boomerang_constellations.rb

#!usr/bin/ruby

# The night sky can be modeled as an infinite 2D plane. There are N stars at distinct positions on this plane, the ith of which is at coordinates (Xi, Yi).

# A boomerang constellation is a pair of distinct equal-length line segments which share a single endpoint, such that both endpoints of each segment coincide with a star's location.

# Two boomerang constellations are distinct if they're not made up of the same unordered pair of line segments. How many distinct boomerang constellations can you spot?

# Input Format

# Input begins with an integer N. Then, N lines follow, the ith of which contains the space-separated integers Xi and Yi.

# Constraints

# 1 ≤ N ≤ 2,000 
# -10,000 ≤ Xi, Yi ≤ 10,000 

# Output Format

# print the number of boomerang constellations in the night sky.

# Example 1:

# Input: 
# 3 
# 0 0 
# 0 1 
# 0 3 

# Expected Output: 
# 0 

# Explanation: Every pair of stars is a unique distance apart, so there are no boomerang constellations.

# Example 2:

# Input: 5 
# 0 0 
# 0 1 
# 0 2 
# 0 3 
# 0 4 

# Expected Output: 4 

# Explanation: There are 4 boomerang constellations. One of them consists of the line segments (0,0)-(0,2) and (0,2)-(0,4).

# Sample Input 0

# 3
# 0 0
# 0 1
# 0 3
# Sample Output 0

# 0
# Sample Input 1

# 5
# 0 0
# 0 1
# 0 2
# 0 3
# 0 4
# Sample Output 1

# 4
# Sample Input 2

# 4
# 0 0
# 0 100
# 100 0
# 100 100
# Sample Output 2

# 4
# Sample Input 3

# 4
# 0 0
# -3 4
# 0 5
# -5 0
# Sample Output 3

# 3
# Sample Input 4

# 6
# 5 6
# 6 5
# 7 6
# 6 7
# 7 8
# 8 7
# Sample Output 4

# 12

class Boomerang
  attr_accessor :num_stars, :stars

  def initialize
    @stars = []
    @num_stars = gets.chop.to_i
    for i in 1..@num_stars
      coordinates = clean_params(gets, ' ')
      @stars << Star.new(coordinates[0].to_i, coordinates[1].to_i)
    end
  end

  def clean_params(str, separator = '')
    str.chop.split(separator)
  end

  def dist2(i, j)
    dx2 = (@stars[i].x - @stars[j].x) * (@stars[i].x - @stars[j].x)
    dy2 = (@stars[i].y - @stars[j].y) * (@stars[i].y - @stars[j].y)
    dx2 + dy2
  end

  def num_constellations
    res = 0
    for i in 0..@num_stars-1 do
      distances = {}
      for j in 0..@num_stars-1 do
        if(i!=j)
          d = dist2(i,j)
          distances[d] = 0 unless distances.has_key?(d)
          res += distances[d]
          distances[d]+=1
        end
      end
    end
    res
  end
end

class Star
  attr_accessor :x, :y

  def initialize(x, y)
    @x = x
    @y = y
  end
end

b = Boomerang.new
puts b.num_constellations