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infinity

infinity.js

Infinite sequences for Node.js

Supports typical array functions and more, all evaluated lazily.

NPM version

Joint effort of Andrew Jones, Maxim Filimonov, Leonardo Borges and Romain Prieto, during a ThoughtWorks Node.js hack night.

Generators

infinite = require 'infinity'
 
infinite.range 9                        # naturals from 9 to infinity 
infinite.cycle [123]                # repeats 1, 2, 3 forever 
infinite.iteration 2(i) -> i*2        # all powers of 2 
infinite.zip s1s2s3                 # a steam pairing all given streams 1 to 1 
infinite.cons 9998stream            # a new stream starting with the given fixed values 
infinite.fromArray [368]            # this one is actually finite, but still lazy 
infinite.primes()                       # sequence of prime numbers 
infinite.fibonacci()                    # fibonacci sequence 

Operations

These operations create a new lazy stream:

stream = infinite.range 1
 
stream.filter (i) -> i%2 == 0       # a new stream with even numbers only 
stream.map (i) -> i*2               # a new stream with all elements doubled 
stream.skip 2                       # a new stream that starts 2 elements further 
stream.takeWhile (i) -> i < 5       # a finite stream that stops at the given condition 

And these ones resolve a stream (non lazy):

# get the first 5 elements as a javascript array 
stream.take 5
 
# reduce the stream to a single value 
# can only be called on a finite stream (after takeWhile) 
sum = (acc, i) -> acc + i
stream.reduce 0sum

Example

FizzBuzz that doesn't use loops or modulo:

naturals = infinite.range 1
threes   = infinite.cycle [''''' fizz']
fives    = infinite.cycle [''''''''' buzz']
concat   = (arr) -> arr.join ''
infinite.zip(naturalsthreesfives).map(concat).take(20)

Custom streams

The basic generators (range, cycle) are not always enough.... but chances are you can express your stream as a head and a tail.

  • The head is a function that returns a value
  • The tail is a function that returns a stream

For example the following sequence outputs every second item it's given, lazily:

skipper = (s) -> infinite.stream (-> s.first())(-> skipper s.tail().skip(1))
 
skipper(infinite.fibonacci()).take 5
# [ 0, 1, 3, 8, 21 ] 

This can be used to implement interesting algorithms. For example, here's the implementation of infinite.primes():

primes = ->
    sieve = (s) ->
        = s.head()
        infinite.stream (-> h)(-> sieve s.tail().filter (x) -> x % h isnt 0)
    sieve(infinite.range 2)
 
infinite.primes().take 5
# [ 2, 3, 5, 7, 11 ] 
 
infinite.primes().filter( (i) -> i > 1000 ).take 5
# [ 1009, 1013, 1019, 1021, 1031 ] 

Curious about the code?

Go to the folder you just cloned:

npm install
npm test