performance - Erlang 二进制文件以降低大小写性能

Question

我的目标是加快仅 ASCII 二进制转换为小写的性能。除了英语，我不需要任何语言。我已经编写并比较了一些变体:

二进制理解:

binary_comprehension(Binary) ->
    << <<if
             C >= $A andalso C =< $Z -> C - $A + $a;
             true -> C
         end >>
       || <<C>> <= Binary >>.

列表理解:

list_comprehension(Binary) ->
    L = binary_to_list(Binary),
    Lower =
        [if
             C >= $A andalso C =< $Z -> C - $A + $a;
             true -> C
         end || C <- L],
    list_to_binary(Lower).

和常规字符串:小写。

令人惊讶的是，列表理解击败了所有其他人:

1> timer:tc(fun() -> lists:foreach(fun(_) -> tolower:list_comprehension(<<"QWEQWEIQEKQHWKEHKQWHEKQHWKEQWEKHQWLKL">>) end, L100000) end).
{267603,ok}

2> timer:tc(fun() -> lists:foreach(fun(_) -> tolower:binary_comprehension(<<"QWEQWEIQEKQHWKEHKQWHEKQHWKEQWEKHQWLKL">>) end, L100000) end).
{324383,ok}

3> timer:tc(fun() -> lists:foreach(fun(_) -> string:lowercase(<<"QWEQWEIQEKQHWKEHKQWHEKQHWKEQWEKHQWLKL">>) end, L100000) end).
{319819,ok}

任何想法为什么双列表转换+理解比二进制转换快得多？

也许你知道更强大的优化？

更新:

我还发现字符串的 list-of-char 版本也很快:

string_lowercase(Binary) ->
    L = binary_to_list(Binary),
    Lower = string:lowercase(L),
    list_to_binary(Lower).

跑:

39> timer:tc(fun() -> lists:foreach(fun(_) -> tolower:string_to_lower(<<"QWEQWEIQEKQHWKEHKQWHEKQHWKEQWEKHQWLKL">>) end, L100000) end).
{277766,ok}

Answer 1

我对代码进行了一些修改并更改了测试用例。测试更改不是强制性的，但我个人更喜欢这种方式:

-module(tolower).
-compile(export_all).

u2l(C) when C >= $A andalso C =< $Z -> C + 32;
u2l(C) -> C.

binary_comprehension(Binary) ->
  << << (u2l(C)) >> || <<C>> <= Binary >>.

list_comprehension(Binary) ->
  list_to_binary([u2l(C) || C <- binary_to_list(Binary)]).

list_recur(Binary) -> list_recur(binary_to_list(Binary), []).

list_recur([], Result) -> lists:reverse(Result);
list_recur([C | Tail], Result) when C >= $A andalso C =< $Z ->
  list_recur(Tail, [(C + 32) | Result]);
list_recur([C | Tail], Result) ->
  list_recur(Tail, [C | Result]).

string_to_lower(Binary) ->
  list_to_binary(string:lowercase(binary_to_list(Binary))).

test() ->
  L100000 = lists:seq(1, 100000),
  TL0 = <<"QWEQWEIQEKQHWKEHKQWHEKQHWKEQWEKHQWLKL">>,
  TL = binary:copy(TL0, 100000),
  {R0, _} = timer:tc(fun() -> lists:foreach(fun(_) -> tolower:binary_comprehension(TL0) end, L100000) end),
  {R1, _} = timer:tc(tolower, binary_comprehension, [TL]),
  {R2, _} = timer:tc(tolower, list_comprehension, [TL]),
  {R3, _} = timer:tc(tolower, list_recur, [TL]),
  {R4, _} = timer:tc(string, lowercase, [TL]),
  {R5, _} = timer:tc(tolower, string_to_lower, [TL]),
  io:format("~n1.binary_comprehension = ~10w~n2.binary_comprehension = ~10w~n3.  list_comprehension = ~10w~n4.          list_recur = ~10w~n5.           lowercase = ~10w~n6.     string_to_lower = ~10w~n", 
  [R0,R1,R2,R3,R4,R5]).

Erlang shell 显示由于系统的并发特性，运行时间不一致。但最佳时间是如预期的 binary_comprehension。

62> c(tolower).    
tolower.erl:2: Warning: export_all flag enabled - all functions will be exported
{ok,tolower}
63> l(tolower).    
{module,tolower}
64> tolower:test().

1.binary_comprehension =     109000
2.binary_comprehension =      94000
3.  list_comprehension =     312001
4.          list_recur =     344001
5.           lowercase =     469002
6.     string_to_lower =     218000
ok
65> tolower:test().

1.binary_comprehension =     140998
2.binary_comprehension =      93999
3.  list_comprehension =     327994
4.          list_recur =     296996
5.           lowercase =     155997
6.     string_to_lower =     280996
ok
66> tolower:test().

1.binary_comprehension =     124998
2.binary_comprehension =      93998
3.  list_comprehension =     327995
4.          list_recur =     296995
5.           lowercase =     452993
6.     string_to_lower =     202997
ok
67> tolower:test().

1.binary_comprehension =     125000
2.binary_comprehension =      94000
3.  list_comprehension =     312000
4.          list_recur =     282000
5.           lowercase =     171000
6.     string_to_lower =     266000
ok

第 5 行的时间与第 6 行的时间不同，因为当您使用二进制参数调用 string:lowercase/1 时，它被处理为 utf8 序列。当您使用字符串参数调用 string:lowercase/1 时，避免了 utf8 处理。有关详细信息，请参阅 OTP 中 string.erl 的代码。