[m-rev.] for review: Simplify string.append(in, in, in) implementation.

[Mark Brown]

This looks fine.

On Wed, Oct 23, 2019 at 3:02 PM Peter Wang <novalazy at gmail.com> wrote:
>
> library/string.m:
>     Replace foreign code implementations with Mercury code.
> ---
>  library/string.m | 47 ++++++++++-------------------------------------
>  1 file changed, 10 insertions(+), 37 deletions(-)
>
> diff --git a/library/string.m b/library/string.m
> index 96566a3dc..8cc572d9c 100644
> --- a/library/string.m
> +++ b/library/string.m
> @@ -3577,43 +3577,16 @@ append(S1::out, S2::out, S3::in) :-
>
>  :- pred append_iii(string::in, string::in, string::in) is semidet.
>
> -:- pragma foreign_proc("C",
> -    append_iii(S1::in, S2::in, S3::in),
> -    [will_not_call_mercury, promise_pure, thread_safe, will_not_modify_trail,
> -        does_not_affect_liveness, no_sharing],
> -"{
> -    size_t len_1 = strlen(S1);
> -    SUCCESS_INDICATOR = (
> -        strncmp(S1, S3, len_1) == 0 &&
> -        strcmp(S2, S3 + len_1) == 0
> -    );
> -}").
> -:- pragma foreign_proc("C#",
> -    append_iii(S1::in, S2::in, S3::in),
> -    [will_not_call_mercury, promise_pure, thread_safe],
> -"{
> -    SUCCESS_INDICATOR = S3.Equals(System.String.Concat(S1, S2));
> -}").
> -:- pragma foreign_proc("Java",
> -    append_iii(S1::in, S2::in, S3::in),
> -    [will_not_call_mercury, promise_pure, thread_safe],
> -"
> -    SUCCESS_INDICATOR = S3.equals(S1.concat(S2));
> -").
> -:- pragma foreign_proc("Erlang",
> -    append_iii(S1::in, S2::in, S3::in),
> -    [will_not_call_mercury, promise_pure, thread_safe],
> -"
> -    S1_length = size(S1),
> -    S2_length = size(S2),
> -    case S1_length + S2_length =:= size(S3) of
> -        true ->
> -            <<Left:S1_length/binary, Right/binary>> = S3,
> -            SUCCESS_INDICATOR = (Left =:= S1 andalso Right =:= S2);
> -        false ->
> -            SUCCESS_INDICATOR = false
> -    end
> -").
> +append_iii(S1, S2, S3) :-
> +    Len1 = length(S1),
> +    Len2 = length(S2),
> +    Len3 = length(S3),
> +    ( if Len3 = Len1 + Len2 then
> +        unsafe_compare_substrings((=), S1, 0, S3, 0, Len1),
> +        unsafe_compare_substrings((=), S2, 0, S3, Len1, Len2)
> +    else
> +        fail
> +    ).
>
>  :- pred append_ioi(string::in, string::uo, string::in) is semidet.
>
> --
> 2.23.0
>
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