View difference between Paste ID: EghuB2BV and jniGqHyp

 -- Demonstrate the difference of time cost iterating over ffi array of double versus native array both with luajit
 
 require "os"
 local ffi = require("ffi")
 local num_ele = 900000
 local num_pass = 50000
 function elap(beg_time, msg)
   local end_time = os.clock()
   print ("elap " .. msg ..  " " .. end_time - beg_time .. "sec")
   return os.clock()
 end
 
 local begt = os.clock()
 local arr = ffi.new("double[?]", num_ele + 1)
 begt = elap(begt, "allocate ffi")
 
 
 -- fill the array for the example, here you would fill it from a file
 local begt = os.clock()
 for i = 0, num_ele do
     arr[i] = i
 end
 
 begt = elap(begt, "fill ffi array")
 
 local sum_start = ffi.cast("double", 0)
-  local sum = 0
+
   local sum = sum_start
   for i = 0, num_ele do
         sum = sum + arr[i]
   end
 end
 begt = elap(begt, "sum the ffi arrray")
 
 
 for pass = 0, num_pass do
   local sum1 = 0
   for i =  num_ele,0,-1 do
         sum1 = sum1 + arr[i]
   end
 end
 begt = elap(begt, "sum the ffi arrray in reverse")
 
 
  --------------
  --- Test with Native Lua Table
  --------------
  tt = {}
 local begt = os.clock()
 for i = 0, num_ele do
     tt[i] = i
 end
 begt = elap(begt, "fill native table ")
 
 
 
 for pass = 0, num_pass do
   local sum2 = 0
   for i = 0, num_ele do
         sum2 = sum2 + tt[i]
   end
 end
 begt = elap(begt, "sum the lua arrray")
 
 
 
 for pass = 0, num_pass do
   local sum3 = 0
   for i = num_ele,0,-1  do
         sum3 = sum3 + tt[i]
   end
 end
 begt = elap(begt, "sum the lua arrray in reverse")
 
 assert(sum == sum1, "warning sum1 ~= sum")
 assert(sum == sum2,  "warning the two sum2 ~= sum")
 assert(sum == sum3,  "warning the two sum3 ~= sum")