added binary search to insert_sorted

Added method tablex.is_sorted

tablex.lua

@@ -48,12 +48,27 @@ function tablex.unshift(t, v)
 	return t
 end
 
+function tablex.is_sorted(t)
+    local sorted = true
+    for i = 1, #t - 1 do
+        if t[i] > t[i + 1] then
+            sorted = false
+            break
+        end
+    end
+    return sorted
+end
+
 --insert to the first position before the first larger element in the table
 --if this is used on an already sorted table, the table will remain sorted and not need re-sorting
 --todo: make it do binary search rather than linear to improve performance
 --return the table for possible chaining
 function tablex.insert_sorted(t, v, less)
 	local inserted = false
+
+	-- to use binary search is necessary as precondition that
+	-- the table is sorted
+	if not tablex.is_sorted(t) then
 			for i = 1, #t do
 				if less(v, t[i]) then
 					table.insert(t, i, v)
@@ -67,6 +82,34 @@ function tablex.insert_sorted(t, v, less)
 		return t
 	end
 
+	local l = 1
+	local r = #t
+	
+	if r < l then
+		table.insert(t,v)
+		return t
+	end
+
+
+	while l <= r do
+		local mid = math.floor(l + (r - l) / 2)
+		if (less(v, t[mid]) or v == t[mid]) and (mid == 1 or less(t[mid - 1], v) or t[mid - 1] == v) then
+			table.insert(t, mid, v)
+			inserted = true
+			break
+		elseif less(t[mid], v) then
+			l = mid + 1
+		elseif less(v, t[mid - 1]) then
+			r  = mid - 1
+		end
+	end
+
+	if not inserted then
+		table.insert(t, l, v)
+	end
+
+	return t
+end
 --find the index in a sequential table that a resides at
 --or nil if nothing was found
 function tablex.index_of(t, a)