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### Running the Engine

The chess engine is started with the command "plisco". You can also run it
from inside the repository like this:

```shell
$ perl -Ilib bin/plisco
```

The engine needs some time to come up because it compiles a number of lookup
tables.  If you run it from a git checkout, it will also need time to parse
its own source code and expand the macros contained.

See the section [Internals](#Internals) below, for details about this.

### Graphical User Interfaces

Like almost all chess engines, plisco does not come with a graphical user
interface.  Try using one of these:

* [Cute Chess](https://cutechess.com/) (Linux, MacOS, and Windows)
* [Banksia GUI](https://banksiagui.com/) (Linux, MacOS, and Windows)
* [Arena](http://www.playwitharena.de/) (Linux, Windows)

### Syzygy Endgame Tablebases

When you want to use Syzygy Endgame Tablebases, make sure that they are stored
on a fast SSD disk. Conventional spinning disks are way too small.

The DTZ files are optional but improve performance. They can be stored on a
slower storage medium. However, try to make sure that the disk does not go to
sleep while playing. Waking the disk up can take several seconds and that can
cause the engine to lose on time.

### Differences to Other UCI Engines

#### Commandline Options

The program understands several commandline options. Try `plisco --help` for
details.

#### Option `SyzygyPath`

Unlike other engines do, directories are searched recursively for tablebase
files.

#### Options `Syzygy7TimeCushion` and `Syzygy3TimeCushion`

Probing the table bases in Perl is a lot slower than in C or similar languages
because the records have to be decompressed.

The engine will therefore not sort root moves if it is likely to be flagged
because of the tablebase probes. But the performance largely depends on the
speed of your storage media.

These two non-standard options control whether the engine will try to order
the root moves with tablebase probes.  The default values for T7
(`Syzygy7TimeCushion`) is 5000 (milliseconds) and the default value for T3
(`Syzygy3TimeCushion`) is 500 (milliseconds). If there are 7 pieces on the
board, and the maximum allocated time for the move is less than T7, no
tablebase will be probed. For fewer pieces, the formula is as follows:

    min_time_left = max(T3, T7 / 5^(7 - n))

Where n is the number of pieces on the board.

If you see that the engine often loses on time, escpecially, with few pieces
on the board, increase the values accordingly.

#### Option `Move Overhead`

You can use the option name `Move Overhead` or `MoveOverhead`, whatever you
prefer.

The default value for the move overhead is displayed as 10 ms. But this is
only the initial value, and in reality, the move overhead is determined
dynamically accurately measured. That has the advantage that the engine
automatically detects network lags or other performance penalties.

The downside of this is that the engine output is not deterministic, which
can be a problem for debugging. You can avoid that by specifying the move
overhead explicitely:

```
setoption name Move Overhead value 10
```

## Internals

Functions and methods (subroutines) are the most fundamental way of avoiding
copy and paste - also known as Don't Repeat Yourself DRY (the mother of all evil)-
in source code, but it often comes at a cost, which is called call overhead.

Often times, the code runs faster if you copy the same snippets over and over
again to the needed locations instead of invoking a subroutine with arguments.
This is called inlining. But it leads, of course, to really ugly code, which is
a nightmare to maintain.

But there are ways to achieve the same results in a more readable form. The
most prominent examples are the commands `m4` and the infamous C preprocessor
`cc -E` (or the equivalent `cpp`). Both are
preprocessors that basically do a pretty smart search and replace on your
source code. Many purists hate these tools but they are key, when you have to
improve performance.

C++ tried to calm down the haters with the `inline` keyword. It is pretty much
a politically correct C preprocessor without its quirks. That turned out to
be quite useful, and eventually, the `inline` keyword found its way back into
C++'s mother language C.

Another such trick is generic programming. Search the internet if you are
interested.

Perl actually allowed you to automatically preprocess your code with the C
preprocessor, but the corresponding option `-P` was dropped with Perl
5.18 because it had many practical issues. But you can, of course, use it
in your own setup. The same goes for `m4`. Both options did not really work
well with `Chess-Plisco`.

The solution was another Perl gimmick, so-called source code filters. These
filters are regular Perl modules that receive some source code as input and