user1512: A friend told me its very hard to implement a bughouse chess engine because it requires much more processor power than a regular chess engine.
Are there any bughouse engines out there ? if not, what are the reasons ?

user70: I don't know of any bughouse engines.
Extra complexities:
In addition to playing the standard game...

Extra tactical vision.  The computer now has to envision good moves that either side could execute if only a certain piece were in-hand.
Drops make the search tree enormous.  There's really no restriction on where pieces may be dropped (pawns? can pawns be dropped?)  That makes the search tree enormous. 
Multi-board sacrifices. If the machine were to find a good move "if only I had a [piece] in hand,", it then has to decide if it is wise to sacrifice on the other board (or even this board) to get that piece.
Tactical clock management.  The bughouse games I played were always on the clock (that is - 5 minute bughouse.)  Tactical clock management is another complexity.  I've seen games won when one side had a clear win on one board, and stopped playing on the other boards when they held the time advantage, to starve the opponents of extra pieces.


user1825: Sunsetter is best seems, but Sjeng (especially 11) is good for many purposes too.
See http://bughousechess.wz.cz/BughouseEngines/

user1312: Sjeng is your best bet but play with an engine is very weird. Typically a human + engine team will have their play revolve around getting up on time (sometimes even by only a few seconds). Then the human player will tell the engine to move fast, and then sit. Note that since the computer can play decent moves instantly, that effectively means it is impossible to sac material against the human player unless also up on time -- even if the sacrifice is very strong and leads to an imminently winning attack or quickly recovering the material. The human player will accept the sacrifice(s) and sit, then let the engine win on time or use the extra material on the board to win. For these reasons none of the strong bughouse players will accept games with engines anymore, really.

user1382: There are certainly several chess engines, but as a result of the additional complexities referenced by @TonyEnnis, these engines are weaker than reasonably strong players. It's extremely difficult to take into account the different scenarios that can arise from two positions as pieces switch from board to board every move. The resulting search tree increases at an exponential rate even greater than that of standard chess.
The engine that I'm most familiar with is Sjeng which claims that it has an ELO of around 2000 on FICS, equivalent to a strong human player.

1512: <p>A friend told me its very hard to implement a <strong>bughouse</strong> chess engine because it requires much more processor power than a regular chess engine.</p>

<p>Are there any bughouse engines out there ? if not, what are the reasons ?</p>
 70: <p>I don't know of any bughouse engines.</p>

<p>Extra complexities:</p>

<p>In addition to playing the standard game...</p>

<ol>
<li>Extra tactical vision.  The computer now has to envision good moves that either side could execute <em>if only</em> a certain piece were in-hand.</li>
<li>Drops make the search tree enormous.  There's really no restriction on where pieces may be dropped (pawns? can pawns be dropped?)  That makes the search tree enormous. </li>
<li>Multi-board sacrifices. If the machine were to find a good move "if only I had a [piece] in hand,", it then has to decide if it is wise to sacrifice on the other board (or even this board) to get that piece.</li>
<li>Tactical clock management.  The bughouse games I played were always on the clock (that is - 5 minute bughouse.)  Tactical clock management is another complexity.  I've seen games won when one side had a clear win on one board, and stopped playing on the other boards when they held the time advantage, to starve the opponents of extra pieces.</li>
</ol>
 1825: <p>Sunsetter is best seems, but Sjeng (especially 11) is good for many purposes too.</p>

<p>See <a href="http://bughousechess.wz.cz/BughouseEngines/" rel="nofollow">http://bughousechess.wz.cz/BughouseEngines/</a></p>
 1312: <p>Sjeng is your best bet but play with an engine is very weird. Typically a human + engine team will have their play revolve around getting up on time (sometimes even by only a few seconds). Then the human player will tell the engine to move fast, and then sit. Note that since the computer can play decent moves instantly, that effectively means it is impossible to sac material against the human player unless also up on time -- even if the sacrifice is very strong and leads to an imminently winning attack or quickly recovering the material. The human player will accept the sacrifice(s) and sit, then let the engine win on time or use the extra material on the board to win. For these reasons none of the strong bughouse players will accept games with engines anymore, really.</p>
 1382: <p>There are certainly several chess engines, but as a result of the additional complexities referenced by <a href="https://chess.stackexchange.com/a/2591/1382">@TonyEnnis</a>, these engines are weaker than reasonably strong players. It's extremely difficult to take into account the different scenarios that can arise from two positions as pieces switch from board to board every move. The resulting search tree increases at an exponential rate even greater than that of standard chess.</p>

<p>The engine that I'm most familiar with is <a href="http://sjeng.org/indexold.html" rel="nofollow noreferrer">Sjeng</a> which claims that it has an ELO of around 2000 on FICS, equivalent to a strong human player.</p>
