# Imperative Random Data Generator and Quickcheck

You can use Rant to generate random test data, and use its Test::Unit extension for property-based testing.

Rant is basically a recursive descent interpreter, each of its method returns a random value of some type (string, integer, float, etc.).

Its implementation has no alien mathematics inside. Completely side-effect-free-free.

# Install

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# Data Generation

## Getting Random Data Values

``````
Rantly#map(n,limit=10,&block)
call the generator n times, and collect values
Rantly#each(n,limit=10,&block)
call a random block n times
Rantly#value(limit=10,&block)
call a random block once, and get its value.
``````

To collect an array of random data,

``````
# we want 5 random integers
> Rantly(5) { integer }
=> [-380638946, -29645239, 344840868, 308052180, -154360970]
``````

To iterate over random data,

``````
> Rantly.each(5) { puts integer }
296971291
504994512
-402790444
113152364
502842783
=> nil
``````

To get one value of random data,

``````
> Rantly { integer }
=> 278101042
``````

The optional argument `limit` is used with generator guard. By default, if you want to generate n items, the generator tries at most n * 10 times.

This almost always succeeds,

``````
> Rantly(5) { i = integer; guard i > 0; i }
=> [511765059, 250554234, 305947804, 127809156, 285960387]
``````

This always fails,

``````
> Rantly(10) { guard integer.is_a?(Float) }
Rantly::TooManyTries: Exceed gen limit 100: 101 failed guards)
``````

## Random Generating Methods

The API is similiar to QuickCheck, but not exactly the same. In particular `choose` picks a random element from an array, and `range` picks a integer from an interval.

### Simple Randomness

``````
Rantly#integer(n=nil)
random positive or negative integer. Fixnum only.
Rantly#range(lo,hi)
random integer between lo and hi.
Rantly#float
random float
Rantly#bool
true or false
Rantly#literal(value)
No-op. returns value.
Rantly#choose(*vals)
Pick one value from among vals.
``````

### Meta Randomness

A rant generator is just a mini interpreter. It’s often useful to go meta,

``````
Rantly#call(gen)
If gen is a Symbol, just do a method call with send.
If gen is an Array, the first element of the array is the method name, the rest are args.
If gen is a Proc, instance_eval it with the generator.
``````
``````
> Rantly { call(:integer) }
=> -240998958
``````
``````
> Rantly { call([:range,0,10]) }
=> 2
``````
``````
> Rantly { call(Proc.new { [integer] })}
=> [522807620]
``````

The `call` method is useful to implement other abstractions (See next subsection).

``````
Rantly#branch(*args)
Pick a random arg among args, and Rantly#call it.
``````

50-50 chance getting an integer or float,

``````
> Rantly { branch :integer, :float }
=> 0.0489446702931332
> Rantly { branch :integer, :float }
=> 494934533
``````

### Frequencies

``````
Rantly#freq(*pairs)
Takes a list of 2-tuples, the first of which is the weight, and the second a Rantly#callable value, and returns a random value picked from the pairs. Follows the distribution pattern specified by the weights.
``````

Twice as likely to get a float than integer. Never gets a ranged integer.

``````
> Rantly { freq [1,:integer], [2,:float], [0,:range,0,10] }
``````

If the “pair” is not an array, but just a symbol, `freq` assumes that the weight is 1.

``````
# 50-50 between integer and float
> Rantly { freq :integer, :float }
``````

If a “pair” is an Array, but the first element is not an Integer, `freq` assumes that it’s a Rantly method-call with arguments, and the weight is one.

``````
# 50-50 chance generating integer limited by 10, or by 20.
> Rantly { freq [:integer,10], [:integer 20] }
``````

### Sized Structure

A Rantly generator keeps track of how large a datastructure it should generate with its `size` attribute.

``````
Rantly#size
returns the current size
Rantly#sized(n,&block)
sets the size for the duration of recursive call of block. Block is instance_eval with the generator.
``````

Rantly provides two methods that depends on the size

``````
Rantly#array(size=default_size,&block)
returns a sized array consisted of elements by Rantly#calling random branches.
Rantly#string(char_class=:print)
returns a sized random string, consisted of only chars from a char_class.
Rantly#dict(size=default_size,&block)
returns a sized random hash. The generator block should generate tuples of keys and values (arrays that have two elements, the first one is used as key, and the second as value).
``````

The avaiable char classes for strings are:

``````
:alnum
:alpha
:blank
:cntrl
:digit
:graph
:lower
:print
:punct
:space
:upper
:xdigit
:ascii
``````
``````
# sized 10 array of integers
> Rantly { array(10) { integer }}
=> [417733046, -375385433, 0.967812380000118, 26478621, 0.888588160450082, 250944144, 305584916, -151858342, 0.308123867823313, 0.316824642414253]
``````

If you set the size once, it applies to all subsequent recursive structures. Here’s a sized 10 array of sized 10 strings,

``````
> Rantly { sized(10) { array {string}} }
=> ["1c}C/,9I#}", "hpA/UWPJ\\j", "H'~ERtI`|]", "%OUaW\\%uQZ", "Z2QdY=G~G!", "H<o|<FARGQ", "g>ojnxGDT3", "]a:L[B>bhb", "_Kl=&{tH^<", "ly]Yfb?`6c"]
``````

Or a sized 10 array of sized 5 strings,

``````
> Rantly {array(10){sized(5) {string}}}
=> ["S\"jf ", "d\\F-\$", "-_8pa", "IN0iF", "SxRV\$", ".{kQ7", "6>;fo", "}.D8)", "P(tS'", "y0v/v"]
``````

Generate a hash that has 5 elements,

``````
> Rantly { dict { [string,integer] }}
{"bR\\qHn"=>247003509502595457,
"-Mp '."=>653206579583741142,
"gY%<SV"=>-888111605212388599,
"+SMn:r"=>-1159506450084197716,
"^3gYfQ"=>-2154064981943219558,
"= :/\\,"=>433790301059833691}
``````

The `dict` generator retries if a key is duplicated. If it fails to generate a unique key after too many tries, it gives up by raising an error:

``````
> Rantly { dict { ["a",integer] }}
Rantly::TooManyTries: Exceed gen limit 60: 60 failed guards)
``````

# Property Testing

Rantly extends Test::Unit for property testing. The extension is in its own module. So you need to require it.

``````
require 'rant/check'
``````

It defines,

``````
Test::Unit::Assertions#property_of(&block)
The block is used to generate random data with a generator. The method returns a Rantly::Property instance, that has the method 'check'.
``````

It’s like this, using the gem ‘shoulda’

``````
# checks that integer only generates fixnum.
should "generate Fixnum only" do
property_of  { integer }.check { |i| assert i.is_a?(Integer) }
end
``````

The check block takes the generated data as its argument. One idiom I find useful is to include a parameter of the random data for the check argument. For example, if I want to check that Rantly#array generates the right sized array, I could say,

``````
should "generate right sized array" do
property_of {
len = integer
[len,array(len){integer}]
}.check { |(len,arr)|
assert_equal len, arr.length
}
end
``````

That’s about it. Enjoy :)

# Copyright

Copyright © 2009 Howard Yeh. See LICENSE for details.