Re: "String builder"-style optimization
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- Subject: Re: "String builder"-style optimization
- From: Martin <eden_martin_fuhrspam@...>
- Date: Fri, 7 Jul 2017 22:35:47 -0700
On 07/03/2017 12:16 PM, J Doe wrote:
> Hi,
>
> In chapter 2 of "Lua Gems", Roberto mentions the fact that building a string in a loop by concatenating more string data to it on each iteration is wasteful as a new string must be allocated, the existing elements copied, the old string GC'ed (at some point) and the addition of the new portion of the string.
>
> The recommendation for optimization is to emulate "string builder" functionality by storing each piece as an element in a table and then calling table.concat(t).
>
> I am wondering if this is also a good pattern for building a regular string that has multiple concatenations . . . but say on the order of 5 to 10 concatenation operations:
>
> t = {"Banned UA:", bad_ua, "reason: ", reason_ua, "stats", clck_elapsed}
> s = table.concat(t, " ")
For your case you'd better to use string:format().
For my measurements, lame chunks concatenation in loop is faster than
insertion them in table up to ~60 elements.
But "faster" not always mean "better". As mentioned, it wastes memory
by storing substrings.
So
msg = 'id: ' .. id .. ', name: ' .. name
is faster (and does not store substrings) than
msg = ('id: %s, name: %s'):format(id, name)
But anyway I mostly prefer second variant.
--
Below is the code I've created for measurements.
<els> is chunks we add.
<test_chunks> is alternative functions to concat <els>.
<test_period_secs> is time to run for each <test_chunks>[i] function.
--
local els =
{
'0', '1', '2', '3', '4', '5', '6', '7', '8', '9', 'A', 'B', 'C',
'D', 'E', 'F',
'0', '1', '2', '3', '4', '5', '6', '7', '8', '9', 'A', 'B', 'C',
'D', 'E', 'F',
'0', '1', '2', '3', '4', '5', '6', '7', '8', '9', 'A', 'B', 'C',
'D', 'E', 'F',
'0', '1', '2', '3', '4', '5', '6', '7', '8', '9', 'A', 'B', 'C',
'D', 'E', 'F',
}
local test_chunks =
{
[1] =
function()
local s = ''
for i = 1, #els do
s = s .. els[i]
end
end,
[2] =
function()
local t = {}
for i = 1, #els do
t[#t + 1] = els[i]
end
local s = table.concat(t)
end,
[3] =
function()
local fmt_str = ('%s'):rep(#els)
local s = fmt_str:format(table.unpack(els))
end,
}
local test_period_secs = 1.0
local start_time
local tic =
function()
start_time = os.clock()
end
local tac =
function()
return os.clock() - start_time
end
local counters = {}
for i = 1, #test_chunks do
tic()
local cnt = 0
local f = test_chunks[i]
while (tac() < test_period_secs) do
f()
cnt = cnt + 1
end
counters[i] = cnt
end
for i = 1, #counters do
print(('%d: %.2f ops/sec'):format(i, counters[i] / test_period_secs))
end
-- Martin