Add Convex Hull in Ruby

archive/r/ruby/convex-hull.rb

@@ -0,0 +1,53 @@
+# frozen_string_literal: true
+
+USAGE = 'Usage: please provide at least 3 x and y coordinates as separate lists (e.g. "100, 440, 210")'
+
+def usage!
+  warn USAGE
+  exit 1
+end
+
+def parse_list(str)
+  str.split(",").map { Integer(it.strip) }
+rescue ArgumentError, NoMethodError
+  usage!
+end
+
+def cross(o, a, b)
+  (a[0] - o[0]) * (b[1] - o[1]) -
+    (a[1] - o[1]) * (b[0] - o[0])
+end
+
+def build_hull(points)
+  hull = []
+
+  points.each do |p|
+    hull.pop while hull.size >= 2 && cross(hull[-2], hull[-1], p) <= 0
+    hull << p
+  end
+
+  hull
+end
+
+def convex_hull(points)
+  points = points.sort
+
+  lower = build_hull(points)
+  upper = build_hull(points.reverse)
+
+  lower + upper[1..-2]
+end
+
+x_str, y_str = ARGV
+usage! if x_str.nil? || y_str.nil?
+
+x = parse_list(x_str)
+y = parse_list(y_str)
+
+usage! if x.size != y.size || x.size < 3
+
+points = x.zip(y)
+
+convex_hull(points).each do |x, y|
+  puts "(#{x}, #{y})"
+end