fib_loop.rb

LT = -1
EQ = 0
GT = 1

def fib n
    case n <=> 1
    when LT then
        0
    when EQ then
        1
    when GT then
        fib(n-1) + fib(n-2)
    end
end

def fib_loop n
    stk = []
    stk.push [true, n]
    n = nil

    a = nil

    loop {
        flg, a = stk.pop
        if flg then
            case a <=> 1
            when LT then
                stk.push [false, 0]
            when EQ then
                stk.push [false, 1]
            when GT then
                stk.push [true, a-2]
                stk.push [true, a-1]
            end
        else
            break if stk.empty?
            flg2, a2 = stk.pop
            if flg2 then
                stk.push [false, a]
                stk.push [true, a2]
            else
                stk.push [false, a+a2]
            end
        end
    }

    return a
end

(0..10).each {|n|
    p fib n
}
puts "===="
(0..10).each {|n|
    p fib_loop n
}