require "queue"
local fmt = require "fmt"

local function rpn_calculate(expr)
    assert(#expr > 0, "expression cannot be empty")
    print($"For expression = {expr}\n")
    print("Token           Action             Stack")
    local tokens = expr:split(" "):filter(|t| -> #t > 0):reorder()
    local stk = new stack()
    for tokens as token do
        local d = tonumber(token)
        if d then
            stk:push(d)
            fmt.print(" %d     Push num onto top of stack  %s", d, fmt.swrite(stk:toarray()))
        elseif #token > 1 or !(token in "+-*/^") then
            error($"{token} is not a valid token.")
        elseif stk:size() < 2 then
            error("Stack contains too few operands.")
        else
            local d1 = stk:pop()
            local d2 = stk:pop()
            stk:push(token == "+" ? d2 + d1 :
                     token == "-" ? d2 - d1 :
                     token == "*" ? d2 * d1 :
                     token == "/" ? d2 / d1 : d2 ^ d1)
            fmt.print(" %s     Apply op to top of stack    %s", token, fmt.swrite(stk:toarray()))
        end
    end
    fmt.print("\nThe final value is %0.14g", stk:pop())
end

local expr = "3 4 2 * 1 5 - 2 3 ^ ^ / +"
rpn_calculate(expr)