Arimaa/Relative Value of Pieces
If each Arimaa piece were given a number, the numbers would often change mid-game. A piece might not even be equal to its enemy counterpart; things must be evaluated in light of the whole board.
Importance of Elephants
As the strongest Arimaa piece, the elephant is unique in its value. Since it cannot be pushed or pulled, an elephant can be captured only when it has chosen to occupy a trap square. With a game still in progress, an elephant loss will almost always be devastating. A strong elephant blockade can also be ruinous. When only one side has a functional elephant, that is the ultimate strategic advantage.
Since it is normally easy to avoid losing one's elephant, it is moot to consider the exchange value of an elephant relative to other pieces. Nevertheless, an elephant loses influence when it can't easily move, whether due to a blockade, hostage, frame, trap control fight, or goal threat. An elephant mobility advantage can amount to a lot.
The one thing worth more than an elephant is a guaranteed goal; an elephant may need to be sacrificed to help a friendly rabbit reach goal, but since the game has been won there is no problem. In rare circumstances, such as move 38s of this game, giving up one's elephant may be the only way to stop an enemy goal.
The practical value of each piece depends on its strength relative to the opponent's remaining forces. If the silver camel is captured, any remaining gold horse becomes stronger, as the silver elephant is now the only threat it faces. There is a similar effect on down the line; the fewer threats a piece faces, the stronger it becomes.
In the endgame, goal threats are often worth more than material; when one side has a path to goal, the opponent must stop that goal at any cost. One forced to prioritize goal defense could lose material simply because there isn't time to save it. Raw quantity may gain importance, since it can be hard for a depleted army to stop a goal.
In an even exchange, each side captures identical material within a few turns. This is not always a wash; such an exchange might actually undermine a strong position. Suppose Gold has lost a dog while Silver has lost her camel, with all other pieces remaining. Each side retains two horses, but Gold's are now functionally stronger, as there is no silver camel to threaten them. An "even" horse trade would leave Gold fewer pieces with which to threaten the two silver dogs and to support the gold camel.
Some general rules of thumb:
- A player holding a blockade, frame, or hostage should make the most of such an advantage, which could be diminished by "even" exchanges.
- A player whose elephant is blockaded will normally benefit from even trades, which will leave the opponent fewer pieces to spare for blockade duty.
- A player whose elephant is pinned to a framed piece will usually benefit from even trades, which will make it harder for the opponent to accomplish anything else while still holding the frame.
- A player whose elephant is defending a hostage will likely benefit from even trades, which will make it harder for the opponent to overload that elephant with a second threat.
- An even piece-for-piece trade weakens every stronger piece, but strengthens every weaker piece.
- When material is even and then a dog is captured outright, the side up a dog will benefit by exchanging camels or horses, as this will reduce the threats to dogs. Cat-for-cat or rabbit-for-rabbit trades, by contrast, might undermine a dog advantage, which makes one's own cats and rabbits stronger than the opponent's.
- A player up by exactly a camel has nothing to gain from any even exchange (elephants are never exchanged). However, a player up by a camel and a rabbit might become even stronger by exchanging dogs, cats, or rabbits. While such an exchange would slightly weaken the camel, it would strengthen the friendly rabbits, which could begin to threaten goal against thinning enemy forces.
- A player with the only remaining camel shouldn't readily trade a horse; a horse and camel should try to overload an opponent who has only one piece which could threaten either.
- Any exchange brings the endgame closer. If one side has a strong latent goal threat or is well up in rabbits, that side may benefit by moving the game along.
When a human plays a bot, any exchange gives the bot fewer possible positions to evaluate, which can be an advantage for the bot.
With all of this said, it is sometimes best to keep things simple even if that allows the opponent to gain a bit of ground. If one has a material lead but is in a jam, even a "bad" exchange may blunt a comeback by the opponent.
When material is traded for non-identical material, the implications likewise depend on what has happened beforehand. When forces have thinned, the looming threat of a goal may greatly increase the value of each remaining rabbit. On a full board, however, a rabbit is thought to be worth slightly less than a cat. A dog is thought to be worth slightly more than a cat. With each step up, the difference is greater. In the opening, two-to-one balances might be figured thus:
- A Dog is worth approximately two Rabbits
- A Horse is worth approximately a Dog and a Rabbit
- A Camel is worth approximately a Horse and a Cat
In the middlegame, such balances depend on how individual piece values have shifted. An initial horse exchange, for example, will strengthen dogs and weaken camels, altering the implications of a subsequent trade involving either. In the endgame, the value of any move relates closely to goal prospects; an otherwise bad exchange is good if it lets a friendly rabbit reach goal first.
When forces become uneven, subsequent gameplay must account for the asymmetry. If one has the only remaining camel but fewer total pieces than the opponent, it is especially important not to weaken the camel with "even" exchanges. Conversely, one with more pieces but no camel might be happy to let the opponent reciprocate any capture. If both camels and three out of four horses remain, the player with two horses will likely benefit from a camel exchange even if he is well behind in weaker pieces; once camels are eliminated, two horses can overload the enemy elephant.
Material vs. Position
By most reckonings, a camel hostage is rarely worth more than a cat. In other words, it would usually be better to capture a dog or save a friendly dog than to take a long-term camel hostage. Holding a weak hostage could be the equivalent of losing material; one should not try to take a hostage, let alone give up anything to get it, unless it would decidedly improve one's overall position.
Likewise, a frame will only benefit the framer if it creates an advantage in free pieces. A strong frame is sometimes worth nearly the value of the framed piece itself, and might become an even greater advantage if the opponent refuses to cut his losses and abandon the piece. In this game, Silver gave up a cat and a rabbit to secure a horse frame which the silver elephant then rotated out of; this paid off for Silver. Any position that gives one the only free elephant is highly valuable.
Strong pieces are often sacrificed in the endgame, when the focus is on goal threats. In this game, Silver gave up his camel at home but soon got a rabbit to goal. This was a type of race position; Silver chose to pursue a southeastern goal attack rather than defend in the northwest.
In a given Arimaa position, it is not always clear which side has the ultimate advantage. For estimates regarding specific material balances, see these material evaluators, of which HarLog is considered the most accurate. This does not however account for positional features; a goal threat or stuck elephant might shift the advantage dramatically. The overall distribution of pieces is also important. Furthermore, any advantage depends on the player knowing how to capitalize; often, the real advantage is with the one who best understands the position at hand.