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1、初始数据：

权重越大，抽取的几率越高

[奖品1， 权重 5], [ 奖品2， 权重6]， [ 奖品3， 权重 7]， [ 奖品4， 权重2]

2、处理步骤：

1）N = 5 + 6 + 7 + 2 = 20

2）然后取1-N的随机数M

3）界定各 奖品的权重范围值 奖品 1 ： 1-5 ; 奖品2 ： 6-11； 奖品3： 12-18； 奖品4： 19-20

4) 如果M在某个奖品的权重范围值内，标识这个奖品被抽取到。

<?php
/**
 * 奖品
 */
class Prize {
  # ID
  public $id = null;
  # 权重
  public $weight = null;
  # 奖品名
  public $name = null;
  
  # 权重范围区间起始值
  protected $start = 0;
  # 权重范围区间结束值
  protected $end = 0;
  
  public function __construct($id, $weight, $name) {
    if (!$id) {
      throw new Exception('奖品ID为空.');
    }
    $this->id = $id;
    $this->weight = $weight ? $weight : 0;
    $this->name = $name ? $name : '随机奖品' . $id;
  }
  
  # id
  public function getId() {
    return $this->id;
  }
  
  # 权重
  public function getWeight() {
    return $this->weight;
  }
  
  # 设置权重范围区间
  public function setRange($start, $end) {
    $this->start = $start;
    $this->end = $end;
  }
  
  # 判断随机数是否在权重范围区间
  public function inRange($num) {
    return ($num >= $this->start) && ($num <= $this->end);
  }
}
  
/**
 * 奖品池
 */
class PrizePoll implements IteratorAggregate, Countable {
  # 奖品集
  protected $items = array();
  
  # 加入奖品
  public function addItem(Prize $item) {
    $this->items[$item->getId()] = $item;
    return $this;
  }
  
  # 删除奖品
  public function removeItem($itemId) {
    if (isset($this->items[$itemId])) {
      unset($this->items[$itemId]);
    }
    return $this;
  }
  
  # 更新奖品
  public function updateItem(Prize $item) {
    if (isset($this->items[$item->getId()])) {
      $this->items[$item->getId()] = $item;
    }
    return $this;
  }
  
  # 获取所有奖品
  public function getItems() {
    return $this->items;
  }
  
  # 所有所有可用奖品(如果权重为0，说明这个奖品永远不可能抽到)
  public function getVisibleItems() {
    $items = array();
    foreach ($this->items as $item) {
      if ($item->getWeight()) {
        $items[$item->getId()] = $item;
      }
    }
    return $items;
  }
  
  # Countable::count
  public function count() {
    return count($this->items);
  }
  
  # IteratorAggregate::getIterator()
  public function getIterator() {
    return new ArrayIterator($this->items);
  }
}
  
/**
 * 简单的抽奖类
 */
class SimpleTurn {
  # 奖池
  protected $poll = null;
    
  public function __construct(PrizePoll $poll) {
    if ($poll) {
      $this->setPoll($poll);
    }
  }
  
  # 抽奖
  public function run(PrizePoll $poll) {
    $poll = $poll ? $poll : $this->poll;
    if ( ! $poll) {
      throw new Exception('奖池未初始化');
    }
  
    if ($poll->count() <= 0) {
      throw new Exception('奖池为空');
    }
  
    $items = $poll->getVisibleItems();
    if (count($items) <= 0) {
      throw new Exception('奖池为空');
    }
  
    $sum = 0;
    foreach ($items as $item) {
      $start = $sum + 1;
      $sum += $item->getWeight();
      $end = $sum;
  
      # 设置奖品的权重范围区间
      $item->setRange($start, $end);
    }
  
    # 随机数
    $rand = $this->getRandNum(1, $sum);
  
    # 区间段判断
    foreach ($items as $item) {
      if ($item->inRange($rand)) {
        return $item;
      }
    }
    return null;
  }
  
  # 获取随机数
  public function getRandNum($min, $max) {
    return mt_rand($min ? $min : 1, $max);
  }
  
  # 设置奖池
  public function setPoll(PrizePoll $poll) {
    $this->poll = $poll;
  }
}
  
# 示例
try {
  $prizePoll = new PrizePoll();
  $prizePoll->addItem(new Prize(1, 5))
    ->addItem(new Prize(2, 6))
    ->addItem(new Prize(3, 7))
    ->addItem(new Prize(4, 2));
  
  $turn = new SimpleTurn($prizePoll);
  $prize = $turn->run();
  var_dump($prize);
} catch (Exception $e) {
  print_r($e);
}