Loading OEIS integer sequences

To show that interpreted languages can be fast when used well, I’m posting this example.

Take the On-line Encyclopedia of Integer Sequences database, which is a collection of integer sequences. That means lists of numbers. You can get a file from http://oeis.org/stripped.gz, which contains the first few numbers of each sequence. Today’s file extracts to about 38MB.

Now I need to do lookups in this file for a program I’m writing, and that program is in PHP. We want to know some sequences based on their A-number, like this:

$primes = oeis("A000040");
$fibonacci = oeis("A000045");

If we’re smart about it, then even a large file can be parsed in fractions of a second. Here’s how we do it:

/* This code needs an extract of the OEIS database to operate.
	I got it from http://oeis.org/stripped.gz
	Just extract that to this folder for lookups */
function oeis($number) {
	/* Return an array of values based on a sequence's OEIS number */
	$number = strtoupper($number);
	$fp = fopen("stripped", "r");
	while($ln = fgets($fp)) { /* Find this sequence and break the loop */
		if(substr($ln, 0, strlen($number) + 1) == $number." ") {
			$res = $ln;
			break;
		}
	}
	fclose($fp);
	/* Exit if we haven't got anything */
	if(!isset($res)) { return false; }
	$rv = explode(" ", $ln); /* Split lines into left and right of space */
	$ln = trim($rv[1]);
	$ln = substr($ln, 1, strlen($ln)-2); /* Slices off extra commas on sides */
	$rv = explode(",", $ln); /* Split by commas */
	return $rv;
}

Note that we don’t use explode() until after we have found the line we need, and also note that file_get_contents() is not used at all. (Multi-megabyte strings will bog you down in any language).