package pool
import (
  "errors"
  "io"
  "sync"
  "time"
)

var (
  ErrInvalidConfig = errors.New("invalid pool config")
  ErrPoolClosed  = errors.New("pool closed")
)

type factory func() (io.Closer, error)

type Pool interface {
  Acquire() (io.Closer, error) // 获取资源
  Release(io.Closer) error   // 释放资源
  Close(io.Closer) error    // 关闭资源
  Shutdown() error       // 关闭池
}

type GenericPool struct {
  sync.Mutex
  pool    chan io.Closer
  maxOpen   int // 池中最大资源数
  numOpen   int // 当前池中资源数
  minOpen   int // 池中最少资源数
  closed   bool // 池是否已关闭
  maxLifetime time.Duration
  factory   factory // 创建连接的方法
}

func NewGenericPool(minOpen, maxOpen int, maxLifetime time.Duration, factory factory) (*GenericPool, error) {
  if maxOpen <= 0 || minOpen > maxOpen {
    return nil, ErrInvalidConfig
  }
  p := &GenericPool{
    maxOpen:   maxOpen,
    minOpen:   minOpen,
    maxLifetime: maxLifetime,
    factory:   factory,
    pool:    make(chan io.Closer, maxOpen),
  }

  for i := 0; i < minOpen; i++ {
    closer, err := factory()
    if err != nil {
      continue
    }
    p.numOpen++
    p.pool <- closer
  }
  return p, nil
}

func (p *GenericPool) Acquire() (io.Closer, error) {
  if p.closed {
    return nil, ErrPoolClosed
  }
  for {
    closer, err := p.getOrCreate()
    if err != nil {
      return nil, err
    }
    // todo maxLifttime处理
    return closer, nil
  }
}

func (p *GenericPool) getOrCreate() (io.Closer, error) {
  select {
  case closer := <-p.pool:
    return closer, nil
  default:
  }
  p.Lock()
  if p.numOpen >= p.maxOpen {
    closer := <-p.pool
    p.Unlock()
    return closer, nil
  }
  // 新建连接
  closer, err := p.factory()
  if err != nil {
    p.Unlock()
    return nil, err
  }
  p.numOpen++
  p.Unlock()
  return closer, nil
}

// 释放单个资源到连接池
func (p *GenericPool) Release(closer io.Closer) error {
  if p.closed {
    return ErrPoolClosed
  }
  p.Lock()
  p.pool <- closer
  p.Unlock()
  return nil
}

// 关闭单个资源
func (p *GenericPool) Close(closer io.Closer) error {
  p.Lock()
  closer.Close()
  p.numOpen--
  p.Unlock()
  return nil
}

// 关闭连接池，释放所有资源
func (p *GenericPool) Shutdown() error {
  if p.closed {
    return ErrPoolClosed
  }
  p.Lock()
  close(p.pool)
  for closer := range p.pool {
    closer.Close()
    p.numOpen--
  }
  p.closed = true
  p.Unlock()
  return nil
}