达夫循环

        //达夫正向循环
        function duffForwardLoop(items, process){
            var idx = 0;
            var len = items.length;
            var itr = len >> 3;
            while (itr--) {
                process(items[idx++]);
                process(items[idx++]);
                process(items[idx++]);
                process(items[idx++]);
                process(items[idx++]);
                process(items[idx++]);
                process(items[idx++]);
                process(items[idx++]);
            }
            
            itr = len & 7;
            
            while (itr--) {
                process(items[idx++]);
            }
        }
        
        //达夫反向循环
        function duffReverseLoop(items, process){
            var len = items.length;
            var idx = len - 1;
            var itr = len >> 3;
            
            while (itr--) {
                process(items[idx--]);
                process(items[idx--]);
                process(items[idx--]);
                process(items[idx--]);
                process(items[idx--]);
                process(items[idx--]);
                process(items[idx--]);
                process(items[idx--]);
            }
            
            itr = len & 7;
            
            while (itr--) {
                process(items[idx--]);
            }
        }