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#!/usr/bin/ruby
#
# See ../README.md
#
#
# Run as: ruby ch-2.rb < input-file
#
#
# See https://stackoverflow.com/questions/2049582/
#
#
# This determines on which side of the line through (x1, y1) and
# (x2, y2) the origin lies. If > 0, then the origin lies to the left
# of the line, if < 0, the origin lies to the right of the line, if
# = 0, the origin lies on the line.
#
def side (x1, y1, x2, y2)
return (y2 - y1) * x2 - (x2 - x1) * y2
end
ARGF . each_line do |_|
#
# Parse input. We need an explicit conversion from string to float
#
x1, y1, x2, y2, x3, y3 = _ . split . map {|_| _ . to_f}
#
# Determine where the origin is relative to the three lines
# through the vertices of the triangle. Note we have to go
# in a specific order through the points. (Either clock wise,
# or counter clockwise, as long as we're consistent).
#
s1 = side x2, y2, x3, y3
s2 = side x3, y3, x1, y1
s3 = side x1, y1, x2, y2
#
# If the origin either lies to the left (or on) each of the
# lines, or to the right (or on) each of the lines, the origin
# lies inside the triangle. If not, it does not.
#
puts s1 <= 0 && s2 <= 0 && s3 <= 0 ||
s1 >= 0 && s2 >= 0 && s3 >= 0 ? 1 : 0
end
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