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A library for simulating dice. Use it to construct dice-rolling systems used in role-playing and board games.


GamesDice can emulate a variety of rules-driven dice systems that are used to generate integer results within a game.

The main features of GamesDice are:

  • Uses string dice descriptions, the basics of which are familiar to many game players e.g. '2d6 + 3'
  • Supports some common features of dice systems:
    • Re-rolls that replace or modify the previous roll
    • Counting number of "successes" from a set of dice
    • Keeping the best, or worst, results from a set of dice
  • Can explain how a result was achieved in terms of the individual die rolls
  • Can calculate probabilities and expected values

There are no game mechanics implemented in GamesDice, such as the chance to hit in a fantasy combat game. There is no support for player interaction within a roll, such as player choice on whether or not to re-roll a specific die within a combined set. These things are of course possible if you use the gem as-is, and add them as features within your project code.

Supported Ruby Versions

GamesDice is tested routinely on MRI Rubies 1.9.3, 2.0.0, 2.1.0, 2.1.1 and JRuby, and the "build passing" badge is based on those tests.

It should also work in versions from 1.8.7, in Ruby Enterprise and in Rubinius, but it is not tested routinely on those Rubies.


Add this line to your application's Gemfile:

gem 'games_dice'

And then execute:

$ bundle

Or install it yourself as:

$ gem install games_dice

When installed, GamesDice will attempt to install Ruby native extensions in C, for speeding up probabilities calculations. However, all the features are available in pure Ruby, and the gem should fall back to that automatically on installation if your system does not support C native extensions. You can verify which is being installed by installing the gem in verbose mode:

$ gem install games_dice --verbose

You can also verify which version you are using in Ruby by calling the class method:


which will return either :ruby or :c. Other than this method, and a speed difference between implementations, there should be no other difference. If you find one, then it will be considered as a bug.


require 'games_dice'

dice = GamesDice.create '4d6+3'
dice.roll  #  => 17 (e.g.)

Library API

Although you can refer to the documentation for the contained classes, and use it if needed to build some exotic dice systems, all you need to know to access the core features is described here.

GamesDice factory method


dice = GamesDice.create dice_description, prng

Converts a string such as '3d6+6' into a GamesDice::Dice object


  • dice_description is a string such as '3d6' or '2d4-1'. See String Dice Descriptions below for possibilities.
  • prng is optional, if provided it should be an object that has a method 'rand( integer )' that works like Ruby's built-in rand method

Returns a GamesDice::Dice object.

GamesDice::Dice instance methods

Example results given for '3d6'. Unless noted, methods do not take any parameters.


Simulates rolling the dice as they were described in the constructor, and keeps a record of how the simulation result was achieved.

dice.roll        # => 12


Returns the value from the last call to roll. This will be nil if no roll has been made yet.

dice.result      # => nil
dice.result      # => 12


Returns a string that attempts to show how the result from the last call to roll was composed from individual results. This will be nil if no roll has been made yet.

dice.explain_result    # => nil
dice.roll              # => 12
dice.explain_result    # => "3d6: 4 + 2 + 6 = 12"

The exact format is the subject of refinement in future versions of the gem.


Returns the maximum possible value from a roll of the dice. Dice with the possibility of rolling progressively higher and higher values will return an arbitrary high value.

dice.max         # => 18


Returns the minimum possible value from a roll of the dice. Dice with the possibility of rolling progressively lower and lower values will return an arbitrary low value.

dice.min         # => 3


Convenience method, returns an array [ dice.min, dice.max ]

dice.minmax      # => [3,18]


Calculates probability distribution for the dice.

Returns a GamesDice::Probabilities object that describes the probability distribution.

probabilities = dice.probabilities

Note that some distributions, involving keeping a number best or worst results, can take significant time to calculate. If the theoretical distribution would contain a large number of very low probabilities due to a possibility of large numbers re-rolls, then the calculations cut short, typically approximating to the nearest 1.0e-10.

GamesDice::Probabilities instance methods


Returns a hash representation of the probability distribution. Each key is a possible result from rolling the dice (an Integer), and the associated value is the probability of a roll returning that result (a Float, between 0.0 and 1.0 inclusive).

distribution = probabilities.to_h
distribution[3]           # => 0.0046296296296


Returns maximum result in the probability distribution. This may not be the theoretical maximum possible on the dice, if for example the dice can roll open-ended high results.

probabilities.max         # => 18


Returns minimum result in the probability distribution. This may not be the theoretical minimum possible on the dice, if for example the dice can roll open-ended low results.

probabilities.min         # => 3

probabilities.p_eql( n )

Returns the probability of a result equal to the integer n.

probabilities.p_eql( 3 )  # => 0.004629629629
probabilities.p_eql( 2 )  # => 0.0

Probabilities below 1e-10 due to requiring long sequences of re-rolls will calculate as 0.0

probabilities.p_gt( n )

Returns the probability of a result greater than the integer n.

probabilities.p_gt( 17 )  # => 0.004629629629
probabilities.p_gt( 2 )   # => 1.0

probabilities.p_ge( n )

Returns the probability of a result greater than or equal to the integer n.

probabilities.p_ge( 17 )  # => 0.0185185185185
probabilities.p_ge( 3 )   # => 1.0

probabilities.p_le( n )

Returns the probability of a result less than or equal to the integer n.

probabilities.p_le( 17 )  # => 0.9953703703703
probabilities.p_le( 3 )   # => 0.0046296296296

probabilities.p_lt( n )

Returns the probability of a result less than the integer n.

probabilities.p_lt( 17 )  # => 0.9953703703703
probabilities.p_lt( 3 )   # => 0.0


Returns the mean result, weighted by probabality of each value.

probabilities.expected  # => 10.5 (rounded to nearest 1e-9)

String Dice Descriptions

The dice descriptions are a mini-language. A simple six-sided die is described like this:


where the first integer is the number of dice to add together, and the second number is the number of sides on each die. Spaces are allowed before the first number, and after the dice description, but not between either number and the "d".

The dice mini-language allows for adding and subtracting integers and groups of dice in a list, e.g.

2d6 + 1d4
1d100 + 1d20 - 5

That is the limit of combining dice and constants though, no multiplications, or bracketed constructs like "(1d8)d8" - you can still use games_dice to help simulate these, but you will need to add your own code to do so.

Die Modifiers

After the number of sides, you may add one or more modifiers, that affect all of the dice in that "NdX" group. A die modifier can be a single character, e.g.


A die modifier can also be a single letter plus an integer value, e.g.


You can add comma-seperated parameters to a modifier by using a ":" (colon) character after the modifier letter, and a "." (full stop) to signify the end of the parameters. What parameters are accepted, and what they mean, depends on the modifier:


You can use more than one modifier. Modifiers should be separated by a "." (full stop) character, although this is optional if you use modifiers without parameters:


are all equivalent.


You can specify that dice rolling certain values should be re-rolled, and how that re-roll should be interpretted.

The simple form specifies a low value that will automatically trigger a re-roll and replace:


When rolled, this die will score from 1 to 6. If it rolls a 1, it will roll again automatically and use that result instead.

The full version of this modifier, allows you to specify from 1 to 3 parameters:



  • VALUE_COMPARISON is one of >, >=, == (default), <= < plus an integer to set conditions on when the reroll should occur
  • REROLL_TYPE is one of
    • replace (default) - use the new value in place of existing value for the die
    • add - add result of reroll to running total, and ignore any subtract rules
    • subtract - subtract result of reroll from running total, and reverse sense of any further add results
    • use_best - use the new value if it is higher than the existing value
    • use_worst - use the new value if it is lower than the existing value
  • LIMIT is an integer that sets the maximum number of times that the rule can be triggered, the default is 1000


1d6r:1.                # Same as "1d6r1"
1d10r:10,replace,1.    # Roll a 10-sided die, re-roll a result of 10 and take the value of the second roll
1d20r:<=10,use_best,1. # Roll a 20-sided die, re-roll a result if 10 or lower, and use best result


You can specify that the value shown on each die is converted to some other set of values. If you add at least one map modifier, all unmapped values will map to 0 by default.

The simple form specifies a value above which the result is considered to be 1, as in "one success":


When rolled, this will score from 0 to 3 - the number of the ten-sided dice that scored 6 or higher.

The full version of this modifier, allows you to specify from 1 to 3 parameters:



  • VALUE_COMPARISON is one of >, >= (default), ==, <= < plus an integer to set conditions on when the map should occur
  • MAP_VALUE is an integer that will be used in place of a result from a die, default value is 1
    • maps are tested in order that they are declared, and first one that matches is applied
    • when at least one map has been defined, all unmapped values default to 0
  • DESCRIPTION is a word or character to use to denote the map in any explanation


9d6x.m:10.                 # Roll 9 six-sided "exploding" dice, and count 1 for any result of 10 or more
9d6x.m:10,1,S.             # Same as above, but with each success marked with "S" in the explanation
5d10m:>=6,1,S.m:==1,-1,F.  # Roll 5 ten-sided dice, count 1 for any result of 6 or more, or -1 for any result of 1


You can specify that only a sub-set of highest or lowest dice values will contribute to the final total.

The simple form indicates the number of highest value dice to keep.


When rolled, this will score from 2 to 20 - the sum of the two highest scoring ten-sided dice, out of five.

The full version of this modifier, allows you to specify from 1 to 2 parameters:



  • KEEP_NUM is an integer specifying the number of dice to keep.
  • KEEP_TYPE is one of
    • best - keep highest values and add them together
    • worst - keep lowest values and add them together


4d6k:3.r:1,replace,1.      # Roll 4 six-sided dice, re-roll any 1s, and keep best 3.
2d20k:1,worst.             # Roll 2 twenty-sided dice, return lowest of the two results.


  • When there are many modifiers, they are applied in strict order:
    • First by type: re-rolls, maps, keepers
    • Then according to the order they were specified
  • A maximum of one re-roll modifier, and one map modifier are applied to each individual die rolled
  • Only one keepers modifier is applied per dice type. Specifying a second one will cause an error


Some combinations of modifiers crop up in well-known games, and have been allocated single-character short codes.

This is an alias for "exploding" dice:

5d10x    # Same as '5d10r:10,add.'

When rolled, this will score from 5 to theoretically any higher number, as results of 10 on any die mean that die rolls again and the result is added on.


  1. Fork it
  2. Create your feature branch (git checkout -b my-new-feature)
  3. Commit your changes (git commit -am 'Add some feature')
  4. Push to the branch (git push origin my-new-feature)
  5. Create new Pull Request

I am always interested to receive information about dice rolling schemes that this library could or should include in its repertoire.