Introduction

LibHashSet is a simple hash set implementation for C99. It uses open addressing and double hashing.

At this time, the only type of elements (keys) supported is uint64_t.

This hash set implementation has been tested to efficiently handle several billions of items 😏

Getting Started

Here is a simple example of how to use LibHashSet in your application:

#include <hash_set.h>
#include <stdio.h>

int main(int argc, char* argv[])
{
	size_t offset = 0U;
	uint64_t value;

	/* create new hash set instance */
	hash_set_t *const hash_set = hash_set_create(0U, -1.0);
	if (!hash_set)
	{
		fputs("Allocation has failed!\n", stderr);
		return EXIT_FAILURE;
	}

	/* add a number of items to the hash set, the set will grow as needed */
	while (have_more_items())
	{
		const errno_t error = hash_set_insert(hash_set, get_next_item());
		if (error)
		{
			fprintf(stderr, "Insert operation has failed! (error: %d)\n", error);
			return EXIT_FAILURE;
		}
	}

	/* test whether hash set contains a specific item */
	if (hash_set_contains(hash_set, 42U) == 0)
	{
		puts("Set contains item!");

		/* remove the existing item from the hash set */
		if (hash_set_remove(hash_set, 42U) == 0)
		{
			puts("Item has been removed!");
		}
	}

	/* print total number of items in the hash set*/
	printf("Total number of items: %zu\n", hash_set_size(hash_set));

	/* print all items in the set */
	while (hash_set_iterate(hash_set, &offset, &value) == 0)
	{
		printf("Item: %016llX\n", value);
	}

	/* destroy the hash set, when it is no longer needed! */
	hash_set_destroy(hash_set);
	return EXIT_SUCCESS;
}

API Reference

This section describes the LibHashSet C99 programming interface.

Types

hash_set_t

A struct that represents a hash set instance. Instances can be allocated and de-allocated via the hash_set_create() and hash_set_destroy() functions, respectively.

Note: Application code shall treat this struct as opaque!

typedef struct _hash_set hash_set_t;

Functions

hash_set_create()

Allocates a new hash set instance. The new hash set instance is empty initially.

hash_set_t *hash_set_create(
	const size_t initial_capacity,
	const double load_factor
);

Parameters

• initial_capacity
  The initial capacity of the hash set (number of values). The given value will be rounded to the next power of two. If the number of values (keys) to be inserted into the hash set can be estimated beforehand, then the initial capacity should be adjusted accordingly to avoid unnecessary re-allocations. In any case, the hash set will be able to grow dynamically as needed. If this parameter is set to zero, the the default initial capacity (8192) is used.

• load_factor
  The load factor to be applied to the hash set. The given value will be clipped to the 0.1 to 1.0 range. Generally, the default load factor (0.8) offers a good trade-off between performance and memory usage. Higher values decrease the memory overhead, but may increase the time required for insert/lookup operations when the hash set is almost completely filled. If this parameter is less than or equal to zero, the default load factor is used.

Return value

On success, this function returns a pointer to a new hash set instance. On error, a NULL pointer is returned.

Note: To avoid a memory leak, the returned pointer must be de-allocated by the application using the hash_set_destroy() function, as soon as the instance is not needed anymore!

hash_set_destroy()

De-allocates an existing hash set instance. All items in the hash set are discarded.

void hash_set_destroy(
	hash_set_t *instance
);

Parameters

• instance
  A pointer to the hash set instance that is to be destroyed, as returned by the hash_set_create() function.
  Note: The given pointer is invalidated by this function, and it must not be used afterwards!

hash_set_insert()

Tries to insert the given value into the hash set. The operation fails, if the set already contains the given value.

Note: If the value is actually inserted, then the hash set may need to grow.

errno_t hash_set_insert(
	hash_set_t *const instance,
	const uint64_t value
);

Parameters

• instance
  A pointer to the hash set instance to be modified, as returned by the hash_set_create() function.

• value
  The value (key) to be inserted into the hash set. It can be any value in the 0U to UINT64_MAX range.

Return value

On success, this function returns zero. On error, the appropriate error code is returned. Possible error codes include:

• EINVAL
  An invalid argument was given, e.g. instance was set to NULL.

• EEXIST
  The given value (key) was not inserted into the hash set (again), because that value was already present.

• ENOMEM
  The value could not be inserted, because the required amount of memory could not be allocated.

• EFAULT
  Something else went wrong. This usually indicates an internal error and is not supposed to happen.

hash_set_remove()

Tries to remove the given value from the hash set. The operation fails, if the set does not contain the given value.

Note: If the value is actually removed, then the hash set may shrink.

errno_t hash_set_remove(
	hash_set_t *const instance,
	const uint64_t value
);

Parameters

• instance
  A pointer to the hash set instance to be modified, as returned by the hash_set_create() function.

• value
  The value (key) to be removed from the hash set. It can be any value in the 0U to UINT64_MAX range.

Return value

On success, this function returns zero. On error, the appropriate error code is returned. Possible error codes include:

• EINVAL
  An invalid argument was given, e.g. instance was set to NULL.

• ENOENT
  The given value (key) could not be removed from the hash set, because no such value was present.

• EFAULT
  Something else went wrong. This usually indicates an internal error and is not supposed to happen.

hash_set_clear()

Discards all items from the hash set at once.

errno_t hash_set_clear(
	hash_set_t *const instance
);

Parameters

• instance
  A pointer to the hash set instance to be modified, as returned by the hash_set_create() function.

Return value

On success, this function returns zero. On error, the appropriate error code is returned. Possible error codes include:

• EINVAL
  An invalid argument was given, e.g. instance was set to NULL.

• EAGAIN
  The hash set was not cleared, because it already was empty. Please try again later!

• EFAULT
  Something else went wrong. This usually indicates an internal error and is not supposed to happen.

hash_set_contains()

Tests whether the hash set contains a value. The operation fails, if the set does not contain the given value.

errno_t hash_set_contains(
	const hash_set_t *const instance,
	const uint64_t value
);

Parameters

• instance
  A pointer to the hash set instance to be examined, as returned by the hash_set_create() function.

• value
  The value (key) to be searched in the hash set. It can be any value in the 0U to UINT64_MAX range.

Return value

On success, this function returns zero. On error, the appropriate error code is returned. Possible error codes include:

• EINVAL
  An invalid argument was given, e.g. instance was set to NULL.

• ENOENT
  The hash set does not contain the specified value (key).

• EFAULT
  Something else went wrong. This usually indicates an internal error and is not supposed to happen.

hash_set_iterate()

Iterates through the values stored in the hash set. The elements are iterated in no particular order.

This function returns one value at a time. It should be called repeatedly, until the end of the set is encountered.

Warning: The result is undefined, if the set is modified while the iteration is in progress!

errno_t hash_set_iterate(
	const hash_set_t *const instance,
	size_t *const offset,
	uint64_t *const value
);

Parameters

• instance
  A pointer to the hash set instance to be examined, as returned by the hash_set_create() function.

• offset
  A pointer to a variable of type size_t where the current position in the set is maintained.
  This variable must be initialized to zero before the first invocation; it is updated on subsequent invocations.

• value
  A pointer to a variable of type uint64_t where the next value in the set is stored on success.
  The content of the variable should be considered undefined, if the invocation has failed.

Return value

On success, this function returns zero. On error, the appropriate error code is returned. Possible error codes include:

• EINVAL
  An invalid argument was given, e.g. instance was set to NULL.

• ENOENT
  No more values. The end of the set has been encountered.

• EFAULT
  Something else went wrong. This usually indicates an internal error and is not supposed to happen.

hash_set_size()

Returns the current number of values in the hash set.

size_t hash_set_size(
	const hash_set_t *const instance
);

Parameters

• instance
  A pointer to the hash set instance to be examined, as returned by the hash_set_create() function.

Return value

This function returns the number of values in the hash set.

hash_set_info()

Returns technical information about the hash set.

errno_t hash_set_info(
	const hash_set_t *const instance,
	size_t *const capacity,
	size_t *const valid,
	size_t *const deleted,
	size_t *const limit
);

Parameters

• instance
  A pointer to the hash set instance to be examined, as returned by the hash_set_create() function.

• capacity
  A pointer to a variable of type size_t where the current total capacity of the hash set is stored.
  This value will always be greater than or equal to the sum of the valid and deleted entries.

• valid
  A pointer to a variable of type size_t where the current number of valid entries in the hash set is stored.
  This value is equivalent to the return value of the hash_set_size() function.

• deleted
  A pointer to a variable of type size_t where the current number of deleted entries in the hash set is stored.
  For technical reasons, entires are not removed from the set immediately, but are marked as "deleted".

• limit
  A pointer to a variable of type size_t where the current "grow" limit of the hash set is stored.
  The hash set is grown automatically, as soon as the sum of the valid and deleted entries exceeds this limit.

Return value

On success, this function returns zero. On error, the appropriate error code is returned. Possible error codes include:

• EINVAL
  An invalid argument was given, e.g. instance was set to NULL.

• EFAULT
  Something else went wrong. This usually indicates an internal error and is not supposed to happen.

Thread Safety

LibHashSet is thread-safe, in the sense that all public functions operate exclusively on the given hash_set_t instance; there is no implicit shared "global" state. This means that no synchronization is required in multi-threaded applications, provided that each hash_set_t instance is created and accessed only by a single thread.

However, LibHashSet does nothing to synchronize access to a particular hash_set_t instance! Consequently, in situations where the same hash_set_t instance needs to be shared across multiple concurrent threads, the calling application is responsible for serializing all access to the "shared" instance, e.g. by using a mutex lock!

License

This work has been released under the CC0 1.0 Universal license.

For details, please refer to:
https://creativecommons.org/publicdomain/zero/1.0/legalcode

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