Random = chooses a valid move at random
Capture = captures when possible, but otherwise plays at random
Capture+Start = captures when possible, otherwise starts a new peg, otherwise random
Statistics
With purely random play, player 1 has a slight advantage, winning 51% of games. Here are some additional stats. The Capture+Start column (for example) represents games in which both players use the Capture+Start strategy described above.
Statistic Random Capture Capture+Start
Turns per game 55 58 62
Captures per game 1.1 1.4 2.7
Starts per game 4.7 5.0 6.3
Passes per game 15 16 14
Choices per game 16 16 26
No-Choice games 1.9% 2.2% 2.7%
On a given turn, a player may be forced to pass. This occurs about 25% of the time. Alternatively, a player might be forced to make a particular move, because only one of its pegs can move the number of spaces rolled (or start when a 6 is rolled). On other turns, a player might have a choice of which peg to move in response to a roll of the die. These choices occur on roughly 30 - 40% of turns. Astonishingly, note that about 2% of the time a player will not be given any choice in the course of an entire game (nearly always resulting in a loss).
Simple Strategies
I simulated various strategies playing against a purely random strategy ("Random"), and here are the win rates I found.
Strategy Win Rate against Random
Behind 44%
Ahead 46%
Random 50%
Capture 55%
Capture+Start 62%
Capture+Start+Ahead 64%
Capture+Start+SafestAhead 68%
The Ahead strategy always moves the peg that's furthest ahead (closest to the finish, or perhaps already in it), while the Behind strategy always moves the peg that is furthest behind (closest to the start, or perhaps starting a new peg). Surprisingly, note that both of these strategies are defeated by Random (which explains why my 4-year-old kept beating me).
The Capture strategy takes us to a 55% win rate. (Note that Capture outperforms Start, which starts a new peg whenever possible.)
We obtain a 62% win rate with Capture+Start. We get to 64% by playing Capture+Start+Ahead, which always moves the peg that's furthest ahead when it can't capture or start a new peg.
Any further improvement defies simple rules, and generally involves choosing to move pegs to escape threats and to threaten opponents. The Capture+Start+SafestAhead strategy listed above moves pegs to minimize the number of threats (in which an opponent's peg could capture the player's peg with a single roll of the die, including rolling a 6 to capture a peg at the opponent's entrance). If two moves lead to the same number of threats, the further ahead peg is moved. An even better strategy would also choose to threaten opponent pegs. It's a good bet that even optimal play can still expect to lose to Random about 30% of the time.
Elements of an Optimal Strategy
Initially, I was unable to write a program to find the optimal Trouble strategy. I therefore began by writing a program to approximate optimal play. (I'll come back to optimal play later in this post.) Each time my program is faced with a choice of which move to make, it tries each possible move and simulates a thousand games from that point. In those simulations, it plays the Capture+Start strategy for both sides, which can be computed quickly and is a good (but imperfect) approximation of optimal play. Here are the rules I extracted from my program's results:
Rule #1: Capture
Always capture an opponent's peg when you can. There are exceptions to this rule, but they are extremely rare. In the game below, the blue player has a roughly 68% chance of winning if it moves the peg at the bottom right, which would escape a pursuer and get the peg to the finish. Nonetheless, the correct move is to capture with the peg at the top left, resulting in a 74% chance of victory.
Rule #2: Start New Pegs
Prefer to start a new peg when you can. This rule and the ones that follow are really guidelines. They are usually correct, but exceptions are common.
In the game below, the blue player has rolled a 6, and has a 46% chance of winning by starting a new peg. This is a significant advantage over the 39% chance of winning by moving the peg already on the board--even though that move would escape a pursuer.
Rule #3: Move the Furthest Ahead
In general, move whichever peg is closest to the finish. In the game below, the blue player should move the peg marked 39 because it is closer to the finish, even though moving the peg marked 34 would escape a pursuer and open up the possibility of starting a new peg.
In general, move whichever peg is closest to the finish. In the game below, the blue player should move the peg marked 39 because it is closer to the finish, even though moving the peg marked 34 would escape a pursuer and open up the possibility of starting a new peg.
Rule #4: Avoid Danger
Most exceptions to the Furthest Ahead rule come down to avoiding danger. A dangerous space is one that could lead to a capture on the opponent's next roll. That means anywhere 1-to-6 spaces ahead of an opponent. It also means the opponent's start space (if the opponent has any pegs left to start).
In the game below, we've rolled a 4. Moving the further ahead peg (marked 47) would place it in danger, so it's significantly better to move the peg marked 51, placing that peg in danger instead.
In the game below, we've rolled a 5. If we move the top-left peg, it will land on the opponent's start space, leaving it vulnerable to being captured on the next turn. Therefore, we should move the peg marked 47 instead.
Note that it's ok to land on the opponent's start space if the opponent has no more pegs to start. It's also ok to land on the opponent's start with a 6, since we'll be able to roll again to get off that space before our opponent has a chance to roll.
Note that it's often helpful to forget about getting the further behind pegs to the finish. Instead, think of those pegs as ones you can move any time a roll would place the further-ahead peg in danger. In general, of all those pegs that can be moved without entering danger, move the one that's furthest ahead.
Most exceptions to the Furthest Ahead rule come down to avoiding danger. A dangerous space is one that could lead to a capture on the opponent's next roll. That means anywhere 1-to-6 spaces ahead of an opponent. It also means the opponent's start space (if the opponent has any pegs left to start).
In the game below, we've rolled a 4. Moving the further ahead peg (marked 47) would place it in danger, so it's significantly better to move the peg marked 51, placing that peg in danger instead.
In the game below, we've rolled a 5. If we move the top-left peg, it will land on the opponent's start space, leaving it vulnerable to being captured on the next turn. Therefore, we should move the peg marked 47 instead.
Note that it's ok to land on the opponent's start space if the opponent has no more pegs to start. It's also ok to land on the opponent's start with a 6, since we'll be able to roll again to get off that space before our opponent has a chance to roll.
Note that it's often helpful to forget about getting the further behind pegs to the finish. Instead, think of those pegs as ones you can move any time a roll would place the further-ahead peg in danger. In general, of all those pegs that can be moved without entering danger, move the one that's furthest ahead.
Rule #5: Play Dead
You've got two pegs on the board, and the one that's further ahead finds itself in danger. In this situation, it's often best to move the peg that's behind. The peg that's furthest ahead "plays dead" until it's safe to move again, either because the threat has leapt over us, or because a subsequent roll lets us move to safety.
In the game below, the further ahead peg is threatened and we've rolled a 2. Moving that peg 2 spaces still leaves it vulnerable to capture on the opponent's next roll. (We would need to have rolled at least a 3 to move our peg beyond our opponent's reach.) Therefore, we'll have it play dead, and we'll choose to move peg 39 instead.
There are multiple rationales for this choice. Advancing the further ahead peg often increases the number of opportunities for an opponent to capture it. In this situation, our opponent has only one active peg. If peg 36 plays dead, it's likely our opponent will be forced to step over our peg on the next couple of turns. Moving peg 39 also increases the chance that it will go on to catch up to and capture the opponent. Finally, it's possible that peg 36 is doomed no matter what. In that case, we're better off giving up on it and using our rolls to benefit peg 39. Of course, we'll re-examine the situation on our next roll.
Rule #6: Desperate Times
Desperate times call for desperate measures. Often, when our opponent has 3 pegs in the finish, we need to stop whatever we're doing to make sure that last peg doesn't reach the finish. If it's unlikely we'll have time to get our remaining pegs to the finish, we're going to have to devote all our energy to capturing our opponent's last peg. That means starting as many pegs as we can and assembling an army to attack it.
In the game below, we should leave peg 11 to threaten our opponent. That's true even if we only rolled a 1. This peg is close enough to attack already. Now let's send in the reinforcements by advancing peg 22. (If we had more pegs left to start, then it'd be even more critical to free up our start space.) As desperate as our situation is, notice that we'll still pull off a win 22% of the time.
Steps Toward a Truly Optimal Strategy
It eventually occurred to me that my program to approximate optimal strategy suffered from a critical weakness. It assumed that I would play randomly in all future moves. That meant that a correct move now might be made incorrect by random play in the future, while an incorrect move now might prove safer in the face of random play. Specifically, I began to notice that the computer rarely suggested I chase my opponent, presumably because I might randomly pass it and get captured.
I therefore set out to write a program to solve for the true optimal strategy. What makes Trouble so tricky is that's its "game tree" is really a "game graph." Due to a capture, an earlier game state can be reached again later in a game, resulting in a cyclic game graph. Attempts to find an algorithm for solving such cyclic games were unsuccessful, so I invented my own algorithm. My idea was inspired by Google's PageRank algorithm. First, I identified all game states, where a state consists of whose turn it is and how the board is arranged. (I did not include the roll of the die.) I associated each game state with a number, indicating the probability that player 1 would win. I initialized all probabilities to 0.5. Then I visited each state exactly once, in no meaningful order, updating each state's probability, according to the following rules.
if player 1 won, record 1.0 and return
if player 2 won, record 0.0 and return
total = 0.0
for each roll 1 to 6:
if no valid moves:
if roll is 6: total += prob(this state)
otherwise: total += 1.0 - prob(other player + this board)
otherwise:
bestValue = -1.0
for each valid move:
nextBoard = board after move is executed
if roll is 6: moveValue = prob(this player + nextBoard)
otherwise: moveValue = 1.0 - prob(other player + nextBoard)
if moveValue > bestValue: bestValue = moveValue
total += bestValue
record total / 6.0
Note that these rules are not recursive. In computing any one state's probability, it assumes all other states are already associated with correct probabilities. I updated each state's probability, and then I repeated the process again and again. The resulting probabilities converge surprisingly quickly to the limits of decimal precision.
Unfortunately, there are an awful lot of possible states in a game of trouble. Each peg can occupy one of roughly 30 possible locations. Let's consider a game where each player already has 3 pegs in the finish, and one peg yet to start. I call this a 1-1-game, since each player has one free peg. There are roughly 2 × 30 × 30 = 1,800 possible states in such a game--2 possible players whose turn it could be, roughly 30 possible locations for the first player's peg, and roughly 30 locations for the second player's peg. If each player has 2 finished pegs and 2 yet to start (a 2-2-game), there are now 2 × 30 × 30 × 30 × 30 = 1,620,000 possible states. The situation is actually a bit better than this. My program computes 381,936 such states. This proved to be highly manageable, and so my program had no trouble solving the 2-2-game for the optimal solution. In fact, it was able to solve m-n-games where m + n ≤ 5. But a full game would require roughly a million times more states than a 2-2-game. The rest of this post describes my understanding of the correct strategy, as gleaned from observing optimal 2-2-games.
More Elements of Optimal Play
All the elements I shared earlier remain true for optimal play. Here are a few more.
- Once a peg is within a single roll of any open places in finish, and that peg is unthreatened, then you should usually wait for a roll that can enter finish, instead of just moving closer to finish.
- Generally avoid moving a peg that has already finished, unless it would be detrimental to move other pegs.
- Generally move a lagging peg off of your opponent's start when your leading peg is safe.
- Here's how to play when no opponent pegs are on the board (all are in start or finish). If you have any pegs beyond your opponent's start, get them to finish. If all of your pegs are before your opponent's start, you should often set up a situation where the leading peg can leap clear across the opponent's start (with a roll of 5 or 6, from just before the opponent's start). Alternatively, set up an ambush, by getting a peg into the space just before your opponent's start, and then leaving it there, allowing your next peg to step over and proceed to finish. Generally abandon any such leap/ambush plans once your opponent starts a new peg.
- A trailing peg can often chase an opponent peg (that is closer to its finish than the trailing peg is) when the leading peg is likely to remain unthreatened.
Whenever you are faced with a decision, consider each possible move separately and identify what type of move it is. Most moves fall into the following categories, sorted by strength of move.
1. Capture your opponent's peg.
2. Start a new peg.
3. Chase your opponent's last peg.
4. Strong moves: enter finish; escape a threat with leading peg (so that no longer a single roll from capture); chase an opponent's peg (that is closer to its finish than the chasing peg is to your finish). I could identify no clear rule for situations when you are faced with a decision between two strong moves. There's probably no more critical decision than determining whether to chase an opponent.
5. Set up a leap over start or ambush if the only pegs on board are yours, and they haven't reached your opponent's start yet.
5. Move unthreatened leading peg until it could enter any open position in finish with a single roll.
6. Move a trailing peg.
7. Move closer to finish with a peg that is already a single roll from any open position in finish.
8. Move within threat instead of playing dead, if the threatening peg is not likely to follow (because it's not your opponent's leader).
9. Move finished peg within finish. This is a neutral move generally made to avoid making a detrimental move.
10. Any other move, other than the following.
11. Move into a threat with your leading peg.
12. Allow opponent's last peg to pass your last defender.
