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IF YOU have ever struggled to complete classic Nintendo games, don't feel bad - they are officially difficult.

An analysis of the computational complexity of video games, including those in the Mario and Legend of Zelda series, proves that many of them belong to a class of mathematical problems called NP-hard. This means that for a given game level, it can be very difficult to work out whether it is possible for a player to reach the end. The results suggest that some hard problems could be solved by playing a game.

The commercial versions of games are designed to be doable, of course, but Erik Demaine of the Massachusetts Institute of Technology and colleagues studied versions without that constraint. "We are working within the same rules, but we are free to design the levels however we like," he says.

The team transformed each game into a type of logical puzzle called the Boolean satisfiability problem. This asks whether the variables in a collection of logical statements can be chosen to make all the statements true, or whether the statements inevitably contradict each other.

For each game, the team constructed sections of a level that force players to choose one of two paths - equivalent to assigning variables in the Boolean satisfiability problem. Arrangements of enemies and power-ups are equivalent to logical statements. If they allow a level to be completed, that is equivalent to all the statements in the Boolean problem being true; if they make the level impossible, that is equivalent to a contradiction.

Many of the games in the Mario, Donkey Kong, Legend of Zelda, Metroid and Pokémon series prove to be NP-hard. That means deciding whether a player can complete them is at least as hard as the hardest problems in NP, a complexity class involved in the tantalising problem of P versus NP (see "Million-dollar proof"). Not every game in each series was included in the proof, as they follow different rules.

For Mario, the team also prove that the games are NP-complete, an additional property with important consequences. Many difficult problems can be converted into any problem in the NP-complete category. Then if you can solve the NP-complete problem - say, by completing a level of Mario - you have solved the original problem too.

That includes the travelling salesman problem - finding the shortest route between a series of points - which is of real interest in the field of logistics, and also the knapsack problem, used in deciding how to allocate resources. So theoretically you could convert an example of either problem into a Mario level, and play the game to solve it. That approach would be fun, says Demaine, although it would probably be simpler to solve the satisfiability problem directly.

The results offer mixed news for game designers, says Giovanni Viglietta, a computer scientist at the University of Pisa, Italy, who was not involved in the research. If it is an NP-hard problem to decide whether a level can be successfully navigated, there is no easy way for a designer to check this. But it does ensure that playing the game is interesting, as players can't easily decide if they are heading the right way or straying into an impassable area of the level. "The game demands some creativity and ingenuity," Viglietta says.

An analysis of the computational complexity of video games, including those in the Mario and Legend of Zelda series, proves that many of them belong to a class of mathematical problems called NP-hard. This means that for a given game level, it can be very difficult to work out whether it is possible for a player to reach the end. The results suggest that some hard problems could be solved by playing a game.

The commercial versions of games are designed to be doable, of course, but Erik Demaine of the Massachusetts Institute of Technology and colleagues studied versions without that constraint. "We are working within the same rules, but we are free to design the levels however we like," he says.

The team transformed each game into a type of logical puzzle called the Boolean satisfiability problem. This asks whether the variables in a collection of logical statements can be chosen to make all the statements true, or whether the statements inevitably contradict each other.

For each game, the team constructed sections of a level that force players to choose one of two paths - equivalent to assigning variables in the Boolean satisfiability problem. Arrangements of enemies and power-ups are equivalent to logical statements. If they allow a level to be completed, that is equivalent to all the statements in the Boolean problem being true; if they make the level impossible, that is equivalent to a contradiction.

Many of the games in the Mario, Donkey Kong, Legend of Zelda, Metroid and Pokémon series prove to be NP-hard. That means deciding whether a player can complete them is at least as hard as the hardest problems in NP, a complexity class involved in the tantalising problem of P versus NP (see "Million-dollar proof"). Not every game in each series was included in the proof, as they follow different rules.

For Mario, the team also prove that the games are NP-complete, an additional property with important consequences. Many difficult problems can be converted into any problem in the NP-complete category. Then if you can solve the NP-complete problem - say, by completing a level of Mario - you have solved the original problem too.

That includes the travelling salesman problem - finding the shortest route between a series of points - which is of real interest in the field of logistics, and also the knapsack problem, used in deciding how to allocate resources. So theoretically you could convert an example of either problem into a Mario level, and play the game to solve it. That approach would be fun, says Demaine, although it would probably be simpler to solve the satisfiability problem directly.

The results offer mixed news for game designers, says Giovanni Viglietta, a computer scientist at the University of Pisa, Italy, who was not involved in the research. If it is an NP-hard problem to decide whether a level can be successfully navigated, there is no easy way for a designer to check this. But it does ensure that playing the game is interesting, as players can't easily decide if they are heading the right way or straying into an impassable area of the level. "The game demands some creativity and ingenuity," Viglietta says.

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