Re: [FFmpeg-devel] [PATCH v2 1/2] lavu/hashtable: create generic robin hood hash table

[Andreas Rheinhardt <andreas.rheinhardt@outlook.com>]

Connor Worley:
> Signed-off-by: Connor Worley <connorbworley@gmail.com>
> ---
>  libavutil/Makefile          |   2 +
>  libavutil/hashtable.c       | 183 ++++++++++++++++++++++++++++++++++++
>  libavutil/hashtable.h       |  47 +++++++++
>  libavutil/tests/hashtable.c | 108 +++++++++++++++++++++
>  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..6fa2af8af3
> --- /dev/null
> +++ b/libavutil/hashtable.c
> @@ -0,0 +1,183 @@
> +/*
> + * 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 <string.h>
> +
> +#include "crc.h"
> +#include "error.h"
> +#include "mem.h"
> +#include "hashtable.h"
> +
> +struct AVHashtableContext {
> +    size_t key_size;
> +    size_t val_size;
> +    size_t entry_size;
> +    size_t max_entries;
> +    size_t utilization;
> +    const AVCRC *crc;
> +    uint8_t *table;
> +    uint8_t *swapbuf;
> +};
> +
> +#define ENTRY_PSL(entry) (entry)
> +#define ENTRY_OCC(entry) (ENTRY_PSL(entry) + sizeof(size_t))
> +#define ENTRY_KEY(entry) (ENTRY_OCC(entry) + 4)
> +#define ENTRY_VAL(entry) (ENTRY_KEY(entry) + FFALIGN(ctx->key_size, 4))
> +

It seems you misunderstood what I said about alignment in my last
review. You should align entry_size and not align the size of key and
val fields. The way you are doing it now does not prevent unaligned
accesses: Imagine the common case of size_t having an alignment
requirement of 8 and key_size and val_size being four. Then entry_size
is not a multiple of 8 and therefore of two consecutive entries, only
one can be properly aligned (e.g. the second entry
ctx->table+ctx->entry_size is misaligned).
(For the same reason, the alignment of a struct is a multiple of the
alignment of every member of said struct.)
The way you are doing it also adds FFALIGN to every access to val.

> +#define KEYS_EQUAL(k1, k2) !memcmp(k1, k2, ctx->key_size)
> +
> +int av_hashtable_alloc(struct AVHashtableContext** ctx, size_t key_size, size_t val_size, size_t max_entries)
> +{
> +    struct AVHashtableContext *res = av_malloc(sizeof(struct AVHashtableContext));
> +    if (!res)
> +        return AVERROR(ENOMEM);
> +    res->key_size = key_size;
> +    res->val_size = val_size;

You mostly only need key_size and key_size+val_size; val_size is not
used that often, so if you only store two sizes, it should not be stored.

> +    res->entry_size = sizeof(size_t) + 4 + FFALIGN(key_size, 4) + FFALIGN(val_size, 4);
> +    res->max_entries = max_entries;
> +    res->utilization = 0;
> +    res->crc = av_crc_get_table(AV_CRC_32_IEEE);
> +    if (!res->crc) {
> +        av_hashtable_freep(&res);
> +        return AVERROR_BUG;
> +    }
> +    res->table = av_calloc(res->max_entries, res->entry_size);
> +    if (!res->table) {
> +        av_hashtable_freep(&res);
> +        return AVERROR(ENOMEM);
> +    }
> +    res->swapbuf = av_calloc(2, FFALIGN(key_size, 4) + FFALIGN(val_size, 4));
> +    if (!res->swapbuf) {
> +        av_hashtable_freep(&res);
> +        return AVERROR(ENOMEM);
> +    }
> +    *ctx = res;
> +    return 0;
> +}
> +
> +static size_t hash_key(const struct AVHashtableContext* ctx, const void* key)
> +{
> +    return av_crc(ctx->crc, 0, key, ctx->key_size) % ctx->max_entries;
> +}
> +
> +int av_hashtable_get(const struct 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) || *(size_t*)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(struct AVHashtableContext* ctx, const void* key, const void* val)
> +{
> +    int swapping = 0;
> +    size_t psl = 0;
> +    size_t hash = hash_key(ctx, key);
> +    uint8_t *set = ctx->swapbuf;
> +    uint8_t *tmp = ctx->swapbuf + FFALIGN(ctx->key_size, 4) + FFALIGN(ctx->val_size, 4);
> +
> +    memcpy(set, key, ctx->key_size);
> +    memcpy(set + FFALIGN(ctx->key_size, 4), 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), set))) {
> +            if (!*ENTRY_OCC(entry))
> +                ctx->utilization++;
> +            *(size_t*)ENTRY_PSL(entry) = psl;
> +            *ENTRY_OCC(entry) = 1;
> +            memcpy(ENTRY_KEY(entry), set, FFALIGN(ctx->key_size, 4) + ctx->val_size);
> +            return 1;
> +        }
> +        if (*(size_t*)ENTRY_PSL(entry) < psl) {
> +            if (ctx->utilization == ctx->max_entries)
> +                return 0;
> +            swapping = 1;
> +            // set needs to swap with entry
> +            memcpy(tmp, ENTRY_KEY(entry), FFALIGN(ctx->key_size, 4) + ctx->val_size);
> +            memcpy(ENTRY_KEY(entry), set, FFALIGN(ctx->key_size, 4) + ctx->val_size);
> +            FFSWAP(uint8_t*, set, tmp);
> +            FFSWAP(size_t, psl, *(size_t*)ENTRY_PSL(entry));
> +        }
> +        psl++;
> +    }
> +    return 0;
> +}
> +
> +int av_hashtable_delete(struct 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) || *(size_t*)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) || !*(size_t*)ENTRY_PSL(next_entry)) {
> +                    ctx->utilization--;
> +                    return 1;
> +                }
> +                memcpy(ENTRY_OCC(entry), ENTRY_OCC(next_entry), ctx->entry_size);
> +                *(size_t*)ENTRY_PSL(entry)--;
> +                *ENTRY_OCC(next_entry) = 0;
> +                entry = next_entry;
> +            }
> +        }
> +    };

This ';' is a null statement.

> +    return 0;
> +}
> +
> +void av_hashtable_clear(struct AVHashtableContext* ctx)
> +{
> +    memset(ctx->table, 0, ctx->entry_size * ctx->max_entries);
> +}
> +
> +void av_hashtable_freep(struct AVHashtableContext** ctx)
> +{
> +    if (*ctx) {
> +        av_freep(&(*ctx)->table);
> +        av_freep(&(*ctx)->swapbuf);
> +    }
> +    av_freep(ctx);
> +}
> diff --git a/libavutil/hashtable.h b/libavutil/hashtable.h
> new file mode 100644
> index 0000000000..90c6cad68a
> --- /dev/null
> +++ b/libavutil/hashtable.h
> @@ -0,0 +1,47 @@
> +/*
> + * 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>
> +
> +/* 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.

This is no longer true and it definitely does not belong in a header.

> + */
> +
> +struct AVHashtableContext;
> +
> +int av_hashtable_alloc(struct AVHashtableContext** ctx, size_t key_size, size_t val_size, size_t max_entries);
> +int av_hashtable_get(const struct AVHashtableContext* ctx, const void* key, void* val);
> +int av_hashtable_set(struct AVHashtableContext* ctx, const void* key, const void* val);
> +int av_hashtable_delete(struct AVHashtableContext* ctx, const void* key);
> +void av_hashtable_clear(struct AVHashtableContext* ctx);
> +void av_hashtable_freep(struct AVHashtableContext** ctx);
> +
> +#endif
> diff --git a/libavutil/tests/hashtable.c b/libavutil/tests/hashtable.c
> new file mode 100644
> index 0000000000..0b38b34c5c
> --- /dev/null
> +++ b/libavutil/tests/hashtable.c
> @@ -0,0 +1,108 @@
> +/*
> + * 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)
> +{
> +    struct AVHashtableContext *ctx;
> +    uint8_t k;
> +    uint64_t v;
> +
> +    // impossibly large allocation should fail gracefully

Actually, av_hashtable_alloc is missing overflow checks.

> +    av_assert0(av_hashtable_alloc(&ctx, -1, -1, -1) < 0);
> +
> +    // hashtable can store up to 3 uint8_t->uint64_t entries
> +    av_assert0(!av_hashtable_alloc(&ctx, sizeof(k), sizeof(v), 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_freep(&ctx);
> +
> +    return 0;
> +}

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