class InterestRateService
{
    private $precision      = 0.001; //夹逼准则 精度范围
    private $principal      = 0; //本金
    private $flow           = []; //还款数据
    private $payment_method = 0; //还款方式 0按月还款 1按年还款
    private $i = 100; //最大循环次数
    public static function getInstance()
    {
        return new self();
    }
    /**
     * @param $principal//本金
     * @param array $flow//还款数据
     * @param int   $payment_method//还款方式 0按月还款 1按年还款
     *
     * @return $this
     */
    public function setPrecision($principal, $flow = [], $payment_method = 0)
    {
        $this->principal           = $principal;
        $this->flow                = $flow;
        $this->payment_method      = $payment_method;
        return $this;
    }
    /**
     * irr利率计算.
     *
     * @return string
     */
    public function irrCalculate()
    {
        $principal = $this->principal;
        $precision = $this->precision;
        $i         = $this->i;
        $flow      = $this->flow;
        $net_present_value  = 1;
        $min   = -10;
        $max   = 10;
        $guess = 0;
        $k     = 0;
        while (abs($net_present_value - $principal) > $precision && $k <= $i) {
            ++$k;
            $net_present_value = 0;
            $guess = ($min + $max) / 2;
            foreach ($flow as $period=>$cash_flow) {
                $net_present_value += $cash_flow / pow(1 + $guess, $period + 1);
            }
            if ($net_present_value - $principal > 0) {
                $min = $guess;
            } else {
                $max = $guess;
            }
        }
        return ($guess * 100) . '%';
    }
    /**
     * 还款数据返回.
     *
     * @return array
     */
    public function remainingAmount()
    {
        $principal = $this->principal;
        $flow      = $this->flow;
        $array = [];
        foreach ($flow as $i=>$value) {
            $array[] = [
                'key'  => $i + 1,
                'money'=> $value,
                'left' => $principal - $value,
            ];
            $principal = $principal - $value;
        }
        return $array;
    }
    /**
     * apr利率计算.
     *
     * @return string
     */
    public function aprCalculate()
    {
        $periods = $this->getPeriods();
        $principal = $this->principal;
        $flow      = $this->flow;
        return (((array_sum($flow) - $principal) / $principal) / ($periods / 12) * 100) . '%';
    }
    /**
     * @return float|int
     */
    private function getPeriods()
    {
        if (0 == $this->payment_method) {
            //按月还款
            return count($this->flow);
        }
        return 12 * count($this->flow);
    }
}