[m-rev.] for review: Simplify string.split implementation.

[Mark Brown]

This looks fine.

On Wed, Oct 23, 2019 at 3:03 PM Peter Wang <novalazy at gmail.com> wrote:
>
> library/string.m:
>     Replace foreign code implementations with Mercury code.
> ---
>  library/string.m | 109 ++++-------------------------------------------
>  1 file changed, 9 insertions(+), 100 deletions(-)
>
> diff --git a/library/string.m b/library/string.m
> index 8cc572d9c..97b005841 100644
> --- a/library/string.m
> +++ b/library/string.m
> @@ -679,8 +679,8 @@
>
>      % split(String, Index, LeftSubstring, RightSubstring):
>      %
> -    % Split a string into two substrings, at the code unit `Index'.
> -    % (If `Count' is out of the range [0, length of `String'], it is treated
> +    % Split a string into two substrings at the code unit offset `Index'.
> +    % (If `Index' is out of the range [0, length of `String'], it is treated
>      % as if it were the nearest end-point of that range.)
>      %
>  :- pred split(string::in, int::in, string::out, string::out) is det.
> @@ -4138,110 +4138,19 @@ join_list_loop(Sep, [H | T]) = Sep ++ H ++ join_list_loop(Sep, T).
>  ").
>
>  %---------------------%
> -%
> -% For some Str and Count inputs, we may return Str as either Left or Right.
> -% Since Str has mode `in', both Left or Right must have mode 'out', not `uo'.
> -%
> -
> -:- pragma foreign_proc("C",
> -    split(Str::in, Count::in, Left::out, Right::out),
> -    [will_not_call_mercury, promise_pure, thread_safe, will_not_modify_trail,
> -        does_not_affect_liveness, may_not_duplicate],
> -"{
> -    MR_Integer  len;
> -
> -    if (Count <= 0) {
> -        MR_make_aligned_string(Left, """");
> -        Right = Str;
> -    } else {
> -        len = strlen(Str);
>
> -        if (Count > len) {
> -            Count = len;
> -        }
> -
> -        MR_allocate_aligned_string_msg(Left, Count, MR_ALLOC_ID);
> -        MR_memcpy(Left, Str, Count);
> -        Left[Count] = '\\0';
> -        // We need to make a copy to ensure that the pointer is word-aligned.
> -        MR_allocate_aligned_string_msg(Right, len - Count, MR_ALLOC_ID);
> -        strcpy(Right, Str + Count);
> -    }
> -}").
> -:- pragma foreign_proc("C#",
> -    split(Str::in, Count::in, Left::out, Right::out),
> -    [will_not_call_mercury, promise_pure, thread_safe],
> -"{
> -    int len;
> -
> -    if (Count <= 0) {
> -        Left = """";
> -        Right = Str;
> -    } else {
> -        len = Str.Length;
> -        if (Count > len) {
> -            Count = len;
> -        }
> -        Left = Str.Substring(0, Count);
> -        Right = Str.Substring(Count);
> -    }
> -}").
> -:- pragma foreign_proc("Java",
> -    split(Str::in, Count::in, Left::out, Right::out),
> -    [will_not_call_mercury, promise_pure, thread_safe],
> -"
> -    if (Count <= 0) {
> -        Left = """";
> -        Right = Str;
> -    } else {
> -        int len = Str.length();
> -        if (Count > len) {
> -            Count = len;
> -        }
> -        Left = Str.substring(0, Count);
> -        Right = Str.substring(Count);
> -    }
> -").
> -:- pragma foreign_proc("Erlang",
> -    split(Str::in, Count::in, Left::out, Right::out),
> -    [will_not_call_mercury, promise_pure, thread_safe],
> -"
> -    if
> -        Count =< 0 ->
> -            Left = <<>>,
> -            Right = Str;
> -        Count > size(Str) ->
> -            Left = Str,
> -            Right = <<>>;
> -        true ->
> -            << Left:Count/binary, Right/binary >> = Str
> -    end
> -").
> -
> -    % XXX ILSEQ from_code_unit_list refuses to create strings containing
> -    % ill-formed sequences.
> -    %
> -split(Str, Count, Left, Right) :-
> -    ( if Count =< 0 then
> +split(Str, Index, Left, Right) :-
> +    ( if Index =< 0 then
>          Left = "",
>          Right = Str
>      else
> -        to_code_unit_list(Str, List),
>          Len = length(Str),
> -        ( if Count > Len then
> -            Num = Len
> -        else
> -            Num = Count
> -        ),
> -        ( if
> -            list.split_list(Num, List, LeftList, RightList),
> -            from_code_unit_list(LeftList, LeftPrime),
> -            from_code_unit_list(RightList, RightPrime)
> -        then
> -            Left = LeftPrime,
> -            Right = RightPrime
> +        ( if Index >= Len then
> +            Left = Str,
> +            Right = ""
>          else
> -            unexpected($pred, "split_list failed")
> +            unsafe_between(Str, 0, Index, Left),
> +            unsafe_between(Str, Index, Len, Right)
>          )
>      ).
>
> --
> 2.23.0
>
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