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IteratedRun.sol

pragma solidity ^0.4.0;

contract IteratedRun {
    struct City {
        bytes32 name;
        bool visited;
    }
    address owner;
    
    City[] cities;
    bytes32 startCity;
    uint nextBlock;
    int cur;
    uint remain;
    uint blockInterval;
    bool returned;
    
    constructor(bytes32 _startCity, bytes32[] names, uint _startBlock, uint _blockInterval) public {
        owner = msg.sender;
        startCity = _startCity;
        nextBlock = _startBlock;
        cur = -1; remain = names.length;
        returned = false;
        blockInterval = _blockInterval;
        for (uint i=0; i<names.length; i++) {
            cities.push(City({
                name: names[i],
                visited: false
            }));
        }
    }
    
    function decideReturn() public {
        require(msg.sender == owner);
        returned = true;
    }
    
    function updateToNextCity() public {
        assert(cities.length > 0);
        require(!returned);
        if (nextBlock > block.number - 3) { // wait for 3 blocks
            return;
        }
        log0(blockhash(nextBlock));
        uint idx = uint(blockhash(nextBlock)) % remain + 1;
        nextBlock = nextBlock + blockInterval;
        remain--;
        uint cnt = 0;
        for (uint i=0; i<cities.length; i++)
            if (!cities[i].visited) {
                cnt++;
                if (cnt == idx) {
                    cur = int(i);
                    cities[i].visited = true;
                    break;
                }
            }
    }
    
    function currentCity() view public returns (bytes32) {
        if (returned) return startCity;
        if (cur >= 0) {
            return cities[uint(cur)].name;
        } else {
            return startCity;
        }
    }
}