(一)设计模式之策略模式

落日映苍穹つ 2022-04-23 05:32 171阅读 0赞

注:**以jdk中的一角来理解设计模式的运用**

## **策略模式:** ##

在不同的情形下选取不同的方式去处理  
比如:jdk中的 java.util.Arrays.sort() 方法,则是根据待排序数组的初始的有序程度,以及规模来选取不同的排序方式处理排序  
具体实现是这个方法:  
策略1:对于小数组,直接使用"快排"1  
策略2:对于不是高度结构化(各区间内有序)的数组,执行"快排"  
策略3:高度结构化的,使用归并排序

//left 与right 框定此次排序的操作区间
        static void sort(int[] a, int left, int right,
                         int[] work, int workBase, int workLen) { 
       策略1:对于小数组,直接使用快排
       // Use Quicksort on small arrays
            if (right - left < QUICKSORT_THRESHOLD) { 
                sort(a, left, right, true);
                return;
            }
    
            /* * Index run[i] is the start of i-th run * (ascending or descending sequence). * run[i]记录第i次run得到的有序序列区间起始下标 * 即:数组索引在区间run[i]至run[i+1],表示数组的这部分片段是有序的 */
            int[] run = new int[MAX_RUN_COUNT + 1];
            int count = 0; run[0] = left;
            
            // Check if the array is nearly sorted
            //检测数组是否接近有序
            for (int k = left; k < right; run[count] = k) { 
            // 框定升序区间
                if (a[k] < a[k + 1]) {  // ascending
                    while (++k <= right && a[k - 1] <= a[k]);
             //框定降序区间,获得降序区间后,从两侧向中间,对换元素,改变成升序区间
                } else if (a[k] > a[k + 1]) {  // descending
                    while (++k <= right && a[k - 1] >= a[k]);
                    for (int lo = run[count] - 1, hi = k; ++lo < --hi; ) { 
                        int t = a[lo];
                         a[lo] = a[hi];
                          a[hi] = t;
                    }
                } else {  // equal
                    for (int m = MAX_RUN_LENGTH; ++k <= right && a[k - 1] == a[k]; ) { 
                    //检测(其实也做了部分的简单排序,将降序序列变为升序序列)次数用完之后,执行排序
                        if (--m == 0) { 
                        
                            sort(a, left, right, true);
                            return;
                        }
                    }
                }
    
                /* * The array is not highly structured, * use Quicksort instead of merge sort. */
                 策略2. 如果数组不是高度的结构化(也就是指区间有序),直接使用快排,而不是使用归并排序
                if (++count == MAX_RUN_COUNT) { 
                    sort(a, left, right, true);
                    return;
                }
            }
    
    
            // Check special cases
            // Implementation note: variable "right" is increased by 1.
            if (run[count] == right++) {  // The last run contains one element
                run[++count] = right;
            } else if (count == 1) {  // The array is already sorted
                return;
            }
         策略3.如果数组是高度结构化(也就是满足归并排序的条件,执行归并排序)
            // Determine alternation base for merge
            byte odd = 0;
            for (int n = 1; (n <<= 1) < count; odd ^= 1);
    
            // Use or create temporary array b for merging
            int[] b;                 // temp array; alternates with a
            int ao, bo;              // array offsets from 'left'
            int blen = right - left; // space needed for b
            if (work == null || workLen < blen || workBase + blen > work.length) { 
                work = new int[blen];
                workBase = 0;
            }
            if (odd == 0) { 
                System.arraycopy(a, left, work, workBase, blen);
                b = a;
                bo = 0;
                a = work;
                ao = workBase - left;
            } else { 
                b = work;
                ao = 0;
                bo = workBase - left;
            }
    
            // Merging
            for (int last; count > 1; count = last) { 
                for (int k = (last = 0) + 2; k <= count; k += 2) { 
                    int hi = run[k], mi = run[k - 1];
                    for (int i = run[k - 2], p = i, q = mi; i < hi; ++i) { 
                        if (q >= hi || p < mi && a[p + ao] <= a[q + ao]) { 
                            b[i + bo] = a[p++ + ao];
                        } else { 
                            b[i + bo] = a[q++ + ao];
                        }
                    }
                    run[++last] = hi;
                }
                if ((count & 1) != 0) { 
                    for (int i = right, lo = run[count - 1]; --i >= lo;
                        b[i + bo] = a[i + ao]
                    );
                    run[++last] = right;
                }
                int[] t = a; a = b; b = t;
                int o = ao; ao = bo; bo = o;
            }
            }

> private static void sort(int\[\] a, int left, int right, boolean  
> leftmost)  
> 第三个参数是比较有意思的:  
> 用来标记是否是左边有序元素更多,如果是则使用插入排序,否则是执行其他策略,优化操作,具体可以参见jdk源码

--------------------

1.  注:上面所说的"快排"是指:**Dual-Pivot Quicksort**,并非经典的快排,但是目前效率却是比经典快排高,可以参考此篇文章:[传送][Link 1] , ↩︎

[Link 1]: https://www.jianshu.com/p/2c6f79e8ce6e