[SOLVED] Random access file in PHP

Random access file in PHP

Is there some PHP function or class that allows me to read a file like an array of characters?

For example:

$string = str_split('blabla');

$i = 0;

switch($string[$i]){

  case 'x':
    do_something();
    $i++;


  case 'y':
    if(isset($string[++$i]))
      do_something_else();
    else
      break;

   case 'z':
      // recursive call of this code etc..

}

I know that I can use $string = file_get_contents($file), but the problem is that I get a huge amount of memory used for a tiny 800K file (like 80MB).

So can I somehow "stream" the file in my code above with some kind of arrayaccess like class that automatically reads data from the file when I call isset() ? :)

Solution

You can use fseek and fgetc to jump around in a file and read single characters at a time.

// Leaves the file handle modified
function get_char($file, $char) {
   fseek($file, $char);
   return fgetc($file);
}

You mentioned you wanted array behavior specifically. You can build a class which implements ArrayAccess to support this.

This could be dangerous for several reasons:

You'll need to guard against $char inputs that request indices past the length of the file
The file handle will be constantly mutated (should be okay, as long as you're not using it elsewhere)
This could be inefficient (offset by caching past requests)

A slightly more efficient alternative would be to "lazily" read the file (i.e., read it in chunks rather than all at once). Here's some (untested) code:

class BufferedReader {
    // The size of a chunk in bytes
    const BUFFER_SIZE = 512;

    protected $file;
    protected $data;

    function __construct($fname) {
        $this->file = fopen($fname, 'r');
    }

    function read_buffer() {
        $this->data .= fread($this->file, self::BUFFER_SIZE);
    }

    function get_char($char) {
        while ( $char >= strlen($this->data) && !feof($this->file) ) {
            $this->read_buffer();
        }

        if ( $char >= strlen($this->data) ) {
            return FALSE;
        }

        return substr($this->data, $char, 1);
    }
}