changeline.lua

--[[
    local str = "Hi"
    print(ChangeLine(str, "OwO", 1)) -- output: OwO
    print(ChangeLine(str, " Srlion!", 1, true)) -- output: Hi Srlion!
    print(ChangeLine(str, "Srlion ", 1, false)) -- output: Srlion Hi
]]

-- Localize functions so accessing them becomes more faster
local sub = string.sub
local rep = string.rep
local find = string.find
local function ChangeLine(str, new_str, line, pos)
    local len, st, en
    len, st, en = str.length, 0, 0
    for i = 1, line do
        -- try to find next new line
        en = find(str, "\n", en + 1, true)
        if (!en) then
            if (line == i) then
                st = len - 1
            else
                -- if we didn't reach the line we need to change then we have to add new lines till we reach the one we need to change
                str = str .. (rep("\n", line - i))
                st = len + line - i
            end
            en = st + 2
            break
        end
        if (i != line) then
            st = en
        end
    end
    
    -- if pos is true then we will add the new string to the old one
    if (pos == true) then
        new_str = sub(str, st + 1, en - 1) .. new_str
    -- if pos is false then we will add the new string before the old one
    elseif (pos == false) then
        new_str = new_str .. (sub(str, st + 1, en - 1))
    end
    return sub(str, 1, st) .. new_str .. sub(str, en, len)
end