eval.lua

--(C) 2017 Taigaclaw some rights reserved (licenced WTFPL).

function table.slice(tbl, first, last, step)
    local sliced = {}
    for i = first or 1, last or #tbl, step or 1 do
        sliced[#sliced+1] = tbl[i]
    end
    return sliced
end

function split(s)
    s = s:gsub('%s+', '')
    local parts = {}
    local i = 1
    while i <= #s do
        local char = s:sub(i, i)
        if tonumber(char) or char == '.' then
            if tonumber(parts[#parts]) and parts[#parts] ~= '-' then
                parts[#parts] = parts[#parts]..char
            else
                table.insert(parts, char)
            end
        else
            table.insert(parts, char)
        end
        i = i + 1
    end
    return parts
end

function evaluate(e)
    if type(e) ~= 'table' then
        e = split(e)
    end
    local total = 0
    local scan = true
    local i = 0
    while i <= #e do
        local char = e[i]
        if scan == true then
            if char == '(' then
                local scope = 0
                local starter = i
                local ender = 0
                local tape = i
                while true do
                    if     e[tape] == '(' then
                        scope = scope + 1
                    elseif e[tape] == ')' then
                        scope = scope - 1
                        if scope == 0 then
                            ender = tape
                            break
                        end
                    end
                    tape = tape + 1
                end
                local ans = evaluate(table.slice(e, starter + 1, ender - 1, 1))
                local first = table.slice(e, 1, starter - 1, 1)
                local last = table.slice(e, ender + 1, #e, 1)
                e = {}
                for _, i in ipairs(first) do
                    table.insert(e, i)
                end
                table.insert(e, tostring(ans))
                for _, i in ipairs(last) do
                    table.insert(e, i)
                end
            end
            i = i + 1
            if i >= #e then
                i = 1
                scan = false
            end
        else
            if char == '+' then
                total = total + e[i + 1]
                i = i + 1
            elseif char == '-' then
                total = total - e[i + 1]
                i = i + 1
            elseif char == '/' then
                total = total / e[i + 1]
                i = i + 1
            elseif char == '*' then
                total = total * e[i + 1]
                i = i + 1
            elseif char == '%' then
                total = total % e[i + 1]
                i = i + 1
            else
                total = e[i]
            end
            i = i + 1
        end
    end
    return total
end

print(evaluate('1 + 1'))