| 음수 - 대체할 문자열의 끝부터 대체할 문자 수
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0 - 대체 대신 삽입
用下来，我是感觉不是很顺手，虽然说满足我现在的需求还是可以的，但是如果将来需要一些扩展的话，耍起来挺吃力的，所以就想到自己构造一个，将来用起来也方便。
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二、自制的星号替换函数
代码如下: replaceStar($str, $start, $length = 0) 
前面的两个参数与上面的一样，最后的参数与上面不同
源码分享
public static function replaceStar($str, $start, $length = 0)
{
$i = 0;
$star = '';
if($start >= 0) {
if($length > 0) {
$str_len = strlen($str);
$count = $length;
if($start >= $str_len) {//当开始的下标大于字符串长度的时候，就不做替换了
$count = 0;
}
}elseif($length < 0){
$str_len = strlen($str);
$count = abs($length);
if($start >= $str_len) {//当开始的下标大于字符串长度的时候，由于是反向的，就从最后那个字符的下标开始
$start = $str_len - 1;
}
$offset = $start - $count + 1;//起点下标减去数量，计算偏移量
$count = $offset >= 0 ? abs($length) : ($start + 1);//偏移量大于等于0说明没有超过最左边，小于0了说明超过了最左边，就用起点到最左边的长度
$start = $offset >= 0 ? $offset : 0;//从最左边或左边的某个位置开始
}else {
$str_len = strlen($str);
$count = $str_len - $start;//计算要替换的数量
}
}else {
if($length > 0) {
$offset = abs($start);
$count = $offset >= $length ? $length : $offset;//大于等于长度的时候 没有超出最右边
}elseif($length < 0){
$str_len = strlen($str);
$end = $str_len + $start;//计算偏移的结尾值
$offset = abs($start + $length) - 1;//计算偏移量，由于都是负数就加起来
$start = $str_len - $offset;//计算起点值
$start = $start >= 0 ? $start : 0;
$count = $end - $start + 1;
}else {
$str_len = strlen($str);
$count = $str_len + $start + 1;//计算需要偏移的长度
$start = 0;
}
}
while ($i < $count) {
$star .= '*';
$i++;
}
return substr_replace($str, $star, $start, $count);
}不擅长算法，这里就用很普通的逻辑来展示啦，没有用到啥数学公式。
if($start >= 0)这里做start大于等于0与小于0的分支 在start 的分之中，分别再做length 大于0，小于0和等于0的三个分支 最后计算出start、count和要替换的星号字符串，最后计算出的start与count都是正数，运用substr_replace做替换
单元测试
public function testReplaceStar()
{
$actual = App_Util_String::replaceStar('123456789', 3, 2);
$this->assertEquals($actual, '123**6789');
$actual = App_Util_String::replaceStar('123456789', 9);
$this->assertEquals($actual, '123456789');
$actual = App_Util_String::replaceStar('123456789', 9, 2);
$this->assertEquals($actual, '123456789');
$actual = App_Util_String::replaceStar('123456789', 9, -9);
$this->assertEquals($actual, '*********');
$actual = App_Util_String::replaceStar('123456789', 9, -10);
$this->assertEquals($actual, '*********');
$actual = App_Util_String::replaceStar('123456789', 9, -11);
$this->assertEquals($actual, '*********');
$actual = App_Util_String::replaceStar('123456789', 3);
$this->assertEquals($actual, '123******');
$actual = App_Util_String::replaceStar('123456789', 0);
$this->assertEquals($actual, '*********');
$actual = App_Util_String::replaceStar('123456789', 0, 2);
$this->assertEquals($actual, '**3456789');
$actual = App_Util_String::replaceStar('123456789', 3, -3);
$this->assertEquals($actual, '1***56789');
$actual = App_Util_String::replaceStar('123456789', 1, -5);
$this->assertEquals($actual, '**3456789');
$actual = App_Util_String::replaceStar('123456789', 3, -3);
$this->assertEquals($actual, '1***56789');
$actual = App_Util_String::replaceStar('123456789', -3, 2);
$this->assertEquals($actual, '123456**9');
$actual = App_Util_String::replaceStar('123456789', -3, 5);
$this->assertEquals($actual, '123456***');
$actual = App_Util_String::replaceStar('123456789', -1, 2);
$this->assertEquals($actual, '12345678*');
$actual = App_Util_String::replaceStar('123456789', -1, -2);
$this->assertEquals($actual, '1234567**');
$actual = App_Util_String::replaceStar('123456789', -4, -7);
$this->assertEquals($actual, '******789');
$actual = App_Util_String::replaceStar('123456789', -1, -3);
$this->assertEquals($actual, '123456***');
$actual = App_Util_String::replaceStar('123456789', -1);
$this->assertEquals($actual, '*********');
$actual = App_Util_String::replaceStar('123456789', -2);
$this->assertEquals($actual, '********9');
$actual = App_Util_String::replaceStar('123456789', -9);
$this->assertEquals($actual, '*23456789');
$actual = App_Util_String::replaceStar('123456789', -10);
$this->assertEquals($actual, '123456789');
$actual = App_Util_String::replaceStar('123456789', -10, -2);
$this->assertEquals($actual, '123456789');
}
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