面试题热个身：5 亿整数的大文件，来排个序？

    private final int cutoff = 8;

    public void perform(Comparable[] a) {
        perform(a,0,a.length - 1);
    }

    private int median3(Comparable[] a,int x,int y,int z) {
        if(lessThan(a[x],a[y])) {
            if(lessThan(a[y],a[z])) {
                return y;
            }
            else if(lessThan(a[x],a[z])) {
                return z;
            }else {
                return x;
            }
        }else {
            if(lessThan(a[z],a[y])){
                return y;
            }else if(lessThan(a[z],a[x])) {
                return z;
            }else {
                return x;
            }
        }
    }

    private void perform(Comparable[] a,int low,int high) {
        int n = high - low + 1;
        //当序列非常小，用插入排序
        if(n <= cutoff) {
            InsertionSort insertionSort = SortFactory.createInsertionSort();
            insertionSort.perform(a,low,high);
            //当序列中小时，使用median3
        }else if(n <= 100) {
            int m = median3(a,low,low + (n >>> 1),high);
            exchange(a,m,low);
            //当序列比较大时，使用ninther
        }else {
            int gap = n >>> 3;
            int m = low + (n >>> 1);
            int m1 = median3(a,low,low + gap,low + (gap << 1));
            int m2 = median3(a,m - gap,m,m + gap);
            int m3 = median3(a,high - (gap << 1),high - gap,high);
            int ninther = median3(a,m1,m2,m3);
            exchange(a,ninther,low);
        }
        if(high <= low)
            return;
        //lessThan
        int lt = low;
        //greaterThan
        int gt = high;
        //中心点
        Comparablepivot =  a[low];
        int i = low + 1;
        /*
        * 不变式：
        *   a[low..lt-1] 小于pivot -> 前部(first)
        *   a[lt..i-1] 等于 pivot -> 中部(middle)
        *   a[gt+1..n-1] 大于 pivot -> 后部(final)
        *
        *   a[i..gt] 待考察区域
        */
        while (i <= gt) {
            if(lessThan(a[i],pivot)) {
                //i-> ,lt ->
                exchange(a,lt++,i++);
            }else if(lessThan(pivot,a[i])) {
                exchange(a,i,gt--);
            }else{
                i++;
            }
        }
        // a[low..lt-1] < v = a[lt..gt] < a[gt+1..high].
        perform(a,low,lt - 1);
        perform(a,gt + 1,high);
    }

    /**
     * 小于等于这个值的时候，交给插入排序
     */
    private final int cutoff = 8;
    /**
     * 对给定的元素序列进行排序
     *
     * @param a 给定元素序列
     */
    @Override
    public void perform(Comparable[] a) {
        Comparable[] b = a.clone();
        perform(b, a, 0, a.length - 1);
    }

    private void perform(Comparable[] src,Comparable[] dest,int low,int high) {
        if(low >= high)
            return;
        //小于等于cutoff的时候,交给插入排序
        if(high - low <= cutoff) {
            SortFactory.createInsertionSort().perform(dest,low,high);
            return;
        }
        int mid = low + ((high - low) >>> 1);
        perform(dest,src,low,mid);
        perform(dest,src,mid + 1,high);
        //考虑局部有序 src[mid] <= src[mid+1]
        if(lessThanOrEqual(src[mid],src[mid+1])) {
            System.arraycopy(src,low,dest,low,high - low + 1);
        }
        //src[low .. mid] + src[mid+1 .. high] -> dest[low .. high]
        merge(src,dest,low,mid,high);
    }

    private void merge(Comparable[] src,Comparable[] dest,int low,int mid,int high) {
        for(int i = low,v = low,w = mid + 1; i <= high; i++) {
            if(w > high || v <= mid && lessThanOrEqual(src[v],src[w])) {
                dest[i] = src[v++];
            }else {
                dest[i] = src[w++];
            }
        }
    }

    private BitSet bits;

    public void perform(
            String largeFileName,
            int total,
            String destLargeFileName,
            Castor<Integer> castor,
            int readerBufferSize,
            int writerBufferSize,
            boolean asc) throws IOException {
        System.out.println("BitmapSort Started.");
        long start = System.currentTimeMillis();
        bits = new BitSet(total);
        InputPart<Integer> largeIn = PartFactory.createCharBufferedInputPart(largeFileName, readerBufferSize);
        OutputPart<Integer> largeOut = PartFactory.createCharBufferedOutputPart(destLargeFileName, writerBufferSize);
        largeOut.delete();
        Integer data;
        int off = 0;
        try {
            while (true) {
                data = largeIn.read();
                if (data == null)
                    break;
                int v = data;
                set(v);
                off++;
            }
            largeIn.close();
            int size = bits.size();
            System.out.println(String.format("lines : %d ,bits : %d", off, size));
            if(asc) {
                for (int i = 0; i < size; i++) {
                    if (get(i)) {
                        largeOut.write(i);
                    }
                }
            }else {
                for (int i = size - 1; i >= 0; i--) {
                    if (get(i)) {
                        largeOut.write(i);
                    }
                }
            }
            largeOut.close();
            long stop = System.currentTimeMillis();
            long elapsed = stop - start;
            System.out.println(String.format("BitmapSort Completed.elapsed : %dms",elapsed));
        }finally {
            largeIn.close();
            largeOut.close();
        }
    }

    private void set(int i) {
        bits.set(i);
    }

    private boolean get(int v) {
        return bits.get(v);
    }