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Robin Hood hash map

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Robin Hood hash map library -- a general purpose hash table, using open addressing with linear probing and Robin Hood hashing for the collision resolution algorithm. Optimal for solving the dictionary problem. The library provides support for the SipHash and Murmurhash3 algorithms. The implementation is written in C99 and distributed under the 2-clause BSD license.


Pedro Celis, 1986, Robin Hood Hashing, University of Waterloo


  • rhashmap_t *rhashmap_create(size_t size, unsigned flags)

    • Construct a new hash map. If size is not zero, then the hash map will be pre-allocated with, at least, the given size as a minimum; otherwise, a default size will be used. Certain hash map behaviour can be specified using any of the following optional flags:
      • RHM_NOCOPY: the keys on insert will not be copied and the given pointers to them will be expected to be valid and the values constant until the key is deleted; by default, the put operation will make a copy of the key.
      • RHM_NONCRYPTO: a non-cryptographic hash function will be used to provide higher performance. By default, half SipHash-2-4 is used to defend against the hash-flooding DoS attacks. With this flag set, the hash function will be switched to the MurmurHash3 algorithm.
  • void rhashmap_destroy(rhashmap_t *hmap)

    • Destroy the hash map, freeing the memory it uses.
  • void *rhashmap_get(rhashmap_t *hmap, const void *key, size_t len)

    • Lookup the key (of a given length) and return the value associated with it. Return NULL if the key is not found (see the caveats section).
  • void *rhashmap_put(rhashmap_t *hmap, const void *key, size_t len, void *val)

    • Insert the key with an arbitrary value. If the key is already present, return the already existing associated value without changing it. Otherwise, on a successful insert, return the given value. Just compare the result against val to test whether the insert was successful.
  • void *rhashmap_del(rhashmap_t *hmap, const void *key, size_t len)

    • Remove the given key. If the key was present, return the associated value; otherwise return NULL.


  • The hash table will grow when it reaches ~85% fill and will shrink when the fill is below ~40%.

  • The key sizes greater than 64 KB are not supported. The hash map supports up to UINT_MAX elements, which is, on any modern CPU architecture, more than 4 billion elements. These limits are expected to be enough for all practical use cases, while allowing this implementation to use less memory.

  • While the NULL values may be inserted, rhashmap_get and rhashmap_del cannot indicate whether the key was not found or a key with a NULL value was found. If the caller needs to indicate an "empty" value, it can use a special pointer value, such as (void *)(uintptr_t)0x1.


With small to medium key sizes, Robin Hood hash map scores above Judy array (JudyHS) and Google Sparse hash map on lookup performance benchmarks. With the very small key sizes, it demonstrates similar performance to JudyHS.

Disclaimer: benchmark results, however, depend on many aspects (workload, hardware characteristics, methodology, etc). Ultimately, readers are encouraged to perform their own benchmarks.


An illustrative code fragment:

#include <rhashmap.h>

rhashmap_t *kvmap;
struct obj *obj;

kvmap = rhashmap_create(0, 0);
assert(kvmap != NULL);
obj = obj_create();
rhashmap_put(kvmap, "test", sizeof("test") - 1, obj);
obj = rhashmap_get(kvmap, "test", sizeof("test") - 1);


Just build the package, install it and link the library using the -lrhashmap flag.

  • RPM (tested on RHEL/CentOS 7): cd pkg && make rpm
  • DEB (tested on Debian 9): cd pkg && make deb

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