""" a queue class implemented using Python list 
"""

class QueueUnderflow(ValueError):
    pass

class SQueue(): 
    def __init__(self, init_len = 8):
        self.len = init_len # length of mem-block
        self.elems = [0] * init_len
        self.head = 0  # index of head element
        self.elnum = 0 # number of elements
        
    def is_empty(self):
        return self.elnum == 0
    
    def first(self):
        if self.elnum == 0:
            raise QueueUnderflow
        return self.elems[self.head]
    
    def dequeue(self):
        if self.elnum == 0:
            raise QueueUnderflow
        e = self.elems[self.head]
        self.head = (self.head+1) % self.len
        self.elnum -= 1
        return e
    
    def enqueue(self, elem):
        if self.elnum == self.len:
            self.__extend()
        self.elems[(self.head+self.elnum)%self.len] = elem
        self.elnum += 1
        
    def __extend(self):
        old_len = self.len
        self.len *= 2        
        new_elems = [0]*(self.len)
        for i in range(old_len):
            new_elems[i] = self.elems[(self.head+i)%old_len]
        self.elems, self.head = new_elems, 0
    
if __name__ == '__main__':
    q = SQueue()
    for i in range(10):
        q.enqueue(i*3)
    print(q.dequeue())
    q.enqueue(100)
    while not q.is_empty():
        print(q.dequeue())