SDL/src/SDL_hashtable.c

/*
  Simple DirectMedia Layer
  Copyright (C) 1997-2024 Sam Lantinga <slouken@libsdl.org>

  This software is provided 'as-is', without any express or implied
  warranty.  In no event will the authors be held liable for any damages
  arising from the use of this software.

  Permission is granted to anyone to use this software for any purpose,
  including commercial applications, and to alter it and redistribute it
  freely, subject to the following restrictions:

  1. The origin of this software must not be misrepresented; you must not
     claim that you wrote the original software. If you use this software
     in a product, an acknowledgment in the product documentation would be
     appreciated but is not required.
  2. Altered source versions must be plainly marked as such, and must not be
     misrepresented as being the original software.
  3. This notice may not be removed or altered from any source distribution.
*/
#include "SDL_internal.h"
#include "SDL_hashtable.h"

typedef struct SDL_HashItem
{
    const void *key;
    const void *value;
    struct SDL_HashItem *next;
} SDL_HashItem;

struct SDL_HashTable
{
    SDL_HashItem **table;
    Uint32 table_len;
    int hash_shift;
    bool stackable;
    void *data;
    SDL_HashTable_HashFn hash;
    SDL_HashTable_KeyMatchFn keymatch;
    SDL_HashTable_NukeFn nuke;
};

SDL_HashTable *SDL_CreateHashTable(void *data, const Uint32 num_buckets, const SDL_HashTable_HashFn hashfn,
                                   const SDL_HashTable_KeyMatchFn keymatchfn,
                                   const SDL_HashTable_NukeFn nukefn,
                                   const bool stackable)
{
    SDL_HashTable *table;

    // num_buckets must be a power of two so we can derive the bucket index with just a bitshift.
    // Need at least two buckets, otherwise hash_shift would be 32, which is UB!
    if ((num_buckets < 2) || !SDL_HasExactlyOneBitSet32(num_buckets)) {
        SDL_SetError("num_buckets must be a power of two");
        return NULL;
    }

    table = (SDL_HashTable *) SDL_calloc(1, sizeof (SDL_HashTable));
    if (!table) {
        return NULL;
    }

    table->table = (SDL_HashItem **) SDL_calloc(num_buckets, sizeof (SDL_HashItem *));
    if (!table->table) {
        SDL_free(table);
        return NULL;
    }

    table->table_len = num_buckets;
    table->hash_shift = 32 - SDL_MostSignificantBitIndex32(num_buckets);
    table->stackable = stackable;
    table->data = data;
    table->hash = hashfn;
    table->keymatch = keymatchfn;
    table->nuke = nukefn;
    return table;
}

static SDL_INLINE Uint32 calc_hash(const SDL_HashTable *table, const void *key)
{
    // Mix the bits together, and use the highest bits as the bucket index.
    const Uint32 BitMixer = 0x9E3779B1u;
    return (table->hash(key, table->data) * BitMixer) >> table->hash_shift;
}


bool SDL_InsertIntoHashTable(SDL_HashTable *table, const void *key, const void *value)
{
    SDL_HashItem *item;
    Uint32 hash;

    if (!table) {
        return false;
    }

    if ( (!table->stackable) && (SDL_FindInHashTable(table, key, NULL)) ) {
        return false;
    }

    // !!! FIXME: grow and rehash table if it gets too saturated.
    item = (SDL_HashItem *) SDL_malloc(sizeof (SDL_HashItem));
    if (!item) {
        return false;
    }

    hash = calc_hash(table, key);

    item->key = key;
    item->value = value;
    item->next = table->table[hash];
    table->table[hash] = item;

    return true;
}

bool SDL_FindInHashTable(const SDL_HashTable *table, const void *key, const void **_value)
{
    Uint32 hash;
    void *data;
    SDL_HashItem *i;

    if (!table) {
        return false;
    }

    hash = calc_hash(table, key);
    data = table->data;

    for (i = table->table[hash]; i; i = i->next) {
        if (table->keymatch(key, i->key, data)) {
            if (_value) {
                *_value = i->value;
            }
            return true;
        }
    }

    return false;
}

bool SDL_RemoveFromHashTable(SDL_HashTable *table, const void *key)
{
    Uint32 hash;
    SDL_HashItem *item = NULL;
    SDL_HashItem *prev = NULL;
    void *data;

    if (!table) {
        return false;
    }

    hash = calc_hash(table, key);
    data = table->data;

    for (item = table->table[hash]; item; item = item->next) {
        if (table->keymatch(key, item->key, data)) {
            if (prev) {
                prev->next = item->next;
            } else {
                table->table[hash] = item->next;
            }

            if (table->nuke) {
                table->nuke(item->key, item->value, data);
            }
            SDL_free(item);
            return true;
        }

        prev = item;
    }

    return false;
}

bool SDL_IterateHashTableKey(const SDL_HashTable *table, const void *key, const void **_value, void **iter)
{
    SDL_HashItem *item;

    if (!table) {
        return false;
    }

    item = *iter ? ((SDL_HashItem *)*iter)->next : table->table[calc_hash(table, key)];

    while (item) {
        if (table->keymatch(key, item->key, table->data)) {
            *_value = item->value;
            *iter = item;
            return true;
        }
        item = item->next;
    }

    // no more matches.
    *_value = NULL;
    *iter = NULL;
    return false;
}

bool SDL_IterateHashTable(const SDL_HashTable *table, const void **_key, const void **_value, void **iter)
{
    SDL_HashItem *item = (SDL_HashItem *) *iter;
    Uint32 idx = 0;

    if (!table) {
        return false;
    }

    if (item) {
        const SDL_HashItem *orig = item;
        item = item->next;
        if (!item) {
            idx = calc_hash(table, orig->key) + 1;  // !!! FIXME: we probably shouldn't rehash each time.
        }
    }

    while (!item && (idx < table->table_len)) {
        item = table->table[idx++];  // skip empty buckets...
    }

    if (!item) {  // no more matches?
        *_key = NULL;
        *iter = NULL;
        return false;
    }

    *_key = item->key;
    *_value = item->value;
    *iter = item;

    return true;
}

bool SDL_HashTableEmpty(SDL_HashTable *table)
{
    if (table) {
        Uint32 i;

        for (i = 0; i < table->table_len; i++) {
            SDL_HashItem *item = table->table[i];
            if (item) {
                return false;
            }
        }
    }
    return true;
}

void SDL_EmptyHashTable(SDL_HashTable *table)
{
    if (table) {
        void *data = table->data;
        Uint32 i;

        for (i = 0; i < table->table_len; i++) {
            SDL_HashItem *item = table->table[i];
            while (item) {
                SDL_HashItem *next = item->next;
                if (table->nuke) {
                    table->nuke(item->key, item->value, data);
                }
                SDL_free(item);
                item = next;
            }
            table->table[i] = NULL;
        }
    }
}

void SDL_DestroyHashTable(SDL_HashTable *table)
{
    if (table) {
        SDL_EmptyHashTable(table);
        SDL_free(table->table);
        SDL_free(table);
    }
}

// this is djb's xor hashing function.
static SDL_INLINE Uint32 hash_string_djbxor(const char *str, size_t len)
{
    Uint32 hash = 5381;
    while (len--) {
        hash = ((hash << 5) + hash) ^ *(str++);
    }
    return hash;
}

Uint32 SDL_HashString(const void *key, void *data)
{
    const char *str = (const char *)key;
    return hash_string_djbxor(str, SDL_strlen(str));
}

bool SDL_KeyMatchString(const void *a, const void *b, void *data)
{
    if (a == b) {
        return true;  // same pointer, must match.
    } else if (!a || !b) {
        return false;  // one pointer is NULL (and first test shows they aren't the same pointer), must not match.
    }
    return (SDL_strcmp((const char *)a, (const char *)b) == 0);  // Check against actual string contents.
}

// We assume we can fit the ID in the key directly
SDL_COMPILE_TIME_ASSERT(SDL_HashID_KeySize, sizeof(Uint32) <= sizeof(const void *));

Uint32 SDL_HashID(const void *key, void *unused)
{
    return (Uint32)(uintptr_t)key;
}

bool SDL_KeyMatchID(const void *a, const void *b, void *unused)
{
    if (a == b) {
        return true;
    }
    return false;
}

void SDL_NukeFreeKey(const void *key, const void *value, void *unused)
{
    SDL_free((void *)key);
}

void SDL_NukeFreeValue(const void *key, const void *value, void *unused)
{
    SDL_free((void *)value);
}