Challenge 1: Correct Hash Keys — Solution

class Point
  attr_reader :x, :y

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

  def ==(other)
    other.is_a?(Point) && x == other.x && y == other.y
  end
  alias eql? ==

  def hash
    [x, y].hash
  end
end

lookup = { Point.new(1, 2) => "Origin-ish" }

puts lookup[Point.new(1, 2)]

=begin
Output:
Origin-ish

Notes:
- == compares two Point instances by value (matching x and y), not by
  object identity.
- alias eql? == makes eql? behave identically to == for this class, so
  Ruby's Hash lookup machinery (which uses eql?, not ==) agrees with the
  value-equality logic already written.
- hash delegates to [x, y].hash — Array already implements hash correctly
  for its elements, so two Points with the same x and y always produce
  the same hash value, satisfying the requirement that eql?-equal objects
  must have equal hash values.
- lookup[Point.new(1, 2)] succeeds even though it's a brand new Point
  instance, different in memory from the one used as the original key —
  it works purely because eql? and hash both agree the two are equal.
=end