class RequestHistory(object):
    def __init__(self, size=10):
        self.size = size
        self.modulus = self.size + 1
        self.history = [''] * (self.modulus)
        self.base = 0
        self.top = 1

    def is_empty(self):
        return self.base == self.decrement(self.top)

    def peek(self):
        if self.is_empty():
            return None
        else:
            return self.history[self.decrement(self.top)]

    def push(self, request):
        self.history[self.top] = request
        if self.top == self.base:
            self.base = self.increment(self.base)
        self.top = self.increment(self.top)

    def pop(self):
        index = self.decrement(self.top)
        if self.is_empty():
            return None
        self.top = index
        return self.history[self.top]

    def export_history(self):
        h = []
        i = self.increment(self.base)
        while i != self.top:
            h.append(self.history[i])
            i = self.increment(i)
        h.append(self.history[i])
        return h

    def import_history(self, history):
        for request in history:
            self.push(request)

    def increment(self, number):
        return (number + 1) % self.modulus

    def decrement(self, number):
        return (number - 1) % self.modulus