Added countingsort.lua

.spec/sorting/sort_spec.lua

@@ -75,3 +75,7 @@ end)
 describe("Bogosort", function()
 	check_sort(require("sorting.bogosort"), 5)
 end)
+describe("Countingsort", function()
+	local countingsort = require("sorting.countingsort")
+	check_sort(require(countingsort()), nil, true)
+end)

src/sorting/countingsort.lua

@@ -0,0 +1,47 @@
+return function(
+    -- list to be sorted in-place
+    list,
+    -- key_function to map elements to integer keys, defaults to identity
+    key_function
+)
+    if list[1] == nil then return end
+
+    key_function = key_function or function(x) return x end
+
+    -- Find the range of keys (min_key and max_key)
+    local min_key, max_key = math.huge, -math.huge
+    for _, elem in ipairs(list) do
+        local key = key_function(elem)
+        min_key = math.min(min_key, key)
+        max_key = math.max(max_key, key)
+    end
+
+    local count = {}
+    for i = 1, max_key - min_key + 1 do
+        count[i] = 0
+    end
+
+    -- Count then occurrences of each key
+    for _, elem in ipairs(list) do
+        local key = key_function(elem)
+        count[key - min_key + 1] = count[key - min_key + 1] + 1
+    end
+
+    -- Compute cumulative counts for final positions
+    for i = 2, ipairs(count) do
+        count[i] = count[i] + count[i - 1]
+    end
+
+    local output = {}
+    for i = #list, 1, -1 do
+        local element = list[i]
+        local key = key_function(element)
+        output[count[key - min_key + 1]] = element
+        count[key - min_key + 1] = count[key - min_key + 1] - 1
+    end
+
+    for i, elem in ipairs(output) do
+        list[i] = elem
+    end
+end
+