Update countingsort.lua

Author: icemberg

src/sorting/countingsort.lua

@@ -1 +1,58 @@
+function counting_sort(arr, key_function)
+     -- If no key_function is provided, use identity function
+     key_function = key_function or function(x) return x end
 
+     -- Step 1: Find the range of keys (min_key and max_key)
+     local min_key, max_key = math.huge, -math.huge
+     for i = 1, #arr do
+         local key = key_function(arr[i])
+         if key < min_key then
+             min_key = key
+         end
+         if key > max_key then
+             max_key = key
+         end
+     end
+
+     -- Step 2: Initialize the count array
+     local count = {}
+     for i = min_key, max_key do
+         count[i] = 0
+     end
+
+      -- Step 3: Count the occurrences of each key. In this case key is same as arr[i]
+     for i = 1, #arr do
+         local key = key_function(arr[i])
+         count[key] = count[key] + 1
+     end
+
+     -- Step 4: Compute cumulative counts to get final positions
+     for i = min_key + 1, max_key do
+         count[i] = count[i] + count[i - 1]
+     end
+
+     -- Step 5: Build the output array (in stable order)
+     local output = {}
+     for i = #arr, 1, -1 do
+         local element = arr[i]
+         local key = key_function(element)
+         output[count[key]] = element
+         count[key] = count[key] - 1
+     end
+
+     -- Step 6: Copy the output array back to the original array
+     for i = 1, #arr do
+         arr[i] = output[i]
+     end
+ end
+
+-- Sample array
+local arr = {100, 2, 2, 8, 3, 10000000, 1}
+
+-- Simple usage with identity key function
+counting_sort(arr)
+
+-- Print sorted array
+for i, v in ipairs(arr) do
+    print(v)
+end