public function compute_string_distance( $string1, $string2 ) {
		// Use an md5 hash of the strings for a count cache, as it's fast to generate, and collisions aren't a concern.
		$count_key1 = md5( $string1 );
		$count_key2 = md5( $string2 );

		// Cache vectors containing character frequency for all chars in each string.
		if ( ! isset( $this->count_cache[ $count_key1 ] ) ) {
			$this->count_cache[ $count_key1 ] = count_chars( $string1 );
		}
		if ( ! isset( $this->count_cache[ $count_key2 ] ) ) {
			$this->count_cache[ $count_key2 ] = count_chars( $string2 );
		}

		$chars1 = $this->count_cache[ $count_key1 ];
		$chars2 = $this->count_cache[ $count_key2 ];

		$difference_key = md5( implode( ',', $chars1 ) . ':' . implode( ',', $chars2 ) );
		if ( ! isset( $this->difference_cache[ $difference_key ] ) ) {
			// L1-norm of difference vector.
			$this->difference_cache[ $difference_key ] = array_sum( array_map( array( $this, 'difference' ), $chars1, $chars2 ) );
		}

		$difference = $this->difference_cache[ $difference_key ];

		// $string1 has zero length? Odd. Give huge penalty by not dividing.
		if ( ! $string1 ) {
			return $difference;
		}

		// Return distance per character (of string1).
		return $difference / strlen( $string1 );
	}