[algo] Optimize insertion sort.

algo.lua

@@ -34,6 +34,24 @@ end
 
 local function lt(a, b) return a < b end
 
+local function _insertionsort(array, lo, hi, less)
+    for i = lo+1,hi do
+        local k = lo
+        local v = array[i]
+
+        for j = i,lo+1,-1 do
+          local el = array[j-1]
+          if less(v, el) then
+              array[j] = el
+          else
+              k = j
+              break
+          end
+        end
+        array[k] = v
+    end
+end
+
 --- Sort array using Insertion Sort - O(n^2).
 --
 -- Provides the most basic sorting algorithm around.
@@ -46,19 +64,7 @@ local function lt(a, b) return a < b end
 --                                  less than its second argument,
 --                                  false otherwise.
 function algo.insertionsort(array, less)
-    less = less or lt
-
-    for i = 2,#array do
-        local val = array[i]
-        local j = i
-
-        while j > 1 and less(val, array[j-1]) do
-            array[j] = array[j-1]
-            j = j - 1
-        end
-
-        array[j] = val
-    end
+    _insertionsort(array, 1, #array, less or lt)
 end
 
 --- Binary search last element where