rubyisms.rb

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Rubyisms
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To get nth bit of a number you can do a = 5; a[0] == 1; 5.downto(0).map { |n| a[n] }.join #"000101"

To average 2 positive integers: (x&y)+((x^y)>>1)
Sum 2 positive integers: (x+y) == (x&y)+(x|y) 

PriorityQueue API 'pqueue' : http://www.rubydoc.info/gems/pqueue/PQueue

Multithreaded example
require 'thread'
work_q = Queue.new
(0..50).to_a.each{|x| work_q.push x }
workers = (0...4).map do
  Thread.new do
    begin
      while x = work_q.pop(true)
        50.times{print [128000+x].pack "U*"}
      end
    rescue ThreadError
    end
  end
end; "ok"
workers.map(&:join); "ok"

Get ascii value of a character
'a'.ord #97
Opposite : 97.chr

All FixNums are default 8bytes ie. 64 bit ints e.g. 0.size # 8

For permutations you can do [1,2,3].permutations(2) # returns 6 possibilities
or if k>n for n permute k you can do [1,2,3].repeated_permutations(5) # returns 243
eval("+1-22") = -21

In ruby
a = [1,2]
b = [4,5,6]
ary = a.map.with_index{ |m,i| m + b[i].to_i }
=> [5, 7]
# IN ruby for set operations you have to "require 'set'"
# For md5 you have to "require 'digest/md5'; Digest::MD5.hexdigest(my_string)

# to initialize an M X N array - THIS IS BAD idea as arrays are going to point to the same instance, 
# fill works only with literals -DONT DO IT
1. [].fill([].fill(0, 0...N),0...M)
2. Preferred Method: (0...M).each {|m| arr[m] ||= []; (0...N).each {|n|  arr[m][n] = 0 }}

# http://stackoverflow.com/questions/7310015/ruby-array-internals
Stack: push/pop i.e. stack operations are O(1) in ruby
Queue: enq(push) is O(1) and deq(shift) is O(1)

Iterate over an array with set increments
(10..100).step(10) do |n|
    # n = 10
    # n = 20
    # n = 30
    # ...
end

String to bits
"a".to_i(2) # binary
"a".to_i(16) # hex

To get all possible combinations of [1.0] in array of n elements i.e. on and off each one
[0,1].repeated_permutation(10).to_a

# when you want to get index of elements that satisfy certain condition
arr.size.times.select {|i| arr[i] == 'x'} # => [0, 2, 6]

To get index of min/max element
ary = [2,3,4,5,1]        # => [2,3,4,5,1]
ary.each_with_index.min  # => [1, 4]

# To store objects as keys in hash you need to override the eql? method like this, then 
you can do fancy things such as keys within range lookups which will return the object
that has key in the range - see http://javieracero.com/blog/the-key-to-ruby-hashes-is-eql-hash
def eql?(key)
    obj_range.include? (key)
end

# For adding binary numbers
base = sum(a,b) % 2 + base
carry = sum(a,b)/ 2;