Algorithms in PHP: Deques | Hacker News Reader

Algorithms in PHP: Deques

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85 pointswithinboredom22 comments

Php's array_shift() does seem unusually slow.

  $ time php -r '$a=array_fill(0,100000,"test");while(array_shift($a)){}' 
  real    0m10.238s
  user    0m10.238s
  sys     0m0.000s

  $ time perl -e 'my @a=("test") x 100000;while(shift(@a)){}'
  real    0m0.027s
  user    0m0.027s
  sys     0m0.000s

  $ time node -e 'a=Array(100000).fill("test");while(a.shift()){}'
  real    0m1.350s
  user    0m1.343s
  sys     0m0.011s

PHP arrays are associative arrays, it has to update the keys for every remaining value after the first element is deleted.

That seems like some kind of local fitness minimum in the space of design choices. Even though there certainly are worse choices one could make, I couldn't imagine anything much worse than that would even stand up to any sort of use.

PHP does at least have an optimised internal representation for arrays with sequential integer keys, where it can avoid having a hash table and just do linear indexing: https://www.npopov.com/2014/12/22/PHPs-new-hashtable-impleme...

But a tradeoff in that design is it still has to keep track of keys and update them if you remove stuff from the start of the array. Adding and removing elements at the end is very fast, though.

Interesting side effect of PHP's magicfull array, which is an array/vector, a linked list and a dictionary/hashmap in a single data structure and in that case copies all elements on each shift into the new location to keep the indexes starting at zero. If you break up the "array", say with an `unset($a[0]);` after the array_fill, which tells that you don't want "array" behavior, it becomes fast but all elements keep their original index instead of being moved forward.

nieve -> naive

Just a small typo, article is good. Clear examples and good explanations.

precisely

That code is so bad. Who does this?!

    return ($this->head === 0 && $this->tail === $this->size - 1) || $this->head === $this->tail + 1;

Or this?

     $return = $this->next?->resetPrevious();

Or this!!!

    $this->tail =
            $this->tail?->setNext(new Node($this->tail, null, $value))->next
            ?? $this->head = new Node(null, null, $value);

Or leaves typos in their code?

    elseif ($this->tail === $this->size - 1) {
            $this->tail = 0;just a tiny
        }

I get that PHP has a bad rep but this ain't helping.

> who does this

Well, I’m doing this for fun, mostly in an illustrative manner. I’m not getting paid for this, nor is anyone reviewing my work. So thanks for volunteering, though your critical analysis could be more constructive.

As to the typos, I suspect it comes from the WordPress editor just being its weird self and perhaps me starting to type in the wrong place, or Grammarly updating the wrong thing. Who knows, but it’s not in the markdown version of this draft.

I also wonder what the purpose of wrapping native PHP array mutation in an object is and what overhead that introduces.

I'm with you on being kind in code reviews but one of the canonical examples of sticky bad PHP code is the addslashes top answer on SO for preventing SQL injection. It stayed there for years/decades and hundreds if not thousands of junior devs copied and pasted it.

So there's some risk when we share code that Google can index and possibly elevate.

I think consulting an algorithms book first would give a more formal example that you could work from.

> purpose of wrapping native PHP array mutation in an object is and what overhead that introduces.

The purpose is to illustrate what we are doing and setting the stage for later implementations. The overhead is practically unmeasurable unless we are filling the object table with these things (we are not).

Property lookup and function calls are pretty darn fast in PHP, especially with JIT enabled.

> The overhead is practically unmeasurable unless we are filling the object table with these things

I think that's typically how one would measure complexity and regardless of JIT I think it introduces some artificial cost.

Sure, it adds a cost, but it’s infinitesimal compared to the cost of what we are measuring.

> I get that PHP has a bad rep

Have you ever tried reading perl code?

I learned that Perl is a write-only language back in the day, and that really helped.

or maybe you're being overly critical.

PHP aint pretty, but that code doesn't seem unreasonable to me. I certainly wouldn't blink at it outside of the typo.

You are not helping either. Show how it is supposed to be :)

That's kind of the whole issue, isn't? Who knows what that kind of code is supposed to mean?

Actually this type of over-usage of ternary operators is not a PHP thing.. it's very prevalent in the JavaScript world

None of those snippets contain a ternary operator.

?? is the Null Coalescing Operator[1]

?-> is the nullsafe operator[2]

[1]: https://www.php.net/manual/en/language.operators.comparison.... [2]: https://www.php.net/manual/en/language.oop5.basic.php#langua...

Oh my mistake. Still stuff like 'return a || b' is the kind of thing I see in JS way more than in PHP codebases

Probably because the Boolean operators in JS return the left or right side of the expression, rather than a Boolean.

It was an okay-ish way to get a default/fallback value before JS got a proper coalescing operator.

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