<?php

class IPv4Subnet
{

    private string $address;
    private int $prefix;

    /**
     * @param string $subnetAddress
     * @param int $prefix
     */
    public function __construct(string $subnetAddress, int $prefix)
    {
        $this->address = $subnetAddress;
        if (ip2long($subnetAddress) === false) {
            throw new RuntimeException("Invalid subnet address: " . $subnetAddress);
        }
        $this->prefix = $prefix;
        if ($prefix < 0 or $prefix > 32) {
            throw new RuntimeException("Invalid subnet prefix: " . $prefix);
        }
    }

    public function getFirstAddress()
    {
        $a = ip2long($this->address);
        $mask = ip2long($this->getNetMask());
        return long2ip($a & $mask);
    }

    public function getLastAddress()
    {
        return long2ip(ip2long($this->getFirstAddress()) + $this->getAddressCount() - 1);
    }

    public function getAddressCount()
    {
        return pow(2, 32 - $this->prefix);
    }

    public function getNetMask()
    {
        return long2ip(-1 << (32 - $this->prefix));
    }
}