# dicebag

A dice expression parser and roller.

## Installation

```
# in a local node_modules/
npm install --save dicebag
# globally, to use the CLI
npm install -g dicebag
```

## Command-line usage

```
dicebag [-p] [<dice expression>]
```

If a dice expression is provided, prints the result of rolling those dice and
exits. Otherwise, reads expressions from `stdin`

in a loop.

`-p`

print dice pools (default behavior is to print the dice's sum)

### Examples

```
$ dicebag 1d6
1
$ dicebag "2d8 + 1d4"
7
$ dicebag -p "2d8 + 1d4"
[ 5, 3, 4 ]
```

## Library usage

```
const { parse, pool, roll } = require('dicebag')
```

The API consists of three functions:

`parse(diceExpression)`

parses an expression into an object understood by the other two functions.`pool(dice)`

rolls the dice and returns an array of their results.`roll(dice)`

rolls the dice and returns their sum.

### Examples

```
const d6 = parse('1d6')
roll(d6) // 4
roll(d6) // 5
pool(d6) // [ 2 ]
const dice = parse('2d6 + 1d8')
roll(dice) // 10
pool(dice) // [ 1, 4, 7 ]
```

## Dice expressions

### Basics

Simple expressions involving standard dice notation as used in most roleplaying games are supported. You can do things like:

`XdY`

: rolls`X`

`Y`

-sided dice (`1d6`

is a single 6-sided die,`2d4`

is two 4-sided dice).`dX`

is the same as`1dX`

(so you can shorten`1d6`

to`d6`

).`dice +/- constant`

: rolls the dice, adds/subtracts the constant.`dice +/- moreDice`

: sums/takes the difference of the results of rolling`dice`

and`moreDice`

.`number K dice`

: rolls the dice and keeps the`number`

highest results. For example,`1K2d20`

is the "rolling with advantage" mechanic from 5th Edition Dungeons and Dragons (roll two d20's, keep the highest).`number k dice`

: like`K`

but keeps the`number`

lowest results.`1k2d20`

is D&D5E's "disadvantage" mechanic.

### Full syntax and semantics

**Note:** this is still an early version. Syntax and semantics will be expanded
in future versions. *Backwards incompatible changes are possible.*

The parser recognizes the following grammar:

```
Die ::= <an integer>
| '(' Die ')'
| Die 'd' Die
| 'd' Die
| Die ' + ' Die
| Die ' - ' Die
| Die ' * ' Die
| Die ' / ' Die
| Die '+' Die
| Die '-' Die
| '-' Die
| Die 'E' Die
| Die 'e' Die
| Die 'K' Die
| Die 'k' Die
| Die 'A' Die
| Die 'a' Die
| Die 'T' Die
| Die 't' Die
| Die ' x ' Die
```

Semantics are defined in terms of the `pool`

function.

`N`

, where`N`

is an integer, is a die that always rolls a single value equal to`N`

.`pool`

returns an array containing just`N`

.`DdE`

, where`D`

and`E`

are dice expressions, is a die that rolls a number of dice equal to the result of rolling`D`

, where each die has a number of sides equal to the result of rolling`E`

.`pool`

returns an array of`roll(D)`

numbers, each between 1 and`roll(E)`

.*Note:*if`D`

or`E`

evaluates to a negative number, the behavior is undefined.`dD`

is equivalent to`1dD`

.`D + E`

appends the dice pool generated by`E`

to the dice pool generated by`D`

.`-D`

returns the opposites of values generated by`D`

.`D - E`

is equivalent to`D + (-E)`

.`D * E`

generates a dicepool with a single value - the product of`roll(D)`

and`roll(E)`

.`D / E`

generates a dicepool with a single value - the result of integer division of`roll(D)`

by`roll(E)`

.`D+E`

is the additive bonus operation. For each die in`D`

's pool, the die is rolled and`roll(E)`

is added to its result.`D-E`

is equivalent to`D+(-E)`

.`DEF`

(here`E`

is the literal symbol`E`

,`D`

and`F`

are dice expressions) is an "exploding die." First`D`

is rolled. Now each die in the dice pool generated by`F`

is rolled repeatedly until it rolls something less than the value rolled on`D`

. The die's result is the sum of all those rolls.*Note:*this could lead to an infinite evaluation if`F`

always rolls higher than a possible result of`D`

.`DeF`

is like`E`

, but explodes on values*less*than what was rolled on`D`

.`DKE`

is the "keep highest" mechanic. First`D`

is rolled. Now each die in the dice pool generated by`E`

is rolled, and the resulting dice pool is composed of those dice that rolled highest, taking up to as many dice as the result of rolling`D`

.`DkE`

is the "keep lowest" mechanic. Like`K`

, but selects the lowest rolling dice.`DAE`

might roll some of the dice in`E`

's pool again. First`D`

is rolled. Now each die in the dice pool generated by`E`

is rolled repeatedly until it rolls something less than the value rolled on`D`

. Each such roll is treated as a separate die, the results for each die are not accumulated like with exploding die.*Note:*this could lead to an infinite evaluation if`E`

always rolls higher than a possible result of`D`

.`DaE`

is like`A`

, but rolls again on values*less*than what was rolled on`D`

.`DTE`

applies a threshold to the dice in`E`

's pool. First`D`

is rolled. Now when a die from`E`

's pool rolls below the value rolled on`D`

, its value is 0, otherwise its value is 1.`DtE`

is like`T`

but dice with values*higher*than what was rolled on`D`

are counted as 0's, the rest as 1's.`D x E`

will repeatedly roll`D`

. First`E`

is rolled, then`D`

is rolled as many times as the value rolled on`E`

. The pools generated by`D`

are concatenated to generate the new pool.*Note:*if`E`

evaluates to a negative number, the behavior is undefined.

Additionally:

- The binary arithmetic operations (
`+`

,`-`

,`*`

,`/`

) are left associative. - The binary arithmetic operations bind as expected (multiplication and division bind stronger than addition and subtraction).
- The bonus operations (
`+`

,`-`

) are left associative. They bind stronger than arithmetic operations. - The die operations (
`d`

,`E`

,`K`

, etc.) are right associative (`1d2d3`

is equivalent to`1d(2d3)`

, use explicit parentheses if you need`(1d2)d3`

). - Die operations bind stronger than the binary arithmetic operations
(
`1d6 + 1d4`

is equivalent to`(1d6) + (1d4)`

) and the bonus operations. - The repetition operation
`x`

is left assosiative. It binds stronger than the standard arithmetic operations, but weaker than the dice operations and the bonus operations.