data header length

使用字节表示数据长度

一个byte等于8bit，最大长度(2«7) - 1, 取值范围(0-255)

多个字节的大小，2 « (8 * 字节数) - 1

根据最大长度计算需要的字节数

function bytes_num(int $count)
{
    return ceil(bcdiv(bcdiv(log($count + 1), log(2), 10), 8, 10));
}

根据长度得到数据长度字节

function bytes(int $count)
{
    $num = bytes_num($count);

    $array = [];

    for ($i = 0; $i < $num; $i++) {
        if ($i == $num - 1) {
            $array[$i] = $count & 0xFF;
        } else {
            $array[$i] = ($count >> (8 * ($num - $i - 1))) & 0xFF;
        }
    }

    return $array;
}

一个字节表示长度(0-255)

pow(2, 8) - 1

$length = 255

$byte = $length & 0xFF

二个字节表示长度(0-65535)

pow(2, 8 * 2) - 1

$length = 65535; 

$highByte = ($length >> 8) & 0xFF;
$lowByte = $length & 0xFF;

$bytes = [$highByte, $lowByte];

三个字节表示长度(0 - 16777215)

pow(2, 8 * 3) - 1

$length = 16777215; 

$highByte = ($length >> (8 * 2)) & 0xFF;
$middleByte = ($length >> (8 * 1)) & 0xFF;
$lowByte = $length & 0xFF;

$bytes = [$highByte, $middleByte, $lowByte];

4个字节表示长度(0 - 4294967295)

pow(2, 8 * 4) - 1

$length = 4294967295; 

$highByte = ($length >> (8 * 3)) & 0xFF;
$middleByte1 = ($length >> (8 * 2)) & 0xFF;
$middleByte2 = ($length >> (8 * 1)) & 0xFF;
$lowByte = $length & 0xFF;

$bytes = [$highByte, $middleByte1, $middleByte2, $lowByte];