import { EventEmitter } from 'events'
import MovingAverage, { type IMovingAverage } from '@vascosantos/moving-average'

export type Op = [string, number, number]

export interface StatOptions {
  enabled: boolean
  computeThrottleTimeout: number
  computeThrottleMaxQueueSize: number
  movingAverageIntervals: number[]
}

export class Stat extends EventEmitter {
  private readonly _options: StatOptions
  private readonly _queue: Op[]
  private _stats: Record<string, bigint>
  private _frequencyLastTime: number
  private _frequencyAccumulators: Record<string, number>
  private _movingAverages: Record<string, Record<number, IMovingAverage>>
  private _enabled: boolean
  private _timeout?: ReturnType<typeof setTimeout>

  constructor (initialCounters: string[], options: StatOptions) {
    super()

    this._options = options
    this._queue = []
    this._stats = {}
    this._frequencyLastTime = Date.now()
    this._frequencyAccumulators = {}
    this._movingAverages = {}

    this._update = this._update.bind(this)

    initialCounters.forEach((key) => {
      this._stats[key] = BigInt(0)
      this._movingAverages[key] = {}
      this._options.movingAverageIntervals.forEach((interval) => {
        const ma = this._movingAverages[key][interval] = MovingAverage(interval)
        ma.push(this._frequencyLastTime, 0)
      })
    })

    this._enabled = this._options.enabled
  }

  enable (): void {
    this._enabled = true
  }

  disable (): void {
    this._enabled = false
  }

  stop (): void {
    if (this._timeout != null) {
      clearTimeout(this._timeout)
    }
  }

  get snapshot (): Record<string, bigint> {
    return Object.assign({}, this._stats)
  }

  get movingAverages (): Record<string, Record<number, IMovingAverage>> {
    return Object.assign({}, this._movingAverages)
  }

  push (counter: string, inc: number): void {
    if (this._enabled) {
      this._queue.push([counter, inc, Date.now()])
      this._resetComputeTimeout()
    }
  }

  _resetComputeTimeout (): void {
    if (this._timeout != null) {
      clearTimeout(this._timeout)
    }
    this._timeout = setTimeout(this._update, this._nextTimeout())
  }

  _nextTimeout (): number {
    // calculate the need for an update, depending on the queue length
    const urgency = this._queue.length / this._options.computeThrottleMaxQueueSize
    return Math.max(this._options.computeThrottleTimeout * (1 - urgency), 0)
  }

  _update (): void {
    this._timeout = undefined

    if (this._queue.length > 0) {
      let last
      while (this._queue.length > 0) {
        const op = last = this._queue.shift()
        ;(op != null) && this._applyOp(op)
      }

      (last != null) && this._updateFrequency(last[2]) // contains timestamp of last op

      this.emit('update', this._stats)
    }
  }

  _updateFrequency (latestTime: number): void {
    const timeDiff = latestTime - this._frequencyLastTime

    if (timeDiff > 0) {
      Object.keys(this._stats).forEach((key) => {
        this._updateFrequencyFor(key, timeDiff, latestTime)
      })
    }

    this._frequencyLastTime = latestTime
  }

  _updateFrequencyFor (key: string, timeDiffMS: number, latestTime: number): void {
    const count = this._frequencyAccumulators[key] ?? 0
    this._frequencyAccumulators[key] = 0
    const hz = (count / timeDiffMS) * 1000

    let movingAverages = this._movingAverages[key]
    if (movingAverages == null) {
      movingAverages = this._movingAverages[key] = {}
    }
    this._options.movingAverageIntervals.forEach((movingAverageInterval) => {
      let movingAverage = movingAverages[movingAverageInterval]
      if (movingAverage == null) {
        movingAverage = movingAverages[movingAverageInterval] = MovingAverage(movingAverageInterval)
      }
      movingAverage.push(latestTime, hz)
    })
  }

  _applyOp (op: Op): void {
    const key = op[0]
    const inc = op[1]

    if (typeof inc !== 'number') {
      throw new Error(`invalid increment number: ${inc}`)
    }

    if (!Object.prototype.hasOwnProperty.call(this._stats, key)) {
      this._stats[key] = BigInt(0)
    }

    this._stats[key] = BigInt(this._stats[key]) + BigInt(inc)

    if (this._frequencyAccumulators[key] == null) {
      this._frequencyAccumulators[key] = 0
    }
    this._frequencyAccumulators[key] += inc
  }
}