Ruby solution for week 101, part 2

2 files changed, 49 insertions, 0 deletions

diff --git a/challenge-101/abigail/README.md b/challenge-101/abigail/README.md
index a6488bf50a..7707028ada 100644
--- a/challenge-101/abigail/README.md
+++ b/challenge-101/abigail/README.md
@@ -106,5 +106,6 @@ Output: 1 because (0,0) is on the edge connecting B and C.
 * [Node.js](node/ch-2.js)
 * [Perl](perl/ch-2.pl)
 * [Python](python/ch-2.py)
+* [Ruby](ruby/ch-2.rb)
 
 ### Blog
diff --git a/challenge-101/abigail/ruby/ch-2.rb b/challenge-101/abigail/ruby/ch-2.rb
new file mode 100644
index 0000000000..8c5a2d542f
--- /dev/null
+++ b/challenge-101/abigail/ruby/ch-2.rb
@@ -0,0 +1,48 @@
+#!/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