Alphabet shifter, for encoded physical letters.

-- i am make alphabet shifter yes? for encoded messages.
-- Ill try to add captilization as well..

local input = [[Pbzchgre fpvrapr vf n ernyyl, ernyyl pbby fhowrpg.  Nf n pbqre, lbh yvggrenyyl unir gur cbjre gb perngr lbhe bja jbeyqf, fvzhyngr lbhe bja raivebzragf, naq unir sha qbvat vg nf jryy!]]
local shift = -13
local time1 = os.clock()

function letter_value(x)
    local alphabet = "abcdefghijklmnopqrstuvwxyz"
    local d = x:lower()
    local value = 0
    for i=1,#alphabet do
        if d == string.sub(alphabet,i,i) then
            value = i
            break
        end
    end
    if value ~= 0 then
        return value
    else
        return x
    end
end

function value_letter(x)
    local alphabet = "abcdefghijklmnopqrstuvwxyz"
    local value = string.sub(alphabet,x,x)
    return value
end

-- if shift is greater than 26 or lower than -26 then
if shift > 26 then
    shift = shift%26
else
    shift = (shift%26)*-1
end

local g = ""
for i=1,#input do
    local x = letter_value(string.sub(input,i,i))
    if x ~= string.sub(input,i,i) then
        local d
        x = x + shift
        if x > 26 then
            x = x - 26
        elseif x < 1 then
            x = x + 26
        end
        if string.sub(input,i,i) == string.sub(input,i,i):upper() then
            d = value_letter(x):upper()
        else
            d = value_letter(x)
        end
        g = g..d
    else
        g = g..string.sub(input,i,i)
    end
end

print(g.."\n\nTime taken: "..string.format("elapsed time: %.2f\n", os.clock() - time1))