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Date: Fri, 5 May 2017 15:19:51 -0600
Message-ID: <CAObuZdsEjkUn_gMAEGQKkQn08W8yFi3SGyQ596fdgE0JRiM0bA@mail.gmail.com>
To: Jesse Schalken <me@jesseschalken.com>
Cc: PHP internals <internals@lists.php.net>
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Subject: Re: [PHP-DEV] Add is_vectorlike($array) function
From: derokorian@gmail.com (Ryan Pallas)

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On Tue, May 2, 2017 at 3:13 AM, Jesse Schalken <me@jesseschalken.com> wrote:

> Related to the optimisation made by Sara Golemon here:
> https://github.com/php/php-src/commit/c74bc87c74f48bc55541b3bf2fc67d
> 595f58a3b5
>
> I often define a function like this, which checks if an array is "vector
> like" i.e. has keys 0,1,2..N:
>
> function is_vectorlike(array $a): bool {
>   $i = 0;
>   foreach ($a as $k => $v) {
>     if ($k !== $i++) {
>       return false;
>     }
>   }
>   return true;
> }
>
> The problem is that this function is O(n), but in PHP7 an array that is
> vector-like is likely to be packed and without holes (HT_IS_PACKED(x) &&
> HT_IS_WITHOUT_HOLES(x)), in which case it is known to be vector-like
> without needing to iterate over it, but PHP code can't check for this (nor
> should it be able to, since it's really an implementation detail).
>
> Would it be a good idea to define this is_vectorlike() function in the PHP
> runtime, so it can short circuit to return true on packed arrays without
> holes? The above code would be a suitable polyfill.
>


I just read this thread and am wondering what exactly is the use case? Like
are you going to do something if it is vector-like, and do something
different (or not do anything at all) if it's not vector-like? I mean, if
you have to crawl it, and need a vector, why not just call array_values and
guarantee you have a vector?

Also, given the implementation above, why does the array have to be ksorted
correctly to count? Specifically, why isn't this array considered vector
like?
$arr = [
    1=> 1,
    0=> 0,
];

Wouldn't the following be a better test?
function is_vectorlike(array $a): bool {
  $l = count($a);
  for (i=0; $i<$l, $i++) {
    if (!array_key_exists($i, $a)) {
      return false;
    }
  }
  return true;
}

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