JS中的 Duff's Device及其变体

loop.js

 /*
    Duff’s device
     */
    function loop1() {
        var result = [];
        function process(value) {
            result.push(value*2);
        }
        var items = [0,1,2,3,4,5,6,7,8];

        var iterations = Math.ceil(items.length / 8),
            startAt = items.length % 8,
            i = 0;
        do {
            switch (startAt) {
                case 0: process(items[i++]); // process(0) i = 1
                case 7: process(items[i++]);
                case 6: process(items[i++]);
                case 5: process(items[i++]);
                case 4: process(items[i++]);
                case 3: process(items[i++]);
                case 2: process(items[i++]);
                case 1: process(items[i++]);
            }
            startAt = 0;
        } while (--iterations);

        console.log(result);
    }

    /*
   Duff’s device的变体，比loop1快 40%
    */
    function loop2() {
        var result = [];
        function process(value) {
            result.push(value*2);
        }
        var items = [0,1,2,3,4,5,6,7,8];

        var iterations = Math.floor(items.length / 8),
            leftover = items.length % 8,
            i = 0;

        if(leftover > 0){
            do {
                process(items[i++])
            }while (--leftover > 0)
        }

        do {
            process(items[i++]);
            process(items[i++]);
            process(items[i++]);
            process(items[i++]);
            process(items[i++]);
            process(items[i++]);
            process(items[i++]);
            process(items[i++]);
        } while (--iterations);

        console.log(result);
    }