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Concatenation.sol

pragma solidity ^0.4.24;

contract Concatenation {
    
    struct Element{
        string first;
        string second;
        string separator;
        bytes concatenatedValue;
        uint concatenatedValueIndex;
    }
    
    Element element;
    
    function concatenate(string _first, string _second, string _separator) public payable returns (string){
        element = Element(_first, _second, _separator, "0X12", 0);
        createConcatenatedValue(element);
        executeConcatenation();
        return getConcatanatedValue();
    }
    
    function createConcatenatedValue(Element _element) private{
        string memory value = new string(bytes(_element.first).length + bytes(_element.second).length + bytes(_element.separator).length);
        element.concatenatedValue = bytes(value);
    }
    
    function add(string _value) private {
        bytes memory value = bytes(_value);
        for(uint index = 0; index < value.length; index++){
            element.concatenatedValue[element.concatenatedValueIndex++] = value[index];
        }
    }
    
    function executeConcatenation() private {
        add(element.first);
        add(element.separator);
        add(element.second);
    }
    
    function getConcatanatedValue() private view returns(string){
        return string(element.concatenatedValue);
    }
    
}

StringUtil.sol

pragma solidity ^0.4.24;

contract Concatenation {
     function concatenate(string _first, string _second, string _separator) public view returns (string) {}
}

contract StringUtil {
    
    address concatenationContractAddress = 0x0dcd2f752394c41875e259e00bb44fd505297caf;
    Concatenation concatenation = Concatenation(concatenationContractAddress);
    
    function concatenate(string _first, string _second, string _separator) public view returns (string){
        return concatenation.concatenate(_first, _second, _separator);
    }
    
}