Separate number by commas, format phone

Separate number by comma with using `gsub`:

def separate_by_comma(number)
  number.to_s.reverse.gsub(/(\d{3})/, '\1,').reverse
end

without:

def spearate_by_comma(number)
  number.to_s.chars.reverse.each_slice(3).map(&:join).join(",").reverse
end

Rails: STI (Single Table Inheritance)

Rust: macros notation

Rust: some notation about Trait methods

has_many :grandchildren trick

class Person < ActiveRecord::Base
  belongs_to :parent, class: Person
  has_many :children, class: Person, foreign_key: :parent_id
  has_many :grandchildren, class: Person, through: :children, source: :children
end

Little bit ruby

% Notation

%q[ ] # Non-interpolated String (except for \\ \[ and \])
%Q[ ] # Interpolated String (default)
%r[ ] # Interpolated Regexp (flags can appear after the closing delimiter)
%i[ ] # Non-interpolated Array of symbols, separated by whitespace
%I[ ] # Interpolated Array of symbols, separated by whitespace
%w[ ] # Non-interpolated Array of words, separated by whitespace
%W[ ] # Interpolated Array of words, separated by whitespace
%x[ ] # Interpolated shell command

Little bit about Graphs

simple - no multiarcs and loop

non-simpl(pseudog) - multiarcs and loop

---

complete - each node conect beetwen each other directly

conected - directly or nondirectli all nodes are conected

tree - conected graph without circle

spanning tree - tree with (nodes-1 == arcs), min arcs for connect all nodes

isomorphick - graph when (arcs == nodes)

sub - part of graph