# Hash tables¶

#include <libcork/ds.h>


This section defines a hash table class. Our hash table implementation is based on the public domain hash table package written in the late 1980’s by Peter Moore at UC Berkeley.

The keys and values of a libcork hash table are both represented by void * pointers. You can also store integer keys or values, as long as you use the intptr_t or uintptr_t integral types. (These are the only integer types guaranteed by the C99 standard to fit within the space used by a void *.) The keys of the hash table can be any arbitrary type; you must provide two functions that control how key pointers are used to identify entries in the table: the hasher (cork_hash_f) and the comparator (cork_equals_f). It’s your responsibility to ensure that these two functions are consistent with each other — i.e., if two keys are equal according to your comparator, they must also map to the same hash value. (The inverse doesn’t need to be true; it’s fine for two keys to have the same hash value but not be equal.)

struct cork_hash_table

A hash table.

struct cork_hash_table *cork_hash_table_new(size_t initial_size, unsigned int flags)

Creates a new hash table instance.

If you know roughly how many entries you’re going to add to the hash table, you can pass this in as the initial_size parameter. If you don’t know how many entries there will be, you can use 0 for this parameter instead.

You will most likely need to provide a hashing function and a comparison function for the new hash table (using cork_hash_table_set_hash() and cork_hash_table_set_equals()), which will be used to compare key values of the entries in the table. If you do not provide your own functions, the default functions will compare key pointers as-is without interpreting what they point to.

The flags field is currently unused, and should be 0. In the future, this parameter will be used to let you customize the behavior of the hash table.

void cork_hash_table_free(struct cork_hash_table *table)

Frees a hash table. If you have provided a free_key or free_value callback for table, then we’ll automatically free any remaining keys and/or values.

struct cork_hash_table_entry

The contents of an entry in a hash table.

void *key

The key for this entry. There won’t be any other entries in the hash table with the same key, as determined by the comparator function that you provide.

void *value

The value for this entry. The entry’s value is completely opaque to the hash table; we’ll never need to compare or interrogate the values in the table.

cork_hash hash

The hash value for this entry’s key. This field is strictly read-only.

## Callback functions¶

You can use the callback functions in this section to customize the behavior of a hash table.

void cork_hash_table_set_user_data(struct cork_hash_table *table, void *user_data, cork_free_f free_user_data)

Lets you provide an opaque user_data pointer to each of the hash table’s callbacks. This lets you provide additional state, other than the hash table itself to those callbacks. If free_user_data is not NULL, then the hash table will take control of user_data, and will use the free_user_data function to free it when the hash table is destroyed.

### Key management¶

void cork_hash_table_set_hash(struct cork_hash_table *table, void *user_data, cork_hash_f hash)

The hash table will use the hash callback to calculate a hash value for each key.

cork_hash (*cork_hash_f)(void *user_data, const void *key)

Note

It’s important to use a hash function that has a uniform distribution of hash values for the set of values you expect to use as hash table keys. In particular, you should not rely on there being a prime number of hash table bins to get the desired uniform distribution. The hash value functions that we provide have uniform distribution (and are fast), and should be safe to use for most key types.

void cork_hash_table_set_equals(struct cork_hash_table *table, void *user_data, cork_equals_f equals)

The hash table will use the equals callback to compare keys.

bool (*cork_equals_f)(void *user_data, const void *key1, const void *key2)

### Built-in key types¶

We also provide a couple of specialized constructors for common key types, which prevents you from having to duplicate common hashing and comparison functions.

struct cork_hash_table *cork_string_hash_table_new(size_t initial_size, unsigned int flags)

Create a hash table whose keys will be C strings.

struct cork_hash_table *cork_pointer_hash_table_new(size_t initial_size, unsigned int flags)

Create a hash table where keys should be compared using standard pointer equality. (In other words, keys should only be considered equal if they point to the same physical object.)

### Automatically freeing entries¶

void cork_hash_table_set_free_key(struct cork_hash_table *table, void *user_data, cork_free_f free_key)
void cork_hash_table_set_free_value(struct cork_hash_table *table, void *user_data, cork_free_f free_value)

If you provide free_key and/or free_value callbacks, then the hash table will take ownership of any keys and values that you add. The hash table will use these callbacks to free each key and value when entries are explicitly deleted (via cork_hash_table_delete() or cork_hash_table_clear()), and when the hash table itself is destroyed.

There are several functions that can be used to add or retrieve entries from a hash table. Each one has slightly different semantics; you should read through them all before deciding which one to use for a particular use case.

Note

Each of these functions comes in two variants. The “normal” variant will use the hash table’s hash callback to calculate the hash value for the key parameter. This is the normal way to interact with a hash table.

When using the _hash variant, you must calculate the hash value for each key yourself, and pass in this hash value as an extra parameter. The hash table’s hash callback is not invoked. This can be more efficient, if you’ve already calculated or cached the hash value. It is your responsibility to make sure that the hash values you provide are consistent, just like when you write a hash callback.

void *cork_hash_table_get(const struct cork_hash_table *table, const void *key)
void *cork_hash_table_get_hash(const struct cork_hash_table *table, cork_hash hash, const void *key)

Retrieves the value in table with the given key. We return NULL if there’s no corresponding entry in the table. This means that, using this function, you can’t tell the difference between a missing entry, and an entry that’s explicitly mapped to NULL. If you need to distinguish those cases, you should use cork_hash_table_get_entry() instead.

struct cork_hash_table_entry *cork_hash_table_get_entry(const struct cork_hash_table *table, const void *key)
struct cork_hash_table_entry *cork_hash_table_get_entry_hash(const struct cork_hash_table *table, cork_hash hash, const void *key)

Retrieves the entry in table with the given key. We return NULL if there’s no corresponding entry in the table.

You are free to update the key and value fields of the entry. However, you must ensure that any new key is considered “equal” to the old key, according to the hasher and comparator functions that you provided for this hash table.

struct cork_hash_table_entry *cork_hash_table_get_or_create(struct cork_hash_table *table, void *key, bool *is_new)
struct cork_hash_table_entry *cork_hash_table_get_or_create_hash(struct cork_hash_table *table, cork_hash hash, void *key, bool *is_new)

Retrieves the entry in table with the given key. If there is no entry with the given key, it will be created. (If we can’t create the new entry, we’ll return NULL.) We’ll fill in the is_new output parameter to indicate whether the entry is new or not.

If a new entry is created, its value will initially be NULL, but you can update this as necessary. You can also update the entry’s key, though you must ensure that any new key is considered “equal” to the old key, according to the hasher and comparator functions that you provided for this hash table. This is necessary, for instance, if the key parameter that we search for was allocated on the stack. We can’t save this stack key into the hash table, since it will disapppear as soon as the calling function finishes. Instead, you must create a new key on the heap, which can be saved into the entry. For efficiency, you’ll only want to allocate this new heap-stored key if the entry is actually new, especially if there will be a lot successful lookups of existing keys.

int cork_hash_table_put(struct cork_hash_table *table, void *key, void *value, bool *is_new, void **old_key, void **old_value)
int cork_hash_table_put_hash(struct cork_hash_table *table, cork_hash hash, void *key, void *value, bool *is_new, void **old_key, void **old_value)

Add an entry to a hash table. If there is already an entry with the given key, we will overwrite its key and value with the key and value parameters. If the is_new parameter is non-NULL, we’ll fill it in to indicate whether the entry is new or already existed in the table. If the old_key and/or old_value parameters are non-NULL, we’ll fill them in with the existing key and value. This can be used, for instance, to finalize an overwritten key or value object.

void cork_hash_table_delete_entry(struct cork_hash_table *table, struct cork_hash_table_entry *entry)

Removes entry from table. You must ensure that entry refers to a valid, existing entry in the hash table. This function can be more efficient than cork_hash_table_delete() if you’ve recently retrieved a hash table entry using cork_hash_table_get_or_create() or cork_hash_table_get_entry(), since we won’t have to search for the entry again.

bool cork_hash_table_delete(struct cork_hash_table *table, const void *key, void **deleted_key, void **deleted_value)
bool cork_hash_table_delete_hash(struct cork_hash_table *table, cork_hash hash, const void *key, void **deleted_key, void **deleted_value)

Removes the entry with the given key from table. If there isn’t any entry with the given key, we’ll return false. If the deleted_key and/or deleted_value parameters are non-NULL, we’ll fill them in with the deleted key and value. This can be used, for instance, to finalize the key or value object that was stored in the hash table entry.

If you have provided a free_key or free_value callback for table, then we’ll automatically free the key and/or value of the deleted entry. (This happens before cork_hash_table_delete returns, so you must not provide a deleted_key and/or deleted_value in this case.)

## Other operations¶

size_t cork_hash_table_size(const struct cork_hash_table *table)

Returns the number of entries in a hash table.

void cork_hash_table_clear(struct cork_hash_table *table)

Removes all of the entries in a hash table, without finalizing the hash table itself.

If you have provided a free_key or free_value callback for table, then we’ll automatically free any remaining keys and/or values.

int cork_hash_table_ensure_size(struct cork_hash_table *table, size_t desired_count)

Ensures that table has enough space to efficiently store a certain number of entries. This can be used to reduce (or eliminate) the number of resizing operations needed to add a large number of entries to the table, when you know in advance roughly how many entries there will be.

## Iterating through a hash table¶

There are two strategies you can use to access all of the entries in a hash table: mapping and iterating.

### Iteration order¶

Regardless of whether you use the mapping or iteration functions, we guarantee that the collection of items will be processed in the same order that they were added to the hash table.

### Mapping¶

With mapping, you write a mapping function that will be applied to each entry in the table. (In this case, libcork controls the loop that steps through each entry.)

void cork_hash_table_map(struct cork_hash_table *table, void *user_data, cork_hash_table_map_f map)

Applies the map function to each entry in a hash table. The map function’s cork_hash_table_map_result return value can be used to influence the iteration.

enum cork_hash_table_map_result (*cork_hash_table_map_f)(void *user_data, struct cork_hash_table_entry *entry)

The function that will be applied to each entry in a hash table. The function’s return value can be used to influence the iteration:

enum cork_hash_table_map_result
CORK_HASH_TABLE_CONTINUE

Continue the current cork_hash_table_map() operation. If there are any remaining elements, the next one will be passed into another call of the map function.

CORK_HASH_TABLE_ABORT

Stop the current cork_hash_table_map() operation. No more entries will be processed after this one, even if there are remaining elements in the hash table.

CORK_HASH_TABLE_DELETE

Continue the current cork_hash_table_map() operation, but first delete the entry that was just processed. If there are any remaining elements, the next one will be passed into another call of the map function.

For instance, you can manually calculate the number of entries in a hash table as follows (assuming you didn’t want to use the built-in cork_hash_table_size() function, of course):

static enum cork_hash_table_map_result
count_entries(void *user_data, struct cork_hash_table_entry *entry)
{
size_t  *count = user_data;
(*count)++;
return CORK_HASH_TABLE_MAP_CONTINUE;
}

struct cork_hash_table  *table = /* from somewhere */;
size_t  count = 0;
cork_hash_table_map(table, &count, count_entries);
/* the number of entries is now in count */


### Iterating¶

The second strategy is to iterate through the entries yourself. Since the internal struture of the cork_hash_table type is opaque (and slightly more complex than a simple array), you have to use a special “iterator” type to manage the manual iteration. Note that unlike when using a mapping function, it is not safe to delete entries in a hash table as you manually iterate through them.

struct cork_hash_table_iterator

A helper type for manually iterating through the entries in a hash table. All of the fields in this type are private. You’ll usually allocate this type on the stack.

void cork_hash_table_iterator_init(struct cork_hash_table *table, struct cork_hash_table_iterator *iterator)

Initializes a new iterator for the given hash table.

struct cork_hash_table_entry *cork_hash_table_iterator_next(struct cork_hash_table_iterator *iterator)

Returns the next entry in iterator‘s hash table. If you’ve already iterated through all of the entries in the table, we’ll return NULL.

With these functions, manually counting the hash table entries looks like:

struct cork_hash_table  *table = /* from somewhere */;
struct cork_hash_table_iterator  iter;
struct cork_hash_table_entry  *entry;
size_t  count = 0;

cork_hash_table_iterator_init(table, &iter);
while ((entry = cork_hash_table_iterator_next(&iter)) != NULL) {
count++;
}
/* the number of elements is now in count */