[m-rev.] for review: Slightly improve string stripping functions.

[Mark Brown]

This looks fine.

On Wed, Oct 30, 2019 at 5:10 PM Peter Wang <novalazy at gmail.com> wrote:
>
> library/string.m:
>     Use unsafe_between for chomp, lstrip_pred, rstrip_pred
>     to avoid range checks.
> ---
>  library/string.m | 16 +++++++++++-----
>  1 file changed, 11 insertions(+), 5 deletions(-)
>
> diff --git a/library/string.m b/library/string.m
> index 37da065af..32d835c8c 100644
> --- a/library/string.m
> +++ b/library/string.m
> @@ -4620,8 +4620,8 @@ pad_right(String0, PadChar, Width, String) :-
>      ).
>
>  chomp(S) = Chomp :-
> -    ( if prev_index(S, length(S), Offset, '\n') then
> -        Chomp = left(S, Offset)
> +    ( if prev_index(S, length(S), Index, '\n') then
> +        Chomp = unsafe_between(S, 0, Index)
>      else
>          Chomp = S
>      ).
> @@ -4629,15 +4629,21 @@ chomp(S) = Chomp :-
>  strip(S0) = S :-
>      L = prefix_length(char.is_whitespace, S0),
>      R = suffix_length(char.is_whitespace, S0),
> -    S = between(S0, L, length(S0) - R).
> +    Start = L,
> +    End = max(L, length(S0) - R),
> +    S = unsafe_between(S0, Start, End).
>
>  lstrip(S) = lstrip_pred(char.is_whitespace, S).
>
>  rstrip(S) = rstrip_pred(char.is_whitespace, S).
>
> -lstrip_pred(P, S) = right(S, length(S) - prefix_length(P, S)).
> +lstrip_pred(P, S0) = S :-
> +    L = prefix_length(P, S0),
> +    S = unsafe_between(S0, L, length(S0)).
>
> -rstrip_pred(P, S) = left(S, length(S) - suffix_length(P, S)).
> +rstrip_pred(P, S0) = S :-
> +    R = suffix_length(P, S0),
> +    S = unsafe_between(S0, 0, length(S0) - R).
>
>  %---------------------%
>
> --
> 2.23.0
>
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