HTTP parser for Lua - http

Question

I'm trying to parse http POST requests in Lua. My implementation works, but eats a hell lot of CPU load. This is critical, hence it is on an embedded platform.
I've looked on other implementations, but they can't fit, because my image is hardly fit in the memory, so I wouldn't use another library. I rolled my own parser, but it uses too much of system resource. Question is how could I optimize this to have lower CPU load.
This is an OpenWRT based system, so I only have Lua 5.1. This is the core function that looks for the boundary (in str variable). It reads the input block by block, and seeks for it.
The other solution would be to use LUCI libraries to do the heavy lifting, but I don't want my code to be integrated in LUCI.
--look for a pattern (str) and copy input until it is found to the output.
local function writeuntil(in_fp, str, out_fp)
local buff = ""
local ret = false
local bs = 4096 --Block size. The amount of data to read at once
local c = in_fp:read(bs)
local strStartPos = 1
while c do
local blockLen = string.len(c) --Not sure that a whole block is read, so get the size of the actual block.
local found = string.find(c, str, 1, true) --Try to locate str, so we don't have much work.
if (found ~= nil) then
if found > 2 then
out_fp:write(string.sub(c, 1, found - 1))
end
ret = true
break --we are done
else --Try to mach str till the end of the block
local strPos = string.find(c, string.sub(str, strStartPos, strStartPos), 1, true) --try to locate the first character
if strPos then --There is a starting character in the block
if (strPos > 1) then
out_fp:write(string.sub(c, 1, strPos - 1))
end
for i = strPos, blockLen do --iterate through the block
local ch = string.sub(c, i, i)
if ch == string.sub(str, strStartPos, strStartPos) then
buff = buff .. ch
if string.len(buff) == string.len(str) then
ret = true
break --We're done
end
strStartPos = strStartPos + 1
else --Lost track. Output.
if string.len(buff) > 0 then
out_fp:write(buff)
buff = ""
end
out_fp:write(ch)
strStartPos = 1
end
end
else
out_fp:write(c)
end
end
if ret then
break
end
c = in_fp:read(bs) --read next block
end
return ret
end

Answer 1

Egor, you were right, but I ended up this solution. It is now uses much less CPU. This not perfect hence scp is faster (although that is implemented in C).
--look for a pattern (str) and copy input until it is found to the output.
local function writeuntil(in_fp, str, out_fp)
local buff = ""
local ret = false
local bs = 4096 --Block size. The amount of data to read at once
local c = in_fp:read(bs)
local strStartPos = 1
local lastStrPos = 1
local needData = true
while c do
local blockLen = string.len(c) --Not sure that a whole block is read, so get the size of the actual block.
local found = string.find(c, str, 1, true) --Try to locate str, so we don't have much work.
if (found ~= nil) then
if found > 1 then
if #buff > 0 then
out_fp:write(buff)
end
out_fp:write(string.sub(c, 1, found - 1))
end
ret = true
break --we are done
else --Try to mach str till the end of the block
local strPos = string.find(c, string.sub(str, strStartPos, strStartPos), lastStrPos, true) --try to locate the first character
if strPos then --There is a starting character in the block
out_fp:write(string.sub(c, lastStrPos, strPos - 1))
for i = strPos, blockLen do --iterate through the block
local ch = string.sub(c, i, i)
if ch == string.sub(str, strStartPos, strStartPos) then
buff = buff .. ch
if string.len(buff) == string.len(str) then
ret = true
break --We're done
end
strStartPos = strStartPos + 1
lastStrPos = i + 1
else --Lost track. Output.
if string.len(buff) > 0 then
out_fp:write(buff)
buff = ""
end
out_fp:write(ch)
strStartPos = 1
if i == blockLen then
needData = true
else
lastStrPos = i + 1
needData = false
end
break
end
end
else
if ret == false then
if string.len(buff) > 0 then
out_fp:write(buff)
buff = ""
end
out_fp:write(string.sub(c, lastStrPos))
lastStrPos = 1
needData = true
else
break
end
end
end
if ret then
break
end
if needData then
c = in_fp:read(bs) --read next block
lastStrPos = 1
end
end
return ret
end