Elf Help

Elf Help: Imroving Search Time

How can I make it go faster?

Even for short touches, half-lead spliced composition can be very slow, simply because there are so many billions of possible ways of putting methods together. In fact, spliced composition of any sort is absolutely the worst job you can give to a computer! However, Elf has a few tricks up its sleeve which can turn days into hours and hours into minutes. First though, it's important to understand the factors that affect the speed of a search.

Things that make it go slower

More leads. It doesn't make much difference how many parts you've chosen - the number of leads in the part has the biggest single impact on the search length. For instance, in a 5-spliced search, every extra lead increases the search time roughly by a factor of 25, because there are 5-times-5 possible combinations of five methods in one lead. So reducing the number of leads (perhaps by increasing the number of parts) is the simplest way of making an impossibly long search more feasible.
Calls. Checking the "Allow bobs" box dramatically increases the search space. Taking the example above, every extra lead in the part now multiplies the search time by 5x5x2 = 50 times. Fortunately there is often no need to use bobs, because splicing methods at the half-lead usually churns the order of the bells around very quickly anyway.
More methods. The more methods you are splicing, the more possibilities there are for arranging each lead. For example, in 3-spliced there are 9 ways of arranging one lead; for 8-spliced that figure rises to 64.
The speed of your computer. A faster computer will of course finish a search quicker than a slower computer. However, in practice sheer processing power isn't as beneficial as you might think, because it has a purely linear effect, whereas the tree search of the composing engine is exponential in running time. For example, it's pretty easy to specify a search that, on a 200MHz PC, might take 100 years. If you upgrade to a 2GHz processor you can reduce that search time to 10 years - but it still doesn't make it very practicable!
The speed of the algorithm. Elf uses a sophisticated rotationally-sorted node-based tree search engine coupled with powerful heuristic pruning systems. It is not a toy - it is a very fast little beast indeed! For more information, see here. It is true that, because Elf is written in Java and runs in a browser, it is not as fast as an equivalent program written in a compiled language such as C++. However, it does get surprisingly close; for instance, it manages some 30-35% of the speed of SMC32, currently the world's fastest composing engine, which is written in hand-optimised assembler.

Pruning

The best way of making a search faster is to ignore parts of the search tree which don't contain good compositions. This is a technique known as "pruning". Elf gives you various composing options which you can play with to alter the pruning parameters. For example, by specifying "Maximum Changes of Method" you can force Elf to consider only those compositions in which the method changes every time. These pruning options can have a huge impact on the search time - so if your search is taking too long, try turning some of them on.

One important thing to bear in mind is that Elf employs an underlying heuristic pruning algorithm which is not affected by the composing options described above. This works by monitoring the output compositions for score, music, changes of method and method balance. As better and better compositions are found, the search is pruned to ignore any touches which do not score as well. This means that the search usually gets faster and faster as it finds the first few compositions, so it is well worth leaving a search to run for several minutes before you start to worry about the "estimated time left" figure. However, if no results at all are being produced you probably do need to worry - it may be worth examining your search parameters to make sure the search is feasible.

Some Rules of Thumb

In general, searches for touches of up to 10 leads are feasible, depending on methods and pruning options. The number of parts doesn't have much bearing on the search time, so this is easily enough for quarter-peals of 4 parts or more. However, you may have to be prepared to wait many hours to get the full set of results - and it really is important to wait, because the best compositions are often found deep into the search! Fortunately, once the Elf web page is loaded, the searches run off-line, so you can disconnect your computer from the internet and even leave Elf running for days at a time if you want. The "pause" button can also be useful if you need to use your computer for other tasks whilst running a long search.

Unfortunately Elf isn't suitable for peal lengths, because there are just too many leads in the part. Remember that for every single extra lead you add to the composition, the search time increases by a factor of n², where n is the number of methods you are splicing. So for peals, it's back to pen and paper!