```Matthew Moss ha scritto:
> -=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
>
> The three rules of Ruby Quiz 2:
>
> 1.  Please do not post any solutions or spoiler discussion for this
> quiz until 48 hours have passed from the time on this message.
>
> 2.  Support Ruby Quiz 2 by submitting ideas as often as you can! (A
> permanent, new website is in the works for Ruby Quiz 2. Until then,
> please visit the temporary website at
>
>      <http://splatbang.com/rubyquiz/>.
> 3.  Enjoy!
>
> Suggestion:  A [QUIZ] in the subject of emails about the problem
> the original quiz message, if you can.
> -=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
>
> ## Circle Drawing (#166)
>
> This week we're going to keep it simple... very simple.
>
> Given a radius, draw an ASCII circle.
>
> For example:
>
>     ruby circle.rb 7
>
> Should produce a circle of radius 7:
>
>          #####
>        ##     ##
>       #         #
>      #           #
>      #           #
>     #             #
>     #             #
>     #             #
>     #             #
>     #             #
>      #           #
>      #           #
>       #         #
>        ##     ##
>          #####
>
>
> Note that most fonts do not have a square aspect ratio, which is why the
> above output may look like an oval, despite my calculations for a circle. It
> is acceptable if your code produces similar output.
>
>
> However, _for extra credit_ you may support an additional argument that
> specifies the aspect ratio (height divided by width).
>
>     ruby circle.rb 7 1.4
>
> This should draw a circle of radius 7 with aspect ratio of 1.4. If done
> correctly, your output will actually look like a circle (assuming 1.4 is an
> accurate measure of the actual aspect ratio).
>
>
>
>
Here my solution. It is available on pastie:

http://pastie.org/215379
http://pastie.org/215380 (specs)

and it is also attached below:

#
# Solution to Ruby Quiz #166 - Circle Drawing
#
# Usage:
#
# Circle.new(5).to_s
#
# or:
#
# Circle.new(5, 2).to_s
#
#
# or:
#
# Circle.new(5, 2, 'x').to_s
#

# Objects of class Circle draw circles on stdout. The aspect ratio
# correction is actually made drawing an ellipse with semimajor axis
# (a) equals to the given circle radius and semiminor axis (b) equals
# to a / aspect_ratio.
#
# Circle class is responsible to
#
#  * initialize a Circle object with the given radius, aspect ratio
#    and drawing char
#
#  * initialize a canvas
#
#  * draw the circle on its internal canvas
#
#  * convert the canvas to string for output on stdout
#
class Circle

# cx, cy are the coordinates of the circle's center.

# w, h are width and height of the canvas

# canvas is a linear array that is initially filled with spaces

#
# Initialize a Circle object passing a value for radius, aspect
# ratio and drawing character.
#
def initialize(radius, aspect_ratio = 1.0, char = '#')

@aspect_ratio = aspect_ratio.to_f
@char = char

fail "Error: aspect ratio must be > 0" if @aspect_ratio <= 0

# a is the semimajor axis of the ellipse and is equal to the given

# b is the semiminor axis of the ellipse and is calculated from a
# and the given aspect ratio
@b = (@a / @aspect_ratio).ceil

# calculate the size of the canvas
@w, @h = (@a + 1) * 2, (@b + 1) * 2

# center coordinates correspond to the size of semiaxis.
@cx, @cy = @a, @b

# initialize the canvas with spaces
@canvas = Array.new(@w * @h, ' ')

# draw ellipse on canvas
draw_ellipse(@a, @b)
end

#
# Print circle on stdout.
#
def to_s
result = ""
(0..@h - 1).each do |line|
result << @canvas[line * @w..line * @w + @w - 1].to_s << "\n"
end
result
end

private

#
# Draw the given character on canvas to the given coordinates.
#
def point(x, y)
@canvas[y * @w + x] = @char
end

#
# Translates and mirrors point (x, y) in the quadrants taking
# advantage of the simmetries in the ellipse. Thus, for a given
# point (x, y) the method plot three other points in the remaining
#
def plot_four_points(x, y)
point(@cx + x, @cy + y)
point(@cx - x, @cy + y)
point(@cx + x, @cy - y)
point(@cx - x, @cy - y)
end

#
# Draw an ellipse on canvas. This method implements a Bresenham
# based algorithm by John Kennedy
# (http://homepage.smc.edu/kennedy_john/BELIPSE.PDF)
#
# The method calculates two set of points in the first quadrant. The
# first set starts on the positive x axis and wraps in a
# counterclockwise direction until the tangent line slope is equal
# to -1.  The second set starts on the positive y axis and wraps in
# a clockwise direction until the tangent line slope is equal to -1.
#
def draw_ellipse(a, b)
a_square = 2 * a**2
b_square = 2 * b**2

draw_first_set(a, b, a_square, b_square)
draw_second_set(a, b, a_square, b_square)
end

#
# The method increments y and decides when to decrement x testing
# the sign of a function.  In this case, the decision function is
# (2*ellipse_error+x_change) and its value is calculated
# iteratively.
#
def draw_first_set(a, b, a_square, b_square)

x, y = a, 0
x_change, y_change = b**2 * (1 - 2 * a), a**2
stopping_x, stopping_y = b_square * a, 0
ellipse_error = 0

while(stopping_x >= stopping_y) do
plot_four_points(x, y)
y += 1
stopping_y += a_square
ellipse_error += y_change
y_change += a_square
if (2 * ellipse_error + x_change) > 0
x -= 1
stopping_x -= b_square
ellipse_error += x_change
x_change += b_square
end
end

end

#
# The method increments x and decides when to decrement y testing
# the sign of a function.  In this case, the decision function is
# (2*ellipse_error+y_change) and its value is calculated
# iteratively.
#
def draw_second_set(a, b, a_square, b_square)

x, y = 0, b
x_change, y_change = b**2, a**2 * (1 - 2 * b)
stopping_x, stopping_y = 0, a_square * b
ellipse_error = 0

while stopping_x <= stopping_y do
plot_four_points(x, y)
x += 1
stopping_x += b_square
ellipse_error += x_change
x_change += b_square
if (2 * ellipse_error + y_change) > 0
y -= 1
stopping_y -= a_square
ellipse_error += y_change
y_change += a_square
end
end

end

end

# Usage:
#
# ruby circle.rb 7 #=> print out a circle of radius 7
#
# ruby circle.rb 7 1.8 #=> print out a circle of radius 7 and aspect
ratio 1.8
#
# ruby circle.rb 7 1.8 x #=> print out a circle of radius 7 and aspect
ratio 1.8
#                            using the ascii char 'x'
#

print Circle.new(ARGV[0], ARGV[1] || 1.0, ARGV[2] || '#').to_s if \$0 ==
__FILE__

```