Re: [PATCH 04/16] refactor skip_prefix to return a boolean

> The skip_prefix function returns a pointer to the content
> past the prefix, or NULL if the prefix was not found. While
> this is nice and simple, in practice it makes it hard to use
> for two reasons:
>
>   1. When you want to conditionally skip or keep the string
>      as-is, you have to introduce a second temporary
>      variable. For example:
>
>        tmp = skip_prefix(buf, "foo");
>        if (tmp)
>                buf = tmp;
>
>   2. It is verbose to check the outcome in a conditional, as
>      you need extra parentheses to silence compiler
>      warnings. For example:
>
>        if ((cp = skip_prefix(buf, "foo"))
>                /* do something with cp */
>
> Both of these make it harder to use for long if-chains, and
> we tend to use starts_with instead. However, the first line
> of "do something" is often to then skip forward in buf past
> the prefix, either using a magic constant or with an extra
> strlen (which is generally computed at compile time, but
> means we are repeating ourselves).
>
> This patch refactors skip_prefix to return a simple boolean,
> and to provide the pointer value as an out-parameter. If the
> prefix is not found, the out-parameter is untouched. This
> lets you write:
>
>   if (skip_prefix(arg, "foo ", &arg))
>           do_foo(arg);
>   else if (skip_prefix(arg, "bar ", &arg))
>           do_bar(arg);
>
> Signed-off-by: Jeff King <peff@peff.net>
> ---
> diff --git a/git-compat-util.h b/git-compat-util.h
> index b6f03b3..556c839 100644
> --- a/git-compat-util.h
> +++ b/git-compat-util.h
> @@ -349,13 +349,31 @@ extern void set_die_is_recursing_routine(int (*routine)(void));
>  extern int starts_with(const char *str, const char *prefix);
>  extern int ends_with(const char *str, const char *suffix);
>
> -static inline const char *skip_prefix(const char *str, const char *prefix)
> +/*
> + * If "str" begins with "prefix", return 1. If out is non-NULL,
> + * it it set to str + strlen(prefix) (i.e., the prefix is skipped).

> + *
> + * Otherwise, returns 0 and out is left untouched.
> + *
> + * Examples:
> + *
> + *   [extract branch name, fail if not a branch]
> + *   if (!skip_prefix(ref, "refs/heads/", &branch)
> + *     return -1;
> + *
> + *   [skip prefix if present, otherwise use whole string]
> + *   skip_prefix(name, "refs/heads/", &name);
> + */
> +static inline int skip_prefix(const char *str, const char *prefix,
> +                             const char **out)
>  {
>         do {
> -               if (!*prefix)
> -                       return str;
> +               if (!*prefix) {
> +                       *out = str;
> +                       return 1;
> +               }
>         } while (*str++ == *prefix++);
> -       return NULL;
> +       return 0;
>  }
>
>  #if defined(NO_MMAP) || defined(USE_WIN32_MMAP)