Add Counting Sort algorithm

algorithms/sorting/counting_sort.rb

@@ -0,0 +1,79 @@
+# A Ruby implementation of the Counting Sort algorithm
+
+require "pp"
+
+def counting_sort(unsorted_array)
+  # Display unsorted array
+  puts "Unsorted array"
+  pp unsorted_array
+  puts "\n"
+
+  range = unsorted_array.max + 1
+  count_array = Array.new(range, 0)
+  sorted_array = Array.new(unsorted_array.length, nil)
+
+  # Count objects and store in matching index
+  unsorted_array.each do |value|
+    count_array[value] = unsorted_array.count(value)
+  end
+
+  # Display count array
+  puts "Array elements counted and stored in index"
+  pp count_array
+  puts "\n"
+
+  # Update each array element to contain sum of previous elements
+  count_array.each_with_index do |value, index|
+    count_array[index] = count_array[index] + count_array[index - 1]
+  end
+
+  # Display count array
+  puts "Index previous elements summed"
+  pp count_array
+  puts "\n"
+
+  # Update count array to include actual position of element in sorted array
+  counter = 1
+  while counter < range
+    count_array[counter] = count_array[counter] + count_array[counter - 1]
+    counter = counter + 1
+  end
+
+  # Build sorted array
+  counter = unsorted_array.length - 1
+  while counter >= 0
+    sorted_array[count_array[unsorted_array[counter]] - 1] = unsorted_array[counter]
+    count_array[unsorted_array[counter]] = count_array[unsorted_array[counter]] - 1
+    counter = counter - 1
+  end
+
+  # Remove nil values from array
+  sorted_array.compact!
+
+  # Display sorted array
+  puts "Sorted array"
+  pp sorted_array
+  puts "\n"
+
+  # Return sorted array
+  sorted_array
+end
+
+# Testing the algorithm
+array = [7, 3, 2, 3, 1, 5, 3, 8, 7]
+counting_sort(array)
+
+# Output
+#
+# Unsorted array
+# [7, 3, 2, 3, 1, 5, 3, 8, 7]
+#
+# Array elements counted and stored in index
+# [0, 1, 1, 3, 0, 1, 0, 2, 1]
+#
+# Index previous elements summed
+# [1, 2, 3, 6, 6, 7, 7, 9, 10]
+#
+# Sorted array
+# [1, 2, 3, 3, 3, 5, 7, 7, 8]
+#