erlang程序解释：冒泡排序

原code参见http://en.literateprograms.org/Bubble_sort_(Erlang)

虽然原文有解释，可能还是不太好理解erlang写得这么间断晦涩的程序。

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25 
26 -module(bubblesort).
27 -export([sort/1]).
28 -import(lists, [reverse/1]).
29 
30 sort(L) -> sort(L, [], true).
31 
32 sort([], L, true) -> reverse(L);
33 
34 sort([], L, false) -> sort(reverse(L), [], true);
35 sort([ X, Y | T ], L, _) when X > Y ->
36     sort([ X | T ], [ Y | L ], false);
37 sort([ X | T ], L, Halt) -> sort(T, [ X | L ], Halt).

我们先看一个例子吧：


sort([2,4,3,5,1])

#round 1

=> sort([2,4,3,5,1], [], true)

=> sort([4,3,5,1], [2], true)

=> sort([4,5,1], [3,2], false)

=> sort([5,1], [4,3,2], false)

=> sort([5], [1,4,3,2], false)

=> sort([], [5,1,4,3,2], false)

=> sort([2,3,4,1,5], [], true)

# round 2

=> sort([3,4,1,5], [2], true)

=> sort([4,1,5], [3,2], true)

=> sort([4,5], [1,3,2], false)

=> sort([5], [4,1,3,2], false)

=> sort([], [5,4,1,3,2], false)

=> sort([2,3,1,4,5], [], true)

# round 3

=> sort([3,1,4,5], [2], true)

=> sort([3,4,5], [1,2], false)

=> sort([4,5], [3,1,2], false)

=> sort([5], [4,3,1,2], false)

=> sort([], [5,4,3,1,2], false)

=> sort([2,1,3,4,5], true)

#round 4

=> sort([2,3,4,5], [1], false)

=> sort([3,4,5], [2,1], false)

=> sort([4,5], [3,2,1], false)

=> sort([5], [4,3,2,1], false)

=> sort([], [5,4,3,2,1], false)

=> sort([1,2,3,4,5], [], true)

#round 5

=> sort([2,3,4,5], [1], true)

=> sort([3,4,5],[2,1], true)

=> sort([4,5],[3,2,1], true)

=> sort([5],[4,3,2,1], true)

=> sort([], [5,4,3,2,1], true)

=> [1,2,3,4,5]

冒泡排序的每一轮，都找出了一个最大值沉到队伍的最后面．如果本轮全部是排好序的，也即不需要下一轮了，那么flag就是true,否则是false.