apache / incubator-weex / daf618fb4a9669b4741236c328b97b030965084f / . / weex_core / Source / include / wtf / BubbleSort.h

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#ifndef BubbleSort_h | |

#define BubbleSort_h | |

namespace WTF { | |

// Why would you want to use bubble sort? When you know that your input is already mostly | |

// sorted! This sort is guaranteed stable (it won't reorder elements that were equal), it | |

// doesn't require any scratch memory, and is the fastest available sorting algorithm if your | |

// input already happens to be sorted. This sort is also likely to have competetive performance | |

// for small inputs, even if they are very unsorted. | |

// We use this sorting algorithm for compiler insertion sets. An insertion set is usually very | |

// nearly sorted. It shouldn't take more than a few bubbles to make it fully sorted. We made | |

// this decision deliberately. Here's the performance of the testb3 Complex(64, 384) benchmark | |

// with the Air::InsertionSet doing no sorting, std::stable_sorting, and bubbleSorting: | |

// | |

// no sort: 8.8222 +- 0.1911 ms. | |

// std::stable_sort: 9.0135 +- 0.1418 ms. | |

// bubbleSort: 8.8457 +- 0.1511 ms. | |

// | |

// Clearly, bubble sort is superior. | |

// | |

// Note that the critical piece here is that insertion sets tend to be small, they must be | |

// sorted, the sort must be stable, they are usually already sorted to begin with, and when they | |

// are unsorted it's usually because of a few out-of-place elements. | |

template<typename IteratorType, typename LessThan> | |

void bubbleSort(IteratorType begin, IteratorType end, const LessThan& lessThan) | |

{ | |

for (;;) { | |

bool changed = false; | |

ASSERT(end >= begin); | |

size_t limit = end - begin; | |

for (size_t i = limit; i-- > 1;) { | |

if (lessThan(begin[i], begin[i - 1])) { | |

std::swap(begin[i], begin[i - 1]); | |

changed = true; | |

} | |

} | |

if (!changed) | |

return; | |

// After one run, the first element in the list is guaranteed to be the smallest. | |

begin++; | |

// Now go in the other direction. This eliminates most sorting pathologies. | |

changed = false; | |

ASSERT(end >= begin); | |

limit = end - begin; | |

for (size_t i = 1; i < limit; ++i) { | |

if (lessThan(begin[i], begin[i - 1])) { | |

std::swap(begin[i], begin[i - 1]); | |

changed = true; | |

} | |

} | |

if (!changed) | |

return; | |

// Now the last element is guaranteed to be the largest. | |

end--; | |

} | |

} | |

template<typename IteratorType> | |

void bubbleSort(IteratorType begin, IteratorType end) | |

{ | |

bubbleSort( | |

begin, end, | |

[](auto& left, auto& right) { | |

return left < right; | |

}); | |

} | |

} // namespace WTF | |

using WTF::bubbleSort; | |

#endif // BubbleSort_h | |