pragma solidity ^0.4.18;

contract QueueExample {

    bytes32 front;
    bytes32 back;
    bool isEmpty;

    struct ElementData {
        // Some Data
        int num;
    }

    struct Element {
        ElementData data;
        bytes32 prev;
    }

    mapping (bytes32 => Element) queue;
    mapping (bytes32 => bool) used;

    function QueueExample() public {
        isEmpty = true;
    }

    function enqueue(int someData) public {
        bytes32 key;
        uint increment = 0;
        do {
            // u can use tx's hash instead of block.number
            key = sha256(someData, msg.sender, block.number, increment++);
        } while (used[key]);
        used[key] = true;
        queue[key] = Element(ElementData(someData), back);
        if (isEmpty) {
            front = back = key;
            isEmpty = false;
        } else {
            queue[back].prev = key;
            back = key;
        }
    }

    function dequeue() public returns(int returnableData) {
        returnableData = getFront();
        bytes32 toDelete = front;
        front = queue[front].prev;
        if (back == toDelete) {
            isEmpty = true;
        }
        delete used[toDelete];
        delete queue[toDelete];
    }

    function getFront() public view returns(int returnableData) {
        require(!isEmpty);
        ElementData storage data = queue[front].data;
        returnableData = data.num;
    }
}