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Subject: Re: [FFmpeg-devel] [PATCH 1/2] lavu/hashtable: create generic robin
hood hash table
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Connor Worley:
> Signed-off-by: Connor Worley <connorbworley@gmail.com>
> ---
> libavutil/Makefile | 2 +
> libavutil/hashtable.c | 172 ++++++++++++++++++++++++++++++++++++
> libavutil/hashtable.h | 62 +++++++++++++
> libavutil/tests/hashtable.c | 104 ++++++++++++++++++++++
> 4 files changed, 340 insertions(+)
> create mode 100644 libavutil/hashtable.c
> create mode 100644 libavutil/hashtable.h
> create mode 100644 libavutil/tests/hashtable.c
>
> diff --git a/libavutil/Makefile b/libavutil/Makefile
> index e7709b97d0..be75d464fc 100644
> --- a/libavutil/Makefile
> +++ b/libavutil/Makefile
> @@ -138,6 +138,7 @@ OBJS = adler32.o \
> fixed_dsp.o \
> frame.o \
> hash.o \
> + hashtable.o \
> hdr_dynamic_metadata.o \
> hdr_dynamic_vivid_metadata.o \
> hmac.o \
> @@ -251,6 +252,7 @@ TESTPROGS = adler32 \
> file \
> fifo \
> hash \
> + hashtable \
> hmac \
> hwdevice \
> integer \
> diff --git a/libavutil/hashtable.c b/libavutil/hashtable.c
> new file mode 100644
> index 0000000000..fb0f0aae99
> --- /dev/null
> +++ b/libavutil/hashtable.c
> @@ -0,0 +1,172 @@
> +/*
> + * Generic hashtable
> + * Copyright (C) 2024 Connor Worley <connorbworley@gmail.com>
> + *
> + * This file is part of FFmpeg.
> + *
> + * FFmpeg is free software; you can redistribute it and/or
> + * modify it under the terms of the GNU Lesser General Public
> + * License as published by the Free Software Foundation; either
> + * version 2.1 of the License, or (at your option) any later version.
> + *
> + * FFmpeg is distributed in the hope that it will be useful,
> + * but WITHOUT ANY WARRANTY; without even the implied warranty of
> + * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
> + * Lesser General Public License for more details.
> + *
> + * You should have received a copy of the GNU Lesser General Public
> + * License along with FFmpeg; if not, write to the Free Software
> + * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
> + */
> +
> +#include <stdint.h>
> +
> +#include "error.h"
> +#include "mem.h"
> +#include "hashtable.h"
> +#include "string.h"
We don't have a string.h header; the string.h from the C stdlib should
of course be included via <>.
> +
> +#define ENTRY_OCC(entry) (entry)
> +#define ENTRY_KEY(entry) (ENTRY_OCC(entry) + 1)
> +#define ENTRY_VAL(entry) (ENTRY_KEY(entry) + ctx->key_size)
> +#define ENTRY_PSL(entry) (size_t*) (ENTRY_VAL(entry) + ctx->val_size)
Unaligned access. You need to align entry_size to the required alignment
of size_t and put this at the start of it the entry to avoid this.
Apart from that: The layout of your entries implies that only two
members are at a fixed offset from the start, so that accessing val and
psl requires actual additions. If you make it psl, occ, key, val, then
only val has a variable offset. This should be beneficial given that it
is the less accessed of key and val (in particular if you combine
several memcpy calls as I outline below).
(This is of course a good reason not to make the layout of the entries
public.)
> +
> +#define KEYS_EQUAL(k1, k2) !memcmp(k1, k2, ctx->key_size)
> +
> +int av_hashtable_init(AVHashtableContext* ctx, size_t key_size, size_t val_size, size_t max_entries)
> +{
> + ctx->key_size = key_size;
> + ctx->val_size = val_size;
> + ctx->entry_size = 1 + key_size + val_size + sizeof(size_t);
> + ctx->max_entries = max_entries;
> + ctx->utilization = 0;
> + ctx->crc = av_crc_get_table(AV_CRC_32_IEEE);
> + if (!ctx->crc)
> + return AVERROR_BUG;
> + ctx->table = av_mallocz(ctx->entry_size * max_entries);
av_calloc()
> + if (!ctx->table)
> + return AVERROR(ENOMEM);
> + ctx->set_key = av_malloc(key_size);
> + if (!ctx->set_key)
> + return AVERROR(ENOMEM);
> + ctx->set_val = av_malloc(key_size);
Should be val_size. Your test should test scenarios with key_size !=
val_size to uncover this.
> + if (!ctx->set_val)
> + return AVERROR(ENOMEM);
> + ctx->tmp_key = av_malloc(key_size);
> + if (!ctx->tmp_key)
> + return AVERROR(ENOMEM);
> + ctx->tmp_val = av_malloc(val_size);
> + if (!ctx->tmp_val)
> + return AVERROR(ENOMEM);
> + return 0;
> +}
(There are btw issues with what happens on error here, but we can ignore
this in the first round.)
> +
> +static size_t hash_key(const AVHashtableContext* ctx, const void* key)
> +{
> + return av_crc(ctx->crc, 0, key, ctx->key_size) % ctx->max_entries;
> +}
> +
> +int av_hashtable_get(const AVHashtableContext* ctx, const void* key, void* val)
> +{
> + size_t hash = hash_key(ctx, key);
> +
> + if (!ctx->utilization)
> + return 0;
> +
> + for (size_t psl = 0; psl < ctx->max_entries; psl++) {
> + size_t wrapped_index = (hash + psl) % ctx->max_entries;
> + uint8_t *entry = ctx->table + wrapped_index * ctx->entry_size;
> + if (!*ENTRY_OCC(entry) || *ENTRY_PSL(entry) < psl)
> + return 0;
> + if (KEYS_EQUAL(ENTRY_KEY(entry), key)) {
> + memcpy(val, ENTRY_VAL(entry), ctx->val_size);
> + return 1;
> + }
> + }
> + return 0;
> +}
> +
> +int av_hashtable_set(AVHashtableContext* ctx, const void* key, const void* val)
> +{
> + int swapping = 0;
> + size_t psl = 0;
> + size_t hash = hash_key(ctx, key);
> +
> + memcpy(ctx->set_key, key, ctx->key_size);
> + memcpy(ctx->set_val, val, ctx->val_size);
> +
> + for (size_t i = 0; i < ctx->max_entries; i++) {
> + size_t wrapped_index = (hash + i) % ctx->max_entries;
> + uint8_t *entry = ctx->table + wrapped_index * ctx->entry_size;
> + if (!*ENTRY_OCC(entry) || (!swapping && KEYS_EQUAL(ENTRY_KEY(entry), ctx->set_key))) {
> + if (!*ENTRY_OCC(entry))
> + ctx->utilization++;
> + *ENTRY_OCC(entry) = 1;
> + memcpy(ENTRY_KEY(entry), ctx->set_key, ctx->key_size);
> + memcpy(ENTRY_VAL(entry), ctx->set_val, ctx->val_size);
> + *ENTRY_PSL(entry) = psl;
> + return 1;
> + }
> + if (*ENTRY_PSL(entry) < psl) {
> + if (ctx->utilization == ctx->max_entries)
> + return 0;
> + swapping = 1;
> + memcpy(ctx->tmp_key, ctx->set_key, ctx->key_size);
> + memcpy(ctx->tmp_val, ctx->set_val, ctx->val_size);
Given that set_key/val are overwritten below, these two can be done via
pointerswaps.
> + memcpy(ctx->set_key, ENTRY_KEY(entry), ctx->key_size);
> + memcpy(ctx->set_val, ENTRY_VAL(entry), ctx->val_size);
> + memcpy(ENTRY_KEY(entry), ctx->tmp_key, ctx->key_size);
> + memcpy(ENTRY_VAL(entry), ctx->tmp_val, ctx->val_size);
> + FFSWAP(size_t, psl, *ENTRY_PSL(entry));
If you allocated foo_key and foo_val jointly, you can combine these
memcpy's. In fact, you can allocate all four them jointly with a
key-val-key-val layout and only keep one pointer in the context. You
would then use an uint8_t *set, *val pointers on the stack of this
function that are initialized at the beginning of this function.
> + }
> + psl++;
> + }
> + return 0;
> +}
> +
> +int av_hashtable_delete(AVHashtableContext* ctx, const void* key)
> +{
> + uint8_t *next_entry;
> + size_t hash = hash_key(ctx, key);
> +
> + if (!ctx->utilization)
> + return 0;
> +
> + for (size_t psl = 0; psl < ctx->max_entries; psl++) {
> + size_t wrapped_index = (hash + psl) % ctx->max_entries;
> + uint8_t *entry = ctx->table + wrapped_index * ctx->entry_size;
> + if (!*ENTRY_OCC(entry) || *ENTRY_PSL(entry) < psl)
> + return 0;
> + if (KEYS_EQUAL(ENTRY_KEY(entry), key)) {
> + *ENTRY_OCC(entry) = 0;
> + for (psl++; psl < ctx->max_entries; psl++) {
> + wrapped_index = (hash + psl) % ctx->max_entries;
> + next_entry = ctx->table + wrapped_index * ctx->entry_size;
> + if (!*ENTRY_OCC(next_entry) || !*ENTRY_PSL(next_entry)) {
> + ctx->utilization--;
> + return 1;
> + }
> + FFSWAP(uint8_t, *ENTRY_OCC(entry), *ENTRY_OCC(next_entry));
> + memcpy(ENTRY_KEY(entry), ENTRY_KEY(next_entry), ctx->key_size);
> + memcpy(ENTRY_VAL(entry), ENTRY_VAL(next_entry), ctx->val_size);
These two memcpy can be combined (if one lays out key and val
sequentially in entry. Can even be combined with occ if you change the
entry layout as I outlined above.
> + *ENTRY_PSL(entry) = *ENTRY_PSL(next_entry) - 1;
> + entry = next_entry;
> + }
> + }
> + };
> + return 0;
> +}
> +
> +void av_hashtable_clear(AVHashtableContext* ctx)
> +{
> + memset(ctx->table, 0, ctx->entry_size * ctx->max_entries);
> +}
> +
> +void av_hashtable_destroy(AVHashtableContext* ctx)
> +{
> + av_freep(&ctx->table);
> + av_freep(&ctx->set_key);
> + av_freep(&ctx->set_val);
> + av_freep(&ctx->tmp_key);
> + av_freep(&ctx->tmp_val);
> +}
> \ No newline at end of file
> diff --git a/libavutil/hashtable.h b/libavutil/hashtable.h
> new file mode 100644
> index 0000000000..aac31ee922
> --- /dev/null
> +++ b/libavutil/hashtable.h
> @@ -0,0 +1,62 @@
> +/*
> + * Generic hashtable
> + * Copyright (C) 2024 Connor Worley <connorbworley@gmail.com>
> + *
> + * This file is part of FFmpeg.
> + *
> + * FFmpeg is free software; you can redistribute it and/or
> + * modify it under the terms of the GNU Lesser General Public
> + * License as published by the Free Software Foundation; either
> + * version 2.1 of the License, or (at your option) any later version.
> + *
> + * FFmpeg is distributed in the hope that it will be useful,
> + * but WITHOUT ANY WARRANTY; without even the implied warranty of
> + * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
> + * Lesser General Public License for more details.
> + *
> + * You should have received a copy of the GNU Lesser General Public
> + * License along with FFmpeg; if not, write to the Free Software
> + * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
> + */
> +
> +#ifndef AVUTIL_HASHTABLE_H
> +#define AVUTIL_HASHTABLE_H
> +
> +#include <stddef.h>
> +
> +#include "crc.h"
> +
> +/* Implements a hash table using Robin Hood open addressing.
> + * See: https://cs.uwaterloo.ca/research/tr/1986/CS-86-14.pdf
> + *
> + * Entry slots are layed out in memory as follows:
> + * - [1 byte] occupancy flag that indicates whether a slot if occupied.
> + * - [key_size bytes] entry key.
> + * - [val_size bytes] entry value.
> + * - [sizeof(size_t) bytes] PSL (probe sequence length), used by the Robin Hood
> + * algorithm.
Such things should not be made public. Even if
sizeof(AVHashtableContext) were part of the ABI.
> + */
> +
> +typedef struct AVHashtableContext {
> + size_t key_size;
> + size_t val_size;
> + size_t entry_size;
> + size_t max_entries;
> + size_t utilization;
> + const AVCRC *crc;
You don't need the complete type here, so by using struct AVCRC *crc you
can avoid the crc.h inclusion.
> + uint8_t *table;
> + /* used for swaps during insertion */
> + uint8_t *set_key;
> + uint8_t *set_val;
> + uint8_t *tmp_key;
> + uint8_t *tmp_val;
> +} AVHashtableContext;
In patch 2/2 you are using this in a way that makes
sizeof(AVHashtableContext) part of the ABI. Is this intended?
Even if it is, the user should be forbidden from accessing anything in it.
> +
> +int av_hashtable_init(AVHashtableContext* ctx, size_t key_size, size_t val_size, size_t max_entries);
We generally use
> +int av_hashtable_get(const AVHashtableContext* ctx, const void* key, void* val);
> +int av_hashtable_set(AVHashtableContext* ctx, const void* key, const void* val);
> +int av_hashtable_delete(AVHashtableContext* ctx, const void* key);
> +void av_hashtable_clear(AVHashtableContext* ctx);
> +void av_hashtable_destroy(AVHashtableContext* ctx);
> +
> +#endif
> \ No newline at end of file
> diff --git a/libavutil/tests/hashtable.c b/libavutil/tests/hashtable.c
> new file mode 100644
> index 0000000000..1cdbd7afd3
> --- /dev/null
> +++ b/libavutil/tests/hashtable.c
> @@ -0,0 +1,104 @@
> +/*
> + * Generic hashtable tests
> + * Copyright (C) 2024 Connor Worley <connorbworley@gmail.com>
> + *
> + * This file is part of FFmpeg.
> + *
> + * FFmpeg is free software; you can redistribute it and/or
> + * modify it under the terms of the GNU Lesser General Public
> + * License as published by the Free Software Foundation; either
> + * version 2.1 of the License, or (at your option) any later version.
> + *
> + * FFmpeg is distributed in the hope that it will be useful,
> + * but WITHOUT ANY WARRANTY; without even the implied warranty of
> + * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
> + * Lesser General Public License for more details.
> + *
> + * You should have received a copy of the GNU Lesser General Public
> + * License along with FFmpeg; if not, write to the Free Software
> + * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
> + */
> +
> +#include "libavutil/avassert.h"
> +#include "libavutil/hashtable.h"
> +
> +int main(void)
> +{
> + AVHashtableContext ctx;
> + int k, v;
> +
> + // hashtable can store up to 3 int->int entries
> + av_assert0(!av_hashtable_init(&ctx, sizeof(int), sizeof(int), 3));
> +
> + // unsuccessful deletes return 0
> + k = 1;
> + av_assert0(!av_hashtable_delete(&ctx, &k));
> +
> + // unsuccessful gets return 0
> + k = 1;
> + av_assert0(!av_hashtable_get(&ctx, &k, &v));
> +
> + // successful sets returns 1
> + k = 1;
> + v = 1;
> + av_assert0(av_hashtable_set(&ctx, &k, &v));
> +
> + // get should now contain 1
> + k = 1;
> + v = 0;
> + av_assert0(av_hashtable_get(&ctx, &k, &v));
> + av_assert0(v == 1);
> +
> + // updating sets should return 1
> + k = 1;
> + v = 2;
> + av_assert0(av_hashtable_set(&ctx, &k, &v));
> +
> + // get should now contain 2
> + k = 1;
> + v = 0;
> + av_assert0(av_hashtable_get(&ctx, &k, &v));
> + av_assert0(v == 2);
> +
> + // fill the table
> + k = 2;
> + v = 2;
> + av_assert0(av_hashtable_set(&ctx, &k, &v));
> + k = 3;
> + v = 3;
> + av_assert0(av_hashtable_set(&ctx, &k, &v));
> +
> + // inserting sets on a full table should return 0
> + k = 4;
> + v = 4;
> + av_assert0(!av_hashtable_set(&ctx, &k, &v));
> +
> + // updating sets on a full table should return 1
> + k = 1;
> + v = 4;
> + av_assert0(av_hashtable_set(&ctx, &k, &v));
> + v = 0;
> + av_assert0(av_hashtable_get(&ctx, &k, &v));
> + av_assert0(v == 4);
> +
> + // successful deletes should return 1
> + k = 1;
> + av_assert0(av_hashtable_delete(&ctx, &k));
> +
> + // get should now return 0
> + av_assert0(!av_hashtable_get(&ctx, &k, &v));
> +
> + // sanity check remaining keys
> + k = 2;
> + v = 0;
> + av_assert0(av_hashtable_get(&ctx, &k, &v));
> + av_assert0(v == 2);
> + k = 3;
> + v = 0;
> + av_assert0(av_hashtable_get(&ctx, &k, &v));
> + av_assert0(v == 3);
> +
> + av_hashtable_destroy(&ctx);
> +
> + return 0;
> +}
> \ No newline at end of file
You should add these newlines, they are AFAIK required by the spec.
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