MOREINFO_RANDSTRING.lua

-- Implementations
local impl = {}

impl[1] = function(size)
  local c,r = string.char,math.random
  local str = {}
  for i = 1,size do
    str[i] = c(r(0,255))
  end
  return table.concat(str)
end

local MAX_UNPACK_SIZE = 50000
impl[2] = function(size)
  local c,r,u,m = string.char,math.random,(table.unpack or unpack),math.min
  local str,temp,n = {},{},1
  while size > 0 do
    local L = m(size,MAX_UNPACK_SIZE)
    for i = 1,L do
      temp[i] = r(0,255)
    end
    str[n] = c(u(temp,1,L))
    n = n + 1
    size = size - L
  end
  return table.concat(str)
end

local MAX_UNPACK_SIZE = 500000
impl[3] = function(size)
  local c,r,u,m = string.char,math.random,(table.unpack or unpack),math.min
  local str,temp,n = {},{},1
  while size > 0 do
    local L = m(size,MAX_UNPACK_SIZE)
    for i = 1,L do
      temp[i] = r(0,255)
    end
    str[n] = c(u(temp,1,L))
    n = n + 1
    size = size - L
  end
  return table.concat(str)
end

impl[4] = function(size)
  local c,r = string.char,math.random
  return ('.'):rep(size):gsub('.',function()
    return c(r(0,255))
  end)
end


-- Profile
local suf = {'KiB', 'MiB', 'GiB', 'TiB'}
function fmtsize(amt)
  if amt == 0 or amt ~= amt then return 0 end
  local m = math.min(math.floor((math.log(math.abs(amt)) / math.log(10)) / 3),#suf)
  local n = amt / 1024 ^ m
  return ('%.2f %s'):format(n,(suf[m] or 'B'))
end

collectgarbage('stop')

local tests = {10^3, 10^4, 10^5, 10^6, 10^7, 10^8, 10^9}
for i,size in ipairs(tests) do
  
  for i,func in ipairs(impl) do
    
    collectgarbage('collect')
    collectgarbage('collect')
    local t = os.clock()
    func(size)
    t = os.clock() - t
    print(('method %d (size %d): %.4f (usage %s)'):format(
      i, size, t, fmtsize(collectgarbage('count')*1024)
    ))
    
  end
  
end


--[[ RESULTS on i5-2520M w/ 6 GiB 1333 MHz RAM on Windows 7 SP1 64-bit, via
     64-bit Lua 5.2.3 from http://joedf.users.sourceforge.net/luabuilds/:
  
  D:\Desktop
  λ lua test.lua
  method 1 (size 1000): 0.0000 (usage 56.21 KiB)
  method 2 (size 1000): 0.0010 (usage 80.90 KiB)
  method 3 (size 1000): 0.0000 (usage 80.90 KiB)
  method 4 (size 1000): 0.0010 (usage 42.52 KiB)
  method 1 (size 10000): 0.0030 (usage 335.26 KiB)
  method 2 (size 10000): 0.0010 (usage 637.26 KiB)
  method 3 (size 10000): 0.0020 (usage 637.26 KiB)
  method 4 (size 10000): 0.0030 (usage 99.07 KiB)
  method 1 (size 100000): 0.0260 (usage 2.38 MiB)
  method 2 (size 100000): 0.0160 (usage 2.98 MiB)
  method 3 (size 100000): 0.0170 (usage 5.46 MiB)
  method 4 (size 100000): 0.0290 (usage 586.86 KiB)
  method 1 (size 1000000): 0.2660 (usage 18.99 MiB)
  method 2 (size 1000000): 0.1270 (usage 8.42 MiB)
  method 3 (size 1000000): 0.1540 (usage 27.58 MiB)
  method 4 (size 1000000): 0.2780 (usage 4.90 MiB)
  method 1 (size 10000000): 2.6880 (usage 297.57 MiB)
  method 2 (size 10000000): 1.2800 (usage 55.55 MiB)
  method 3 (size 10000000): 1.4300 (usage 81.94 MiB)
  method 4 (size 10000000): 2.7770 (usage 60.65 MiB)
  method 1 (size 100000000): 26.5250 (usage 2.34 GiB)
  method 2 (size 100000000): 13.2950 (usage 484.08 MiB)
  method 3 (size 100000000): 13.9930 (usage 553.07 MiB)
  method 4 (size 100000000): 27.9750 (usage 542.14 MiB)
  lua: not enough memory

NOTES: The test was also repeated the the garbage collector turned on, and the
time was (surprisingly) the same across the board.

--]]