This describes an optimization for "binary insertion sort" (BINSORT for short).
BINSORT has been implemented in Python, CyThon, and Timsort (the default Array.sort() in JAVA SE 7 and JAVA SE 8) I have read the BINSORT in Timsort http://grepcode.com/file/repository.grepcode.com/java/root/jdk/openjdk/8-b132/java/util/TimSort.java#TimSort.binarySort%28java.lang.Object%5B%5D%2Cint%2Cint%2Cint%2Cjava.util.Comparator%29 and I think that I can make a little more optimization. ================= The old BINSORT: The basic idea is to use binary search on the sorted list to find a final position for a new element X, then insert X to the sorted list. [SORTED_LIST], [X in UNSORTED_LIST] // pick X in UNSORTED_LIST index = binarySeach([SORTED_LIST]) // use binary search to find appropriate location for X in SORTED_LIST [SORTED_LIST].add(index, X) // insert X to index location ================== New BINSORT: [SORTED_LIST], [A] // A is an UNSORTED_LIST j = compare(A[i], A[i+1]) // pick A[i], compare to next element index = binarySeach([SORTED_LIST], A[i]) // use binary search to find // appropriate location for A[i] in SORTED_LIST, and remember the index [SORTED_LIST].add(index, A[i]) // insert A[i] to index location // Now for A[+1], where already know where it should be, compare to A[i] if j >= 0: // A[i] > A[i+1], so A[i+1] will be in the right side of A[i] // We only have to search on a reduced Array: index = binarySearch(SORTED_LIST[index : length(SORTED_LIST)], A[i+1]) else: // we search on the left side of of A[i] index = binarySearch(SORTED_LIST[0 : index], A[i+1]) [SORTED_LIST].add(index, A[i+1]) // insert A[i+1] to index location //repeat the loop ================== Run test. Intuitively, new BINSORT will be better if the Array become bigger, because it reduce the array to search with the cost of only 1 more comparison. I only care about small array, with the length < 100 (we have known that in Timsort, the list is divided to chunks with length 64, then apply BINSORT to them). So I make a big Array, divide them, and apply new BINSORT in each chunk, then compare to OLD BINSORT. The source code is in the bottom of this document. Here is the results: cpuinfo: model name : Intel(R) Core(TM) i3 CPU M 350 @ 2.27GHz stepping : 2 microcode : 0xc cpu MHz : 933.000 cache size : 3072 KB ----- random array: ARRAY_SIZE: 1000000 CHUNK_SIZE: 100 DATA: randint(0, 1000000) OLD BINSORT: 81.45754 new BINSORT: 5.26754 RATIO: (OLD - new) / new = 14.464 --- incremental array: ARRAY_SIZE: 1000000 CHUNK_SIZE: 100 DATA: range(0, 1000000) OLD BINSORT: 81.87927 new BINSORT: 5.41651 RATIO: (OLD - new) / new = 14.11659 --- decremental array: ARRAY_SIZE: 1000000 CHUNK_SIZE: 100 DATA: range(0, 1000000) OLD BINSORT: 81.45723 new BINSORT: 5.09823 RATIO: (OLD - new) / new = 14.97753 ---- all equal array: ARRAY_SIZE: 1000000 CHUNK_SIZE: 100 DATA: 50000 OLD BINSORT: 40.46027 new BINSORT: 5.41221 RATIO: (OLD - new) / new = 6.47573 ==================== What should we do next: - Tuning my test code (I have just graphed it). - Test other cases, and bigger array (my laptop cannot handle array more than 10^6.) - Modifying Timsort in java.util. and test if it is better. ==================== My test code, written by Python. from timeit import Timer setup ="""\ import bisect from random import randint from timeit import Timer SIZE = 1000000 CHUNK = 100 NUM_CHUNK = SIZE/CHUNK data = [] data2 = [] data3 = [] for i in range(0,SIZE): data.append(randint(0,1000000)) #data.append(i) #data = data[::-1] """ sample ="""\ for j in range(0,NUM_CHUNK): low = CHUNK*j high= low + CHUNK data2.append(data[low]) index = low for i in range(low,high): x = data[i] index = bisect.bisect_right(data2[low:], x, low, len(data2) - low-1) data2.insert(index, x) """ new ="""\ for j in range(0,NUM_CHUNK): low = CHUNK*j high= low + CHUNK data3.append(data[low]) index = low for i in range(low,high): x = data[i] if x >= data[i-1]: index = bisect.bisect_right(data3[low:len(data3) - low-1], x, index, len(data3) - low-1) else: index = bisect.bisect_right(data3[low:index], x, low, index) data3.insert(index, x) """ t2 = Timer(stmt = sample, setup=setup) a = t2.timeit(1) print a t3 = Timer(stmt = new, setup=setup) b = t3.timeit(1) print b print (str((a - b)/b)) ==================================== Nha Pham Mar 07 2015
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