Simplest Python K-Way Merging Sort|最简单的Python k路归并排序

想做这个好长时间了，因为有一篇Dreamworks的论文《Coherent Out-of-Core Point-Based Global Illumination》提到了这个，一直没时间做，于是今天抽了十几分钟搞了一下，基于Python的，非常简单。由于基于点云的GI已经毫无疑问的不会使用了，所以Weta Digital的PantaRay和Dreamworks的这些方法都会直接宣布作废，权当一个过渡阶段（从REYES到纯RT）的折中方法了。

k是线程数目，n是每个线程处理的数字个数。可以直接替换为外部的IO访问代码，每一个线程读取外部待排序数据的索引，写出各自排序后的结果，主线程再来读取所有的文件做一次归并输出即可。这里用了Python的heapq，C++的话直接就是std::priority_queue，也非常方便。

 import heapq
 import random
 import threading

 # k threads
 k = 

 # Generate n numbers for each thread to process.
 n = 

 def GenerateNumbers(a, b):
     numbers = []
     for i in xrange(n):
         numbers.append(random.randint(a, b))
     return numbers

 class SortingThread(threading.Thread):
     def __init__(self):
         super(SortingThread, self).__init__()
         self.numbers = GenerateNumbers(, )

     def run(self):
         self.numbers.sort()

 if __name__ == '__main__':
     # Parallel sort with multiple threads.
     #
     sortingThreads = []
     for i in xrange(k):
         sortingThreads.append(SortingThread())
     for i in xrange(k):
         sortingThreads[i].start()
     for i in xrange(k):
         sortingThreads[i].join()
         print i , sortingThreads[i].numbers

     # Use heap to output the final sorted list.
     # We fill the heap with several numbers firstly
     #
     theHeap = []
     for i in xrange():
         for t in xrange(len(sortingThreads)):
             number = sortingThreads[t].numbers[i]
             heapq.heappush(theHeap, number)

     sortedNumbers = []

     # In the while loop, people may replace the code by reading file routines.
     # Since IO is always the bottleneck, so single thread should be enough.
     p =
     while (len(theHeap) > ):
         if p < len(sortingThreads[t].numbers):
             for t in xrange(len(sortingThreads)):
                 number = sortingThreads[t].numbers[p]
                 heapq.heappush(theHeap, number)
             p +=
         sortedNumbers.append(heapq.heappop(theHeap))

     # Done
     print sortedNumbers