/**
 * Queue Manager
 *
 * Manages task queue for concurrency control.
 * Implements FIFO queue with configurable max concurrent tasks.
 *
 * @see Requirements 10.1, 10.2, 10.4
 */

import {
  TaskQueueState,
  MAX_CONCURRENT,
  createInitialQueueState,
} from '../streams/taskQueue'

/**
 * Result of checkAndEnqueue operation
 */
export interface EnqueueResult {
  status: 'RUNNING' | 'QUEUED'
  queuePosition: number | null
}

/**
 * Queue Manager class for managing task concurrency
 *
 * This class provides methods to:
 * - Check if a task can run immediately or needs to be queued
 * - Dequeue the next task when a slot becomes available
 * - Get the current queue position for a task
 */
export class QueueManager {
  private state: TaskQueueState

  constructor(initialState?: TaskQueueState) {
    this.state = initialState ?? createInitialQueueState()
  }

  /**
   * Get the current queue state
   */
  getState(): TaskQueueState {
    return { ...this.state }
  }

  /**
   * Check if a new task can run immediately or needs to be queued.
   * If the task can run, increments runningCount.
   * If the task needs to be queued, adds it to the queue.
   *
   * @param sessionId - The unique session ID for the task
   * @returns EnqueueResult with status and queue position
   *
   * @see Requirements 10.1, 10.4
   */
  checkAndEnqueue(sessionId: string): EnqueueResult {
    // Check if task is already in queue or running
    if (this.state.queue.includes(sessionId)) {
      const position = this.state.queue.indexOf(sessionId) + 1
      return { status: 'QUEUED', queuePosition: position }
    }

    // Check if we can run immediately
    if (this.state.runningCount < this.state.maxConcurrent) {
      this.state.runningCount++
      return { status: 'RUNNING', queuePosition: null }
    }

    // Need to queue the task
    this.state.queue.push(sessionId)
    const queuePosition = this.state.queue.length
    return { status: 'QUEUED', queuePosition }
  }

  /**
   * Dequeue the next task when a running task completes.
   * Decrements runningCount and returns the next queued sessionId if available.
   *
   * @returns The sessionId of the next task to run, or null if queue is empty
   *
   * @see Requirements 10.2
   */
  dequeueNext(): string | null {
    // Decrement running count (a task just completed)
    if (this.state.runningCount > 0) {
      this.state.runningCount--
    }

    // Check if there's a queued task to start
    if (this.state.queue.length === 0) {
      return null
    }

    // Get the next task from the front of the queue (FIFO)
    const nextSessionId = this.state.queue.shift()!

    // Increment running count for the new task
    this.state.runningCount++

    return nextSessionId
  }

  /**
   * Get the queue position for a specific session.
   * Returns 1-based position, or 0 if not in queue.
   *
   * @param sessionId - The session ID to look up
   * @returns Queue position (1-based), or 0 if not in queue
   *
   * @see Requirements 10.4
   */
  getQueuePosition(sessionId: string): number {
    const index = this.state.queue.indexOf(sessionId)
    return index === -1 ? 0 : index + 1
  }

  /**
   * Remove a task from the queue (e.g., if cancelled)
   *
   * @param sessionId - The session ID to remove
   * @returns true if the task was removed, false if not found
   */
  removeFromQueue(sessionId: string): boolean {
    const index = this.state.queue.indexOf(sessionId)
    if (index === -1) {
      return false
    }
    this.state.queue.splice(index, 1)
    return true
  }

  /**
   * Mark a task as no longer running (without dequeuing next)
   * Used when a task fails or is cancelled while running
   */
  markTaskCompleted(): void {
    if (this.state.runningCount > 0) {
      this.state.runningCount--
    }
  }

  /**
   * Check if the queue has capacity for immediate execution
   */
  hasCapacity(): boolean {
    return this.state.runningCount < this.state.maxConcurrent
  }

  /**
   * Get the number of tasks currently running
   */
  getRunningCount(): number {
    return this.state.runningCount
  }

  /**
   * Get the number of tasks in the queue
   */
  getQueueLength(): number {
    return this.state.queue.length
  }
}

// ============================================================================
// Standalone Functions (for use without class instance)
// ============================================================================

/**
 * Check if a new task can run immediately or needs to be queued.
 * Returns a new state and the result.
 *
 * @param state - Current queue state
 * @param sessionId - The unique session ID for the task
 * @returns Tuple of [newState, result]
 */
export function checkAndEnqueue(
  state: TaskQueueState,
  sessionId: string
): [TaskQueueState, EnqueueResult] {
  const newState = { ...state, queue: [...state.queue] }

  // Check if task is already in queue
  if (newState.queue.includes(sessionId)) {
    const position = newState.queue.indexOf(sessionId) + 1
    return [newState, { status: 'QUEUED', queuePosition: position }]
  }

  // Check if we can run immediately
  if (newState.runningCount < newState.maxConcurrent) {
    newState.runningCount++
    return [newState, { status: 'RUNNING', queuePosition: null }]
  }

  // Need to queue the task
  newState.queue.push(sessionId)
  const queuePosition = newState.queue.length
  return [newState, { status: 'QUEUED', queuePosition }]
}

/**
 * Dequeue the next task when a running task completes.
 * Returns a new state and the next sessionId.
 *
 * @param state - Current queue state
 * @returns Tuple of [newState, nextSessionId or null]
 */
export function dequeueNext(
  state: TaskQueueState
): [TaskQueueState, string | null] {
  const newState = { ...state, queue: [...state.queue] }

  // Decrement running count (a task just completed)
  if (newState.runningCount > 0) {
    newState.runningCount--
  }

  // Check if there's a queued task to start
  if (newState.queue.length === 0) {
    return [newState, null]
  }

  // Get the next task from the front of the queue (FIFO)
  const nextSessionId = newState.queue.shift()!

  // Increment running count for the new task
  newState.runningCount++

  return [newState, nextSessionId]
}

/**
 * Get the queue position for a specific session.
 * Returns 1-based position, or 0 if not in queue.
 *
 * @param state - Current queue state
 * @param sessionId - The session ID to look up
 * @returns Queue position (1-based), or 0 if not in queue
 */
export function getQueuePosition(
  state: TaskQueueState,
  sessionId: string
): number {
  const index = state.queue.indexOf(sessionId)
  return index === -1 ? 0 : index + 1
}