Multiset
Multiset
Multiset

Multiset (bag) is a generalized version set structure. Analogously to set, multiset is a container used for storing values and it does not guarantee any particular ordering of its contents. On the other hand, it allows storing of multiple items with the same value (ie. supports non-unique keys).

Implementation

The easiest way to implement a multiset is to use a list a forget its ordering. But this approach is very memory inefficient, because it needs to store all instances of repeating items. A better solution uses two lists – one for data and second for storing numbers of occurrences of corresponding items. This implementation performs better, but is still not optimal, because the contains operation needs to iterate over the whole array, which takes O(n) steps, where n is number of distinct elements stored. The optimal implementation uses for storing items a hash table structure, which guarantees O(1) average access time for all elements.

Code

/**
 * Multiset implemented using two lists (list of values, list of occurrences)
 * @author Pavel Micka
 * @param <ENTITY> type parameter of the contained value
 */
public class Multiset<VALUE> {

    private List<VALUE> values;
    private List<Integer> occurences;

    /**
     * Constructor
     * @param initialCapacity initial capacity of underlying lists
     */
    public Multiset(int initialCapacity) {
        values = new ArrayList<VALUE>(initialCapacity);
        occurences = new ArrayList<Integer>(initialCapacity);
    }

    /**
     * Inserts a value into the multiset
     * @param val value
     * @return number of occurences of the value in the set after the addition
     */
    public int put(VALUE val) {
        int index = values.indexOf(val);
        if (index == -1) {
            values.add(val);
            occurences.add(1);
            return 1;
        } else {
            int currCount = occurences.get(index);
            occurences.set(index, currCount + 1);
            return currCount + 1;
        }
    }

    /**
     * Returns and deletes (decrements number of uccurences) some value from the multiset
     * @return value, null if the multiset is empty
     */
    public VALUE pick() {
        if (values.size() == 0) {
            return null;
        }
        if (occurences.get(0) == 1) {
            VALUE v = values.remove(0);
            occurences.remove(0);
            return v;
        } else {
            VALUE v = values.get(0);
            occurences.set(0, occurences.get(0) - 1);
            return v;
        }
    }

    /**
     * Deletes a given value from the multiset (removes one occurrence)
     * @param val value
     * @return number of occurences of the value in the set after the deletion
     */
    public int remove(VALUE e) {
        int index = values.indexOf(e);
        int curr = occurences.get(index);
        if (curr != 1) {
            occurences.set(index, curr - 1);
            return curr - 1;
        } else {
            values.remove(index);
            occurences.remove(index);
            return 0;
        }
    }

    /**
     * Query, if the multiset contains a given value
     * @param val value
     * @return number of occurences of the given value in the set
     */
    public int contains(VALUE e) {
        int index = values.indexOf(e);
        if(index == -1) return 0;
        return occurences.get(index);
    }

    /**
     * Size of the multiset (including all multiplicities)
     * @return number of stored entities (values*occurences)
     */
    public int size() {
        int count = 0;
        for(Integer i : occurences){
            count += i;
        }
        return count;
    }
}

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