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From: Jeff King <peff@peff.net>
To: Elijah Newren via GitGitGadget <gitgitgadget@gmail.com>
Cc: git@vger.kernel.org, Elijah Newren <newren@gmail.com>
Subject: Re: [PATCH v2 05/10] strmap: new utility functions
Date: Fri, 30 Oct 2020 10:12:33 -0400	[thread overview]
Message-ID: <20201030141233.GE3277724@coredump.intra.peff.net> (raw)
In-Reply-To: <5c7507f55b09e24c0bfe87cc3df06213cfd1235b.1602549650.git.gitgitgadget@gmail.com>

On Tue, Oct 13, 2020 at 12:40:45AM +0000, Elijah Newren via GitGitGadget wrote:

> Add strmap as a new struct and associated utility functions,
> specifically for hashmaps that map strings to some value.  The API is
> taken directly from Peff's proposal at
> https://lore.kernel.org/git/20180906191203.GA26184@sigill.intra.peff.net/

This looks overall sane to me. I mentioned elsewhere that we could be
using FLEXPTR_ALLOC to save an extra allocation. I think it's easy and
worth doing here, as the logic would be completely contained within
strmap_put():

  if (strdup_strings)
	FLEXPTR_ALLOC_STR(entry, key, str);
  else {
	entry = xmalloc(sizeof(*entry));
	entry->key = str;
  }

And free_entries() then doesn't even have to care about strdup_strings.

> A couple of items of note:
> 
>   * Similar to string-list, I have a strdup_strings setting.  However,
>     unlike string-list, strmap_init() does not take a parameter for this
>     setting and instead automatically sets it to 1; callers who want to
>     control this detail need to instead call strmap_ocd_init().

That seems reasonable. It could just be a parameter, but I like that you
push people in the direction of doing the simple and safe thing, rather
than having them wonder whether they ought to set strdup_strings or not.

>   * I do not have a STRMAP_INIT macro.  I could possibly add one, but
>       #define STRMAP_INIT { { NULL, cmp_str_entry, NULL, 0, 0, 0, 0, 0 }, 1 }
>     feels a bit unwieldy and possibly error-prone in terms of future
>     expansion of the hashmap struct.  The fact that cmp_str_entry needs to
>     be in there prevents us from passing all zeros for the hashmap, and makes
>     me worry that STRMAP_INIT would just be more trouble than it is worth.

You can actually omit everything after cmp_str_entry, and those fields
would all get zero-initialized. But we also allow C99 designed
initializers these days. Coupled with the HASHMAP_INIT() I mentioned in
the earlier email, you'd have:

  #define STRMAP_INIT { \
		.map = HASHMAP_INIT(cmp_strmap_entry, NULL), \
		.strdup_strings = 1, \
	  }

which seems pretty maintainable.

> +static int cmp_strmap_entry(const void *hashmap_cmp_fn_data,
> +			    const struct hashmap_entry *entry1,
> +			    const struct hashmap_entry *entry2,
> +			    const void *keydata)
> +{
> +	const struct strmap_entry *e1, *e2;
> +
> +	e1 = container_of(entry1, const struct strmap_entry, ent);
> +	e2 = container_of(entry2, const struct strmap_entry, ent);
> +	return strcmp(e1->key, e2->key);
> +}

I expected to use keydata here, but it's pretty easy to make a fake
strmap_entry because of the use of the "key" pointer. So that makes
sense.

> +static void strmap_free_entries_(struct strmap *map, int free_util)

You use the term "value" for the mapped-to value in this iteration. So
perhaps free_values here (and in other functions) would be a better
name?

> +	/*
> +	 * We need to iterate over the hashmap entries and free
> +	 * e->key and e->value ourselves; hashmap has no API to
> +	 * take care of that for us.  Since we're already iterating over
> +	 * the hashmap, though, might as well free e too and avoid the need
> +	 * to make some call into the hashmap API to do that.
> +	 */
> +	hashmap_for_each_entry(&map->map, &iter, e, ent) {
> +		if (free_util)
> +			free(e->value);
> +		if (map->strdup_strings)
> +			free((char*)e->key);
> +		free(e);
> +	}
> +}

Yep, makes sense.

> +void strmap_clear(struct strmap *map, int free_util)
> +{
> +	strmap_free_entries_(map, free_util);
> +	hashmap_free(&map->map);
> +}

This made me wonder about a partial_clear(), but it looks like that
comes later.

> +void *strmap_put(struct strmap *map, const char *str, void *data)
> +{
> +	struct strmap_entry *entry = find_strmap_entry(map, str);
> +	void *old = NULL;
> +
> +	if (entry) {
> +		old = entry->value;
> +		entry->value = data;

Here's a weird hypothetical. If strdup_strings is not set and I do:

  const char *one = xstrdup("foo");
  const char *two = xstrdup("foo");

  hashmap_put(map, one, x);
  hashmap_put(map, two, y);

it's clear that the value should be pointing to "y" afterwards (and you
return "x" so the caller can free it or whatever, good).

But which key should the entry be pointing to? The old one or the new
one? I'm trying and failing to think of a case where it would matter.
Certainly I could add a free() to the toy above where it would, but it
feels like a real caller would have to have pretty convoluted memory
lifetime semantics for it to make a difference.

So I'm not really asking for a particular behavior, but just bringing it
up in case you can think of something relevant.

> +	} else {
> +		/*
> +		 * We won't modify entry->key so it really should be const.
> +		 */
> +		const char *key = str;

The "should be" here confused me. It _is_ const. I'd probably just
delete the comment entirely, but perhaps:

  /*
   * We'll store a const pointer. For non-duplicated strings, they belong
   * to the caller and we received them as const in the first place. For
   * our duplicated ones, they do point to memory we own, but they're
   * still conceptually constant within the lifetime of an entry.
   */

Though it might make more sense in the struct definition, not here.

> +void *strmap_get(struct strmap *map, const char *str)
> +{
> +	struct strmap_entry *entry = find_strmap_entry(map, str);
> +	return entry ? entry->value : NULL;
> +}

Just noting that the caller can't tell the difference between "no such
entry" and "the entry is storing NULL". I think the simplicity offered
by this interface makes it worth having (and being the primary one). If
some caller really needs to tell the difference between the two, we can
add another function later.

Obviously they could use strmap_contains(), but that would mean two hash
lookups.

> +/*
> + * Same as strmap_init, but for those who want to control the memory management
> + * carefully instead of using the default of strdup_strings=1.
> + * (OCD = Obsessive Compulsive Disorder, a joke that those who use this function
> + * are obsessing over minor details.)
> + */
> +void strmap_ocd_init(struct strmap *map,
> +		     int strdup_strings);

I'm not personally bothered by this name, but I wonder if some people
may be (because they have or know somebody who actually has OCD).

Perhaps strmap_init_with_options() would be a better name? It likewise
would extend well if we want to add other non-default options later.

-Peff

  reply	other threads:[~2020-10-30 14:12 UTC|newest]

Thread overview: 144+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-08-21 18:52 [PATCH 0/5] Add struct strmap and associated " Elijah Newren via GitGitGadget
2020-08-21 18:52 ` [PATCH 1/5] hashmap: add usage documentation explaining hashmap_free[_entries]() Elijah Newren via GitGitGadget
2020-08-21 19:22   ` Jeff King
2020-08-21 18:52 ` [PATCH 2/5] strmap: new utility functions Elijah Newren via GitGitGadget
2020-08-21 19:48   ` Jeff King
2020-08-21 18:52 ` [PATCH 3/5] strmap: add more " Elijah Newren via GitGitGadget
2020-08-21 19:58   ` Jeff King
2020-08-21 18:52 ` [PATCH 4/5] strmap: add strdup_strings option Elijah Newren via GitGitGadget
2020-08-21 20:01   ` Jeff King
2020-08-21 20:41     ` Elijah Newren
2020-08-21 21:03       ` Jeff King
2020-08-21 22:25         ` Elijah Newren
2020-08-28  7:08           ` Jeff King
2020-08-28 17:20             ` Elijah Newren
2020-08-21 18:52 ` [PATCH 5/5] strmap: add functions facilitating use as a string->int map Elijah Newren via GitGitGadget
2020-08-21 20:10   ` Jeff King
2020-08-21 20:51     ` Elijah Newren
2020-08-21 21:05       ` Jeff King
2020-08-21 20:16 ` [PATCH 0/5] Add struct strmap and associated utility functions Jeff King
2020-08-21 21:33   ` Elijah Newren
2020-08-21 22:28     ` Elijah Newren
2020-08-28  7:03     ` Jeff King
2020-08-28 15:29       ` Elijah Newren
2020-09-01  9:27         ` Jeff King
2020-10-13  0:40 ` [PATCH v2 00/10] " Elijah Newren via GitGitGadget
2020-10-13  0:40   ` [PATCH v2 01/10] hashmap: add usage documentation explaining hashmap_free[_entries]() Elijah Newren via GitGitGadget
2020-10-30 12:50     ` Jeff King
2020-10-30 19:55       ` Elijah Newren
2020-11-03 16:26         ` Jeff King
2020-11-03 16:48           ` Elijah Newren
2020-10-13  0:40   ` [PATCH v2 02/10] hashmap: adjust spacing to fix argument alignment Elijah Newren via GitGitGadget
2020-10-30 12:51     ` Jeff King
2020-10-13  0:40   ` [PATCH v2 03/10] hashmap: allow re-use after hashmap_free() Elijah Newren via GitGitGadget
2020-10-30 13:35     ` Jeff King
2020-10-30 15:37       ` Elijah Newren
2020-11-03 16:08         ` Jeff King
2020-11-03 16:16           ` Elijah Newren
2020-10-13  0:40   ` [PATCH v2 04/10] hashmap: introduce a new hashmap_partial_clear() Elijah Newren via GitGitGadget
2020-10-30 13:41     ` Jeff King
2020-10-30 16:03       ` Elijah Newren
2020-11-03 16:10         ` Jeff King
2020-10-13  0:40   ` [PATCH v2 05/10] strmap: new utility functions Elijah Newren via GitGitGadget
2020-10-30 14:12     ` Jeff King [this message]
2020-10-30 16:26       ` Elijah Newren
2020-10-13  0:40   ` [PATCH v2 06/10] strmap: add more " Elijah Newren via GitGitGadget
2020-10-30 14:23     ` Jeff King
2020-10-30 16:43       ` Elijah Newren
2020-11-03 16:12         ` Jeff King
2020-10-13  0:40   ` [PATCH v2 07/10] strmap: enable faster clearing and reusing of strmaps Elijah Newren via GitGitGadget
2020-10-30 14:27     ` Jeff King
2020-10-13  0:40   ` [PATCH v2 08/10] strmap: add functions facilitating use as a string->int map Elijah Newren via GitGitGadget
2020-10-30 14:39     ` Jeff King
2020-10-30 17:28       ` Elijah Newren
2020-11-03 16:20         ` Jeff King
2020-11-03 16:46           ` Elijah Newren
2020-10-13  0:40   ` [PATCH v2 09/10] strmap: add a strset sub-type Elijah Newren via GitGitGadget
2020-10-30 14:44     ` Jeff King
2020-10-30 18:02       ` Elijah Newren
2020-10-13  0:40   ` [PATCH v2 10/10] strmap: enable allocations to come from a mem_pool Elijah Newren via GitGitGadget
2020-10-30 14:56     ` Jeff King
2020-10-30 19:31       ` Elijah Newren
2020-11-03 16:24         ` Jeff King
2020-11-02 18:55   ` [PATCH v3 00/13] Add struct strmap and associated utility functions Elijah Newren via GitGitGadget
2020-11-02 18:55     ` [PATCH v3 01/13] hashmap: add usage documentation explaining hashmap_free[_entries]() Elijah Newren via GitGitGadget
2020-11-02 18:55     ` [PATCH v3 02/13] hashmap: adjust spacing to fix argument alignment Elijah Newren via GitGitGadget
2020-11-02 18:55     ` [PATCH v3 03/13] hashmap: allow re-use after hashmap_free() Elijah Newren via GitGitGadget
2020-11-02 18:55     ` [PATCH v3 04/13] hashmap: introduce a new hashmap_partial_clear() Elijah Newren via GitGitGadget
2020-11-02 18:55     ` [PATCH v3 05/13] hashmap: provide deallocation function names Elijah Newren via GitGitGadget
2020-11-02 18:55     ` [PATCH v3 06/13] strmap: new utility functions Elijah Newren via GitGitGadget
2020-11-02 18:55     ` [PATCH v3 07/13] strmap: add more " Elijah Newren via GitGitGadget
2020-11-04 20:13       ` Jeff King
2020-11-04 20:24         ` Elijah Newren
2020-11-02 18:55     ` [PATCH v3 08/13] strmap: enable faster clearing and reusing of strmaps Elijah Newren via GitGitGadget
2020-11-02 18:55     ` [PATCH v3 09/13] strmap: add functions facilitating use as a string->int map Elijah Newren via GitGitGadget
2020-11-04 20:21       ` Jeff King
2020-11-02 18:55     ` [PATCH v3 10/13] strmap: add a strset sub-type Elijah Newren via GitGitGadget
2020-11-04 20:31       ` Jeff King
2020-11-02 18:55     ` [PATCH v3 11/13] strmap: enable allocations to come from a mem_pool Elijah Newren via GitGitGadget
2020-11-02 18:55     ` [PATCH v3 12/13] strmap: take advantage of FLEXPTR_ALLOC_STR when relevant Elijah Newren via GitGitGadget
2020-11-04 20:43       ` Jeff King
2020-11-02 18:55     ` [PATCH v3 13/13] Use new HASHMAP_INIT macro to simplify hashmap initialization Elijah Newren via GitGitGadget
2020-11-04 20:48       ` Jeff King
2020-11-04 20:52     ` [PATCH v3 00/13] Add struct strmap and associated utility functions Jeff King
2020-11-04 22:20       ` Elijah Newren
2020-11-05  0:22     ` [PATCH v4 " Elijah Newren via GitGitGadget
2020-11-05  0:22       ` [PATCH v4 01/13] hashmap: add usage documentation explaining hashmap_free[_entries]() Elijah Newren via GitGitGadget
2020-11-05  0:22       ` [PATCH v4 02/13] hashmap: adjust spacing to fix argument alignment Elijah Newren via GitGitGadget
2020-11-05  0:22       ` [PATCH v4 03/13] hashmap: allow re-use after hashmap_free() Elijah Newren via GitGitGadget
2020-11-05  0:22       ` [PATCH v4 04/13] hashmap: introduce a new hashmap_partial_clear() Elijah Newren via GitGitGadget
2020-11-05  0:22       ` [PATCH v4 05/13] hashmap: provide deallocation function names Elijah Newren via GitGitGadget
2020-11-05  0:22       ` [PATCH v4 06/13] strmap: new utility functions Elijah Newren via GitGitGadget
2020-11-05  0:22       ` [PATCH v4 07/13] strmap: add more " Elijah Newren via GitGitGadget
2020-11-05  0:22       ` [PATCH v4 08/13] strmap: enable faster clearing and reusing of strmaps Elijah Newren via GitGitGadget
2020-11-05  0:22       ` [PATCH v4 09/13] strmap: add functions facilitating use as a string->int map Elijah Newren via GitGitGadget
2020-11-05  0:22       ` [PATCH v4 10/13] strmap: add a strset sub-type Elijah Newren via GitGitGadget
2020-11-05  0:22       ` [PATCH v4 11/13] strmap: enable allocations to come from a mem_pool Elijah Newren via GitGitGadget
2020-11-05  0:22       ` [PATCH v4 12/13] strmap: take advantage of FLEXPTR_ALLOC_STR when relevant Elijah Newren via GitGitGadget
2020-11-05  0:22       ` [PATCH v4 13/13] Use new HASHMAP_INIT macro to simplify hashmap initialization Elijah Newren via GitGitGadget
2020-11-05 13:29       ` [PATCH v4 00/13] Add struct strmap and associated utility functions Jeff King
2020-11-05 20:25         ` Junio C Hamano
2020-11-05 21:17           ` Jeff King
2020-11-05 21:22           ` Elijah Newren
2020-11-05 22:15             ` Junio C Hamano
2020-11-06  0:24       ` [PATCH v5 00/15] " Elijah Newren via GitGitGadget
2020-11-06  0:24         ` [PATCH v5 01/15] hashmap: add usage documentation explaining hashmap_free[_entries]() Elijah Newren via GitGitGadget
2020-11-06  0:24         ` [PATCH v5 02/15] hashmap: adjust spacing to fix argument alignment Elijah Newren via GitGitGadget
2020-11-06  0:24         ` [PATCH v5 03/15] hashmap: allow re-use after hashmap_free() Elijah Newren via GitGitGadget
2020-11-06  0:24         ` [PATCH v5 04/15] hashmap: introduce a new hashmap_partial_clear() Elijah Newren via GitGitGadget
2020-11-06  0:24         ` [PATCH v5 05/15] hashmap: provide deallocation function names Elijah Newren via GitGitGadget
2020-11-06  0:24         ` [PATCH v5 06/15] strmap: new utility functions Elijah Newren via GitGitGadget
2020-11-06  0:24         ` [PATCH v5 07/15] strmap: add more " Elijah Newren via GitGitGadget
2020-11-06  0:24         ` [PATCH v5 08/15] strmap: enable faster clearing and reusing of strmaps Elijah Newren via GitGitGadget
2020-11-06  0:24         ` [PATCH v5 09/15] strmap: add functions facilitating use as a string->int map Elijah Newren via GitGitGadget
2020-11-06  0:24         ` [PATCH v5 10/15] strmap: split create_entry() out of strmap_put() Elijah Newren via GitGitGadget
2020-11-06  0:24         ` [PATCH v5 11/15] strmap: add a strset sub-type Elijah Newren via GitGitGadget
2020-11-06  0:24         ` [PATCH v5 12/15] strmap: enable allocations to come from a mem_pool Elijah Newren via GitGitGadget
2020-11-11 17:33           ` Phillip Wood
2020-11-11 18:49             ` Elijah Newren
2020-11-11 19:01             ` Jeff King
2020-11-11 20:34               ` Chris Torek
2020-11-06  0:24         ` [PATCH v5 13/15] strmap: take advantage of FLEXPTR_ALLOC_STR when relevant Elijah Newren via GitGitGadget
2020-11-06  0:24         ` [PATCH v5 14/15] Use new HASHMAP_INIT macro to simplify hashmap initialization Elijah Newren via GitGitGadget
2020-11-06  0:24         ` [PATCH v5 15/15] shortlog: use strset from strmap.h Elijah Newren via GitGitGadget
2020-11-06  2:00         ` [PATCH v5 00/15] Add struct strmap and associated utility functions Junio C Hamano
2020-11-06  2:42           ` Elijah Newren
2020-11-06  2:48             ` Jeff King
2020-11-06 17:32               ` Junio C Hamano
2020-11-11 20:02         ` [PATCH v6 " Elijah Newren via GitGitGadget
2020-11-11 20:02           ` [PATCH v6 01/15] hashmap: add usage documentation explaining hashmap_free[_entries]() Elijah Newren via GitGitGadget
2020-11-11 20:02           ` [PATCH v6 02/15] hashmap: adjust spacing to fix argument alignment Elijah Newren via GitGitGadget
2020-11-11 20:02           ` [PATCH v6 03/15] hashmap: allow re-use after hashmap_free() Elijah Newren via GitGitGadget
2020-11-11 20:02           ` [PATCH v6 04/15] hashmap: introduce a new hashmap_partial_clear() Elijah Newren via GitGitGadget
2020-11-11 20:02           ` [PATCH v6 05/15] hashmap: provide deallocation function names Elijah Newren via GitGitGadget
2020-11-11 20:02           ` [PATCH v6 06/15] strmap: new utility functions Elijah Newren via GitGitGadget
2020-11-11 20:02           ` [PATCH v6 07/15] strmap: add more " Elijah Newren via GitGitGadget
2020-11-11 20:02           ` [PATCH v6 08/15] strmap: enable faster clearing and reusing of strmaps Elijah Newren via GitGitGadget
2020-11-11 20:02           ` [PATCH v6 09/15] strmap: add functions facilitating use as a string->int map Elijah Newren via GitGitGadget
2020-11-11 20:02           ` [PATCH v6 10/15] strmap: split create_entry() out of strmap_put() Elijah Newren via GitGitGadget
2020-11-11 20:02           ` [PATCH v6 11/15] strmap: add a strset sub-type Elijah Newren via GitGitGadget
2020-11-11 20:02           ` [PATCH v6 12/15] strmap: enable allocations to come from a mem_pool Elijah Newren via GitGitGadget
2020-11-11 20:02           ` [PATCH v6 13/15] strmap: take advantage of FLEXPTR_ALLOC_STR when relevant Elijah Newren via GitGitGadget
2020-11-11 20:02           ` [PATCH v6 14/15] Use new HASHMAP_INIT macro to simplify hashmap initialization Elijah Newren via GitGitGadget
2020-11-11 20:02           ` [PATCH v6 15/15] shortlog: use strset from strmap.h Elijah Newren via GitGitGadget
2020-11-11 20:07           ` [PATCH v6 00/15] Add struct strmap and associated utility functions Jeff King

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