find specific Byte in File and read until specific byte in Lua - file

Question

Is it possible to search at first inside the file after an specific byte and find the position and read just the bytes from the file in until that specific byte?
At the moment it is just possible for me to read some bytes or the whole file in and afterwards search for that specific byte.
like this:
local function read_file(path)
local file = open(path, "r") -- r read mode and b binary mode
if not file then return nil end
local content = file:read(64) -- reading 64 bytes
file:close()
return content
end
local fileContent = read_file("../test/l_0.dat");
print(fileContent)
function parse(line)
if line then
len = 1
a = line:find("V", len +1) --find V in content
return a
else
return false
end
end
a = parse(fileContent) --position of V in content
print(a)
print(string.sub(fileContent, a)) -- content until first found V
In this example i find at position 21 the first V. So it would be cool to read in only 21 bytes except of 64 bytes or the whole file. But then i need to find the position before reading something in. Is this possible ? (The 21byte are variable, it could be 20 or 50 or so on)

Answer 1

You can specify a file position using file:seek and read a certain number of characters (bytes) by providing an integer to file:read
local file = file:open(somePath)
if file then
-- set cursor to -5 bytes from the file's end
file:seek("end", -5)
-- read 3 bytes
print(file:read(3))
file:close()
end
You cannot search in a file without reading it. If you don't want to read the entire file you can read it in chunks either by reading it linewise (if there are lines in your file) or by reading a specific number of bytes each time until you find something.
Of course you can also read it byte-wise.
You can argue if it makes more sense to read a 64 byte file as a whole or in chunks. I mean in most scenarios you won't notice any difference.
So you could file:read(1) in a loop that terminates once you found a V or reach the end of the file.
local file = io.open(somePath)
if file then
local data = ""
for i = 1, 64 do
local b = file:read(1)
if not b then print("no V in file") data = nil break end
data = data .. b
if b == "V" then print(data) break end
end
file:close()
end
vs
local file = io.open("d:/test.txt", "r")
if file then
local data = file:read("a")
local pos = data:find("V")
if pos then
print(data:sub(1, pos))
end
file:close()
end

Answer 2

(Or) Correct your code to...
local function read_file(path)
local file = io.open(path, "r") -- r read mode and b binary mode
if not file then return nil end
local content = file:read(64) -- reading 64 bytes
file:close()
return content
end
local fileContent = read_file("test/l_0.dat") -- '../' causing error
print(fileContent)
local function parse(line)
if line then
local len = 1
local a = line:find("V", len +1) --find V in content
return a
else
return false
end
end
print(fileContent:sub(1, parse(fileContent))) -- content until first found V
That puts out...
0123456789VabcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ
0123456789V
If you want that V is a (single) delimiter you probably dont want to put it out.
Meet the strength of string.sub(text, start, stop)...
print(fileContent:sub(1, parse(fileContent) - 1)) -- before V
-- 0123456789
print(fileContent:sub(parse(fileContent) + 1, -1)) -- after V
-- abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ