState.startedAt = startedAt;
        await this.valkeyService.setAgentState(updatedState);
      }

      // Emit event
      await this.publishStateChangeEvent("agent.running", updatedState);

      this.logger.log(`Agent ${agentId} transitioned to running`);
      return updatedState;
    });
  }

  /**
   * Transition agent to completed state
   * @param agentId Unique agent identifier
   * @returns Updated agent state
   * @throws Error if agent not found or invalid transition
   */
  async transitionToCompleted(agentId: string): Promise<AgentState> {
    return this.withAgentLock(agentId, async () => {
      this.logger.log(`Transitioning agent ${agentId} to completed`);

      const currentState = await this.getAgentState(agentId);
      this.validateTransition(currentState.status, "completed");

      // Set completedAt timestamp
      const completedAt = new Date().toISOString();

      // Update state in Valkey
      const updatedState = await this.valkeyService.updateAgentStatus(
        agentId,
        "completed",
        undefined
      );

      // Ensure completedAt is set
      if (!updatedState.completedAt) {
        updatedState.completedAt = completedAt;
        await this.valkeyService.setAgentState(updatedState);
      }

      // Emit event
      await this.publishStateChangeEvent("agent.completed", updatedState);

      // Schedule session cleanup
      this.spawnerService.scheduleSessionCleanup(agentId);

      this.logger.log(`Agent ${agentId} transitioned to completed`);
      return updatedState;
    });
  }

  /**
   * Transition agent to failed state with error
   * @param agentId Unique agent identifier
   * @param error Error message
   * @returns Updated agent state
   * @throws Error if agent not found or invalid transition
   */
  async transitionToFailed(agentId: string, error: string): Promise<AgentState> {
    return this.withAgentLock(agentId, async () => {
      this.logger.log(`Transitioning agent ${agentId} to failed: ${error}`);

      const currentState = await this.getAgentState(agentId);
      this.validateTransition(currentState.status, "failed");

      // Set completedAt timestamp
      const completedAt = new Date().toISOString();

      // Update state in Valkey
      const updatedState = await this.valkeyService.updateAgentStatus(agentId, "failed", error);

      // Ensure completedAt is set
      if (!updatedState.completedAt) {
        updatedState.completedAt = completedAt;
        await this.valkeyService.setAgentState(updatedState);
      }

      // Emit event
      await this.publishStateChangeEvent("agent.failed", updatedState, error);

      // Schedule session cleanup
      this.spawnerService.scheduleSessionCleanup(agentId);

      this.logger.error(`Agent ${agentId} transitioned to failed: ${error}`);
      return updatedState;
    });
  }

  /**
   * Transition agent to killed state
   * @param agentId Unique agent identifier
   * @returns Updated agent state
   * @throws Error if agent not found or invalid transition
   */
  async transitionToKilled(agentId: string): Promise<AgentState> {
    return this.withAgentLock(agentId, async () => {
      this.logger.log(`Transitioning agent ${agentId} to killed`);

      const currentState = await this.getAgentState(agentId);
      this.validateTransition(currentState.status, "killed");

      // Set completedAt timestamp
      const completedAt = new Date().toISOString();

      // Update state in Valkey
      const updatedState = await this.valkeyService.updateAgentStatus(agentId, "killed", undefined);

      // Ensure completedAt is set
      if (!updatedState.completedAt) {
        updatedState.completedAt = completedAt;
        await this.valkeyService.setAgentState(updatedState);
      }

      // Emit event
      await this.publishStateChangeEvent("agent.killed", updatedState);

      // Schedule session cleanup
      this.spawnerService.scheduleSessionCleanup(agentId);

      this.logger.warn(`Agent ${agentId} transitioned to killed`);
      return updatedState;
    });
  }

  /**
   * Get current agent lifecycle state
   * @param agentId Unique agent identifier
   * @returns Agent state or null if not found
   */
  async getAgentLifecycleState(agentId: string): Promise<AgentState | null> {
    return this.valkeyService.getAgentState(agentId);
  }

  /**
   * List all agent lifecycle states
   * @returns Array of all agent states
   */
  async listAgentLifecycleStates(): Promise<AgentState[]> {
    return this.valkeyService.listAgents();
  }

  /**
   * Get agent state and throw if not found
   * @param agentId Unique agent identifier
   * @returns Agent state
   * @throws Error if agent not found
   */
  private async getAgentState(agentId: string): Promise<AgentState> {
    const state = await this.valkeyService.getAgentState(agentId);

    if (!state) {
      throw new Error(`Agent ${agentId} not found`);
    }

    return state;
  }

  /**
   * Validate state transition is allowed
   * @param from Current state
   * @param to Target state
   * @throws Error if transition is invalid
   */
  private validateTransition(from: AgentStatus, to: AgentStatus): void {
    if (!isValidAgentTransition(from, to)) {
      throw new Error(`Invalid state transition from ${from} to ${to}`);
    }
  }

  /**
   * Publish state change event
   * @param eventType Type of event
   * @param state Updated agent state
   * @param error Optional error message
   */
  private async publishStateChangeEvent(
    eventType: "agent.running" | "agent.completed" | "agent.failed" | "agent.killed",
    state: AgentState,
    error?: string
  ): Promise<void> {
    const event: AgentEvent = {
      type: eventType,
      agentId: state.agentId,
      taskId: state.taskId,
      timestamp: new Date().toISOString(),
      error,
    };

    await this.valkeyService.publishEvent(event);
  }
}