PhutilBufferedIterator.php
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	<?php

	/**
	* Simple iterator that loads results page-by-page and handles buffering. In
	* particular, this maps well to iterators that load database results page
	* by page and allows you to implement an iterator over a large result set
	* without needing to hold the entire set in memory.
	*
	* For an example implementation, see @{class:PhutilExampleBufferedIterator}.
	*
	* @task impl Methods to Implement
	* @task config Configuration
	* @task iterator Iterator Implementation
	*/
	abstract class PhutilBufferedIterator extends Phobject implements Iterator {

	private $data;
	private $pageSize = 100;
	private $naturalKey;


	/* -( Methods to Implement )----------------------------------------------- */


	/**
	* Called when @{method:rewind} is invoked. You should reset any internal
	* cursor your implementation holds.
	*
	* @return void
	* @task impl
	*/
	abstract protected function didRewind();


	/**
	* Called when the iterator needs a page of results. You should load the next
	* result page and update your internal cursor to point past it.
	*
	* If possible, you should use @{method:getPageSize} to choose a page size.
	*
	* @return list<wild> List of results.
	* @task impl
	*/
	abstract protected function loadPage();


	/* -( Configuration )------------------------------------------------------ */


	/**
	* Get the configured page size.
	*
	* @return int Page size.
	* @task config
	*/
	final public function getPageSize() {
	return $this->pageSize;
	}


	/**
	* Configure the page size. Note that implementations may ignore this.
	*
	* @param int $size Page size.
	* @return $this
	* @task config
	*/
	final public function setPageSize($size) {
	$this->pageSize = $size;
	return $this;
	}


	/* -( Iterator Implementation )-------------------------------------------- */


	/**
	* @task iterator
	*/
	final public function rewind() {
	$this->didRewind();
	$this->data = array();
	$this->naturalKey = 0;
	$this->next();
	}


	/**
	* @task iterator
	*/
	final public function valid() {
	return (bool)count($this->data);
	}


	/**
	* @task iterator
	*/
	final public function current() {
	return end($this->data);
	}


	/**
	* By default, the iterator assigns a "natural" key (0, 1, 2, ...) to each
	* result. This method is intentionally nonfinal so you can substitute a
	* different behavior by overriding it if you prefer.
	*
	* @return scalar Key for the current result (as per @{method:current}).
	* @task iterator
	*/
	public function key() {
	return $this->naturalKey;
	}


	/**
	* @task iterator
	*/
	final public function next() {
	if ($this->data) {
	$this->naturalKey++;
	array_pop($this->data);
	if ($this->data) {
	return;
	}
	}

	$data = $this->loadPage();

	// NOTE: Reverse the results so we can use array_pop() to discard them,
	// since it doesn't have the O(N) key reassignment behavior of
	// array_shift().

	$this->data = array_reverse($data);
	}

	}

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