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LinesOfALarge.php
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Thu, Nov 21, 19:14

LinesOfALarge.php
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<?php
/**
* Abstraction for processing large inputs without holding them in memory. This
* class implements line-oriented, buffered reads of some external stream, where
* a "line" is characterized by some delimiter character. This provides a
* straightforward interface for most large-input tasks, with relatively good
* performance.
*
* If your stream is not large, it is generally more efficient (and certainly
* simpler) to read the entire stream first and then process it (e.g., with
* `explode()`).
*
* This class is abstract. The concrete implementations avialable are:
*
* - @{class:LinesOfALargeFile}, for reading large files; and
* - @{class:LinesOfALargeExecFuture}, for reading large output from
* subprocesses.
*
* For example:
*
* foreach (new LinesOfALargeFile('/path/to/file.log') as $line) {
* // ...
* }
*
* By default, a line is delimited by "\n". The delimiting character is
* not returned. You can change the charater with @{method:setDelimiter}. The
* last part of the file is returned as the last $line, even if it does not
* include a terminating character (if it does, the terminating character is
* stripped).
*
* @task config Configuration
* @task internals Internals
* @task iterator Iterator Interface
* @group filesystem
*/
abstract class LinesOfALarge implements Iterator {
private $pos;
private $buf;
private $num;
private $line;
private $valid;
private $eof;
private $delimiter = "\n";
/* -( Configuration )------------------------------------------------------ */
/**
* Change the "line" delimiter character, which defaults to "\n". This is
* used to determine where each line ends.
*
* @param string A one-byte delimiter character.
* @return this
* @task config
*/
final public function setDelimiter($character) {
if (strlen($character) !== 1) {
throw new Exception("Delimiter character MUST be one byte in length.");
}
$this->delimiter = $character;
return $this;
}
/* -( Internals )---------------------------------------------------------- */
/**
* Hook, called before @{method:rewind()}. Allows a concrete implementation
* to open resources or reset state.
*
* @return void
* @task internals
*/
abstract protected function willRewind();
/**
* Called when the iterator needs more data. The subclass should return more
* data, or empty string to indicate end-of-stream.
*
* @return string Data, or empty string for end-of-stream.
* @task internals
*/
abstract protected function readMore();
/* -( Iterator Interface )------------------------------------------------- */
/**
* @task iterator
*/
final public function rewind() {
$this->willRewind();
$this->buf = '';
$this->pos = 0;
$this->num = 0;
$this->eof = false;
$this->valid = true;
$this->next();
}
/**
* @task iterator
*/
final public function key() {
return $this->num;
}
/**
* @task iterator
*/
final public function current() {
return $this->line;
}
/**
* @task iterator
*/
final public function valid() {
return $this->valid;
}
/**
* @task iterator
*/
final public function next() {
// Consume the stream a chunk at a time into an internal buffer, then
// read lines out of that buffer. This gives us flexibility (stream sources
// only need to be able to read blocks of bytes) and performance (we can
// read in reasonably-sized chunks of many lines), at the cost of some
// complexity in buffer management.
// We do this in a loop to avoid recursion when consuming more bytes, in
// case the size of a line is very large compared to the chunk size we
// read.
while (true) {
if (strlen($this->buf)) {
// If we already have some data buffered, try to get the next line from
// the buffer. Search through the buffer for a delimiter. This should be
// the common case.
$endl = strpos($this->buf, $this->delimiter, $this->pos);
if ($endl !== false) {
// We found a delimiter, so return the line it delimits. We leave
// the buffer as-is so we don't need to reallocate it, in case it is
// large relative to the size of a line. Instead, we move our cursor
// within the buffer forward.
$this->num++;
$this->line = substr($this->buf, $this->pos, ($endl - $this->pos));
$this->pos = $endl + 1;
return;
}
// We only have part of a line left in the buffer (no delimiter in the
// remaining piece), so throw away the part we've already emitted and
// continue below.
$this->buf = substr($this->buf, $this->pos);
$this->pos = 0;
}
// We weren't able to produce the next line from the bytes we already had
// buffered, so read more bytes from the input stream.
if ($this->eof) {
// NOTE: We keep track of EOF (an empty read) so we don't make any more
// reads afterward. Normally, we'll return from the first EOF read,
// emit the line, and then next() will be called again. Without tracking
// EOF, we'll attempt another read. A well-behaved impelmentation should
// still return empty string, but we can protect against any issues
// here by keeping a flag.
$more = '';
} else {
$more = $this->readMore();
}
if (strlen($more)) {
// We got some bytes, so add them to the buffer and then try again.
$this->buf .= $more;
continue;
} else {
// No more bytes. If we have a buffer, return its contents. We
// potentially return part of a line here if the last line had no
// delimiter, but that currently seems reasonable as a default
// behaivor. If we don't have a buffer, we're done.
$this->eof = true;
if (strlen($this->buf)) {
$this->num++;
$this->line = $this->buf;
$this->buf = null;
} else {
$this->valid = false;
}
break;
}
}
}
}

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