64 lines
2.6 KiB
ReStructuredText
64 lines
2.6 KiB
ReStructuredText
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.. _maps:
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Maps and Dictionaries
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=====================
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MicroPython dictionaries and maps use techniques called open addressing and linear probing.
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This chapter details both of these methods.
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Open addressing
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---------------
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`Open addressing <https://en.wikipedia.org/wiki/Open_addressing>`_ is used to resolve collisions.
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Collisions are very common occurrences and happen when two items happen to hash to the same
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slot or location. For example, given a hash setup as this:
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.. image:: img/collision.png
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If there is a request to fill slot ``0`` with ``70``, since the slot ``0`` is not empty, open addressing
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finds the next available slot in the dictionary to service this request. This sequential search for an alternate
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location is called *probing*. There are several sequence probing algorithms but MicroPython uses
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linear probing that is described in the next section.
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Linear probing
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--------------
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Linear probing is one of the methods for finding an available address or slot in a dictionary. In MicroPython,
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it is used with open addressing. To service the request described above, unlike other probing algorithms,
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linear probing assumes a fixed interval of ``1`` between probes. The request will therefore be serviced by
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placing the item in the next free slot which is slot ``4`` in our example:
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.. image:: img/linprob.png
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The same methods i.e open addressing and linear probing are used to search for an item in a dictionary.
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Assume we want to search for the data item ``33``. The computed hash value will be 2. Looking at slot 2
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reveals ``33``, at this point, we return ``True``. Searching for ``70`` is quite different as there was a
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collision at the time of insertion. Therefore computing the hash value is ``0`` which is currently
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holding ``44``. Instead of simply returning ``False``, we perform a sequential search starting at point
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``1`` until the item ``70`` is found or we encounter a free slot. This is the general way of performing
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look-ups in hashes:
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.. code-block:: c
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// not yet found, keep searching in this table
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pos = (pos + 1) % set->alloc;
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if (pos == start_pos) {
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// search got back to starting position, so index is not in table
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if (lookup_kind & MP_MAP_LOOKUP_ADD_IF_NOT_FOUND) {
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if (avail_slot != NULL) {
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// there was an available slot, so use that
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set->used++;
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*avail_slot = index;
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return index;
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} else {
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// not enough room in table, rehash it
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mp_set_rehash(set);
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// restart the search for the new element
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start_pos = pos = hash % set->alloc;
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}
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}
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} else {
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return MP_OBJ_NULL;
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}
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