Subject: Re: [Outreachy][PATCH 1/2] strbuf: introduce strbuf_addstrings() to repeatedly add a string

From: Junio C Hamano <[email-masked]>
Date: Mon, 26 Feb 2024 08:39:25 -0800

Achu Luma <[email-masked]> writes:

> In a following commit we are going to port code from
> "t/helper/test-sha256.c", t/helper/test-hash.c and "t/t0015-hash.sh" to
> a new "t/unit-tests/t-hash.c" file using the recently added unit test
> framework.
>
> To port code like: perl -e "$| = 1; print q{aaaaaaaaaa} for 1..100000;"
> we are going to need a new strbuf_addstrings() function that repeatedly
> adds the same string a number of times to a buffer.

We do not need to call such a function "addstrings", though.  The
name on the subject line made me expect a varargs function:

 (bad)	strbuf_addstrings(&sb, "foo", "bar", "baz", NULL);

It would have been clearer if the name hinted what it does, clearer
than just a single "s" that says it is talking about plural.  What
would be a good name that hints "n times add a single same string"?
I dunno.

I also would have expected that the order of parameters are
repeat-count followed by what gets repeated.

Having said all of the above, we already have "addchars" that is
equally strange, so let's let it pass ;-).

> diff --git a/strbuf.c b/strbuf.c
> index 7827178d8e..eb2b3299ce 100644
> --- a/strbuf.c
> +++ b/strbuf.c
> @@ -302,6 +302,17 @@ void strbuf_add(struct strbuf *sb, const void *data, size_t len)
>  	strbuf_setlen(sb, sb->len + len);
>  }
>
> +void strbuf_addstrings(struct strbuf *sb, const char *s, size_t n)
> +{
> +	size_t len = strlen(s);

Let's have a blank line here to separate decls from the first
statement.

> +	if (unsigned_mult_overflows(len, n))
> +		die("you want to use way too much memory");
> +	strbuf_grow(sb, len * n);

The error message given by

	strbuf_grow(sb, st_mult(len, n));

would be equally informative and takes only a single line.

> +	for (size_t i = 0; i < n; i++)
> +		memcpy(sb->buf + sb->len + len * i, s, len);

Wouldn't it be sufficient to run strbuf_add() n times at this point,
as we have already called strbuf_grow() to avoid repeated
reallocation?  Repeated manual memcpy() that involves manual offset
computation makes me nervous.

> +	strbuf_setlen(sb, sb->len + len * n);
> +}
> +
>  void strbuf_addbuf(struct strbuf *sb, const struct strbuf *sb2)
>  {
>  	strbuf_grow(sb, sb2->len);
> diff --git a/strbuf.h b/strbuf.h
> index e959caca87..0fb1b5e81e 100644
> --- a/strbuf.h
> +++ b/strbuf.h
> @@ -310,6 +310,11 @@ static inline void strbuf_addstr(struct strbuf *sb, const char *s)
>  	strbuf_add(sb, s, strlen(s));
>  }
>
> +/**
> + * Add a NUL-terminated string the specified number of times to the buffer.
> + */
> +void strbuf_addstrings(struct strbuf *sb, const char *s, size_t n);
> +
>  /**
>   * Copy the contents of another buffer at the end of the current one.
>   */
> --
> 2.43.0.windows.1