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Dice Bag: The Ruby Dice Rolling Library

Name : Dice Bag Library for Ruby

Author : Randy Carnahan

Version: 3.2.2

License: LGPL OR MIT

The dice library for Ruby is an attempt to bring a standard interface to every gamer's (RPG and otherwise) need to have dice rolled. The centralized concept to this is taking a standard formatted string and parsing it, returning values from that string.

The original inspiration for this library was the 'rolldice' Unix command-line application. Since then, it's been added to to allow additional elements in the dice string.

Note: prior versions of this library allowed for dice strings to be "complex" in that there was more than one dice string, separated by commas, to be parsed and returned. This feature was never really used, so it's been removed to keep this newer version (3.0+) cleaner.

Starting with Version 3.0, this library now uses parslet, the excellent Ruby syntax parser. It's made the internals a bit more complicated, but has allowed greater flexibility in constructing dice strings. For example, previous version, you had to do '4d6 e6 k3' if you wanted the dice to explode (see below) and to keep the 3 highest rolls. Now, the e6 and k3 (and the other options) can be in any order after the xDx part of the string.

Installation

gem install dicebag

Dice Strings

Simple Dice String

A simple dice string, also called the xDx string, that represents a single group of dice, such as 3d10 or 4d6. Optional parts of the Simple Dice string are given below.

A dice string is made up of one or more parts that consist of either:

  • an optional label, which must be the first part of the string. Labels are defined within parenthesis.
  • an xDx (such as 3d6) definition and options for that definition.
  • modifiers that is applied to the current total. These are either static values or additional xDx strings.

The allowed static modifiers are add (+), subtract (-), multiply (*), and divide (/). As the library (in the Roll class) iterates over the parsed tree, a total for that roll is kept; the static modifiers are applied in the order they are gotten, which is to say, the standard order of arithmetic calculations do not apply.

For example:

2d6 + 5 * 3 - 6

...would roll the 2d6 for the total, then add 5 to it, then multiply that total by 3, and finally subtract 6. The 5 is not multiplied by 3 for a 15 and then added to the roll result. Just something to keep in mind.

Simple Dice String Options

In the following section, note that # is used to denote the number part of a option.

xDx - denotes how many dice to roll and how many sides the dice should have. This is the standard RPG dice syntax. This must come before any options for a given set of dice.

e# - the explode value. Some game systems call this 'open ended' dice. If the number rolled is greater than or equal to the value given for this option, the die is rolled again and added to the total. If no number is given for this option, it is assumed to be the same as the number of sides on the die. Thus, '1d6 e' is the same as '1d6 e6'.

d# - this denotes how many dice to drop from the tally. These dice are dropped before any dice are kept with k# below. So, '5d6 d2' means roll five 6-sided dice and drop the lowest 2 values. If the given value (combined with how many dice to keep) are greater than the number of dice in the xDx string, this value will be reset to 0.

k# - this denotes how many dice to keep out of the number of dice rolled, keeping the highest values from the roll. Thus, '4d6 k3' means to roll four 6-sided dice and keep the best 3 values. If the given value (combined with how many dice to drop) are greater than the number of dice in the xDx string, this value will be reset to 0.

r# - this denotes a reroll value. If the die rolls this value or less, then the die is rolled again. Thus, '1d6 r3' will only return a result of 4, 5, or 6. If the given value is larger than the number of sides on the die, then it is reset to 0.

t# - this denotes a target number that each die in the roll must match or exceed to count as a 'success'. That is, the dice in the roll are not added together for a total, but any die that meets or exceeds the target number is added to a total of successes. For example, '5d10 t8' means roll five 10-sided dice and each die that is 8 or higher is a success. (Like in WhiteWolf games.) If this option is given a 0 value, that is the same as not having the option at all; that is, a normal sum of all dice in the roll is performed instead.

Note: if any value is reset because of a validation failure, a note is attached to the Roll.

Dice String Limitations

Within the dice library itself, simple (xDx) strings are limited to 3 digits for all parts of the string. This is to prevent honkin' huge numbers that some users abuse to lag out the dice rolling process.

Using the Dice Library

Using the library is rather straight forward:

require 'dicebag'

dstr   = "(Damage) 2d8 + 5 + 1d6"
dice   = DiceBag::Roll.new(dstr)
result = dice.result()

puts result

This would output something like the following:

Damage: 15

The returned result from Roll#result is an instance of the Result class, which has methods to access the label (if any), the total of the roll, and also each of the sections that made up the roll.

It is possible to get the individual sections values as well:

result.each do |section|
  puts "%s: %s" % [section, section.total]
end

For the above given dice string, would print something like this:

2d8: 7
  5: 5
1d6: 3

Also, if you are curious to see how the dice string was parsed, you can retrieved the parsed value from the Roll instance using the #tree method:

parsed = dice.tree()

For the above given dice string, this returns a nested array of values:

[[:label, #<Dice::LabelPart:0x1091f2980 @value="Damage"],
[:start,
  #<Dice::RollPart:0x1091f2840
    @count=2,
    @notes=[],
    @options={:keep=>0, :reroll=>0, :explode=>0, :drop=>0},
    @sides=8,
    @tally=[5, 2],
    @total=7,
    @value={:sides=>8, :count=>2}>],
[:add, #<Dice::StaticPart:0x1091f2750 @value=5>],
[:add,
  #<Dice::RollPart:0x1091f2688
    @count=1,
    @notes=[],
    @options={:keep=>0, :reroll=>0, :explode=>0, :drop=>0},
    @sides=6,
    @tally=[3],
    @total=3,
    @value={:sides=>6, :count=>1}>]]

Typically, you won't have to deal with the internals of a dice roll if all you want are the results. However, you can dig down into the returned result's classes to obtain pretty much any data you want. Most, if not all, of the instance properties have attr accessors set up.

For example, if you wanted to know the actual dice tally of a '4d6 k3' roll, you could do this, after getting the result from Dice::Roll.result():

result = Dice::Roll.new("4d6 k3").result()
tally  = result[0].sections[0].tally()

puts "[%s]" % tally.join("][")

All of the classes have to_s methods that'll work for most cases.

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