zeitwerk: move our Array extension to initializers/core_ext

lib/core_ext/array.rb

@@ -0,0 +1,31 @@
+# Adapted from https://github.com/thirtysixthspan/descriptive_statistics
+
+# Copyright (c) 2010-2014 Derrick Parkhurst (derrick.parkhurst@gmail.com)
+#
+# Permission is hereby granted, free of charge, to any person obtaining a copy
+# of this software and associated documentation files (the "Software"), to deal
+# in the Software without restriction, including without limitation the rights
+# to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+# copies of the Software, and to permit persons to whom the Software is
+# furnished to do so, subject to the following conditions:
+#
+# The above copyright notice and this permission notice shall be included in
+# all copies or substantial portions of the Software.
+
+class Array
+  def percentile(p)
+    values = self.sort
+
+    if values.empty?
+      return []
+    elsif values.size == 1
+      return values.first
+    elsif p == 100
+      return values.last
+    end
+
+    rank = p / 100.0 * (values.size - 1)
+    lower, upper = values[rank.floor, 2]
+    lower + (upper - lower) * (rank - rank.floor)
+  end
+end