I need API (or easily modifiable source code) for a chess ..

As part of an artificial life experiment, I need to run a tournament
where different computer programs play each other, in games like chess
or checkers (I'm looking for games that are less brute-force and more
on interesting AI algorithms). There will be no human players (so no
GUI needed), and I need an API or some source code (preferably in C)
so that my main tournament program can call the chess engines and have
them play a game against each other and see who wins. I'd also like to
be able to modify the "level" of play of each artificial player on the
fly: that is, when choosing among a set of moves, or deciding how far
to look ahead, I'd like to pass it a parameter "Q" (quality of
player), say from 1 to 10, which tells it how good a move to generate.
So, in pseudocode, something like this:

declare common chess board C;
initialize players p1, p2;
while (no winner)
{
Q=decide_quality();
move(p1,C,Q);
Q=decide_quality();
move(p2,C,Q);
}
print who won;

this may seem like a weird thing to do, and it doesn't really matter
how the Q will be decided each time. I need advice: what is the chess
engine/source code set that will make it easiest to do something like
this (play artificial players against each other in a loop, adjusting
the "strength" of each player during a game).

Thanks,

Mike