Downloads a file from a given url and converts it into a data url

url_to_base64_data_url.lua

function base64Encode(data)
    local function encodeChar(n)
        if n <= 25 then
            return string.char(65 + n)
        end
        if n <= 51 then
            return string.char(97 + n - 26)
        end
        if n <= 61 then
            return string.char(48 + n - 52)
        end
        if n == 62 then
            return "+"
        end
        return "/"
    end
    if type(data) == "string" then
        local newData = {}
        for i=1,string.len(data) do
            table.insert(newData, data:sub(i, i))
        end
        data = newData
    end
    local res = ""
    local pos = 1
    while pos <= #data do
        local c1 = string.byte(data[pos])
        local c2 = 0
        if pos + 1 <= #data then
            c2 = string.byte(data[pos + 1])
        end
        local c3 = 0
        if pos + 2 <= #data then
            c3 = string.byte(data[pos + 2])
        end
        res = res .. encodeChar(c1 >> 2) .. encodeChar(((c1 & 3) << 4) | (c2 >> 4))
        if pos + 1 <= #data then
            res = res .. encodeChar(((c2 & 15) << 2) | (c3 >> 6))
            if pos + 2 <= #data then
                res = res .. encodeChar(c3 & 63)
            else
                res = res .. "="
            end
        else
            res = res .. "=="
        end
        pos = pos + 3
    end
    return res
end

function downloadFile(url)
    local tmpFile = "tmp-" .. tostring(math.random(1000,9999))
    local h = io.popen("curl -is --output " .. tmpFile .. " " .. url)
    h:close()
    h = io.open(tmpFile, "rb")
    local t = ""
    local l = h:read("*l")
    while l ~= "\r" do
        if l:sub(1, 13):lower() == "content-type:" then
            t = l:sub(15, l:len() - 1)
        end
        l = h:read("*l")
    end
    local data = h:read("*a")
    h:close()
    os.remove(tmpFile)
    return {
        type = t,
        data = data
    }
end

function urlToBase64DataUrl(url)
    local file = downloadFile(url)
    return "data:" .. file.type .. ";base64," .. base64Encode(file.data)
end