implement lazy deletion

import edu.princeton.cs.algs4.Queue;

/** * The {@code LinearProbingHashST} class represents a symbol table of generic * key-value pairs. * It supports the usual put, get, contains, * delete, size, and is-empty methods. * It also provides a keys method for iterating over all of the keys. * A symbol table implements the associative array abstraction: * when associating a value with a key that is already in the symbol table, * the convention is to replace the old value with the new value. * Unlike {@link java.util.Map}, this class uses the convention that * values cannot be {@code null}setting the * value associated with a key to {@code null} is equivalent to deleting the key * from the symbol table. *

* This implementation uses a linear probing hash table. It requires that * the key type overrides the {@code equals()} and {@code hashCode()} methods. * The expected time per put, contains, or remove * operation is constant, subject to the uniform hashing assumption. * The size, and is-empty operations take constant time. * Construction takes constant time. *

* For additional documentation, see Section 3.4 of * Algorithms, 4th Edition by Robert Sedgewick and Kevin Wayne. * For other implementations, see {@link ST}, {@link BinarySearchST}, * {@link SequentialSearchST}, {@link BST}, {@link RedBlackBST}, and * {@link SeparateChainingHashST}, * * @author Robert Sedgewick * @author Kevin Wayne */ public class LinearProbingHashST { private static final int INIT_CAPACITY = 4;

private int n; // number of key-value pairs in the symbol table private int m; // size of linear probing table private Key[] keys; // the keys private Value[] vals; // the values

/** * Initializes an empty symbol table. */ public LinearProbingHashST() { this(INIT_CAPACITY); }

/** * Initializes an empty symbol table with the specified initial capacity. * * @param capacity the initial capacity */ public LinearProbingHashST(int capacity) { m = capacity; n = 0; keys = (Key[]) new Object[m]; vals = (Value[]) new Object[m]; }

/** * Returns the number of key-value pairs in this symbol table. * * @return the number of key-value pairs in this symbol table */ public int size() { return n; }

/** * Returns true if this symbol table is empty. * * @return {@code true} if this symbol table is empty; * {@code false} otherwise */ public boolean isEmpty() { return size() == 0; }

/** * Returns true if this symbol table contains the specified key. * * @param key the key * @return {@code true} if this symbol table contains {@code key}; * {@code false} otherwise * @throws IllegalArgumentException if {@code key} is {@code null} */ public boolean contains(Key key) { if (key == null) throw new IllegalArgumentException("argument to contains() is null"); return get(key) != null; }

// hash function for keys - returns value between 0 and M-1 private int hash(Key key) { return (key.hashCode() & 0x7fffffff) % m; }

// resizes the hash table to the given capacity by re-hashing all of the keys private void resize(int capacity) { LinearProbingHashST temp = new LinearProbingHashST(capacity); for (int i = 0; i < m; i++) { if (keys[i] != null) { temp.put(keys[i], vals[i]); } } keys = temp.keys; vals = temp.vals; m = temp.m; }

/** * Inserts the specified key-value pair into the symbol table, overwriting the old * value with the new value if the symbol table already contains the specified key. * Deletes the specified key (and its associated value) from this symbol table * if the specified value is {@code null}. * * @param key the key * @param val the value * @throws IllegalArgumentException if {@code key} is {@code null} */ public void put(Key key, Value val) { if (key == null) throw new IllegalArgumentException("first argument to put() is null");

if (val == null) { delete(key); return; }

// double table size if 50% full if (n >= m/2) resize(2*m);

int i; for (i = hash(key); keys[i] != null; i = (i + 1) % m) { if (keys[i].equals(key)) { vals[i] = val; return; } } keys[i] = key; vals[i] = val; n++; }

/** * Returns the value associated with the specified key. * @param key the key * @return the value associated with {@code key}; * {@code null} if no such value * @throws IllegalArgumentException if {@code key} is {@code null} */ public Value get(Key key) { int i; for (i=hash(key); keys[i] != null; i = (i + 1) % m) { if (keys[i].equals(key)) return vals[i]; } return null; }

/** * Removes the specified key and its associated value from this symbol table * (if the key is in this symbol table). * * @param key the key * @throws IllegalArgumentException if {@code key} is {@code null} */ public void delete(Key key) { if (key == null) throw new IllegalArgumentException("argument to delete() is null"); if (!contains(key)) return;

// find position i of key int i = hash(key); while (!key.equals(keys[i])) { i = (i + 1) % m; }

// delete key and associated value keys[i] = null; vals[i] = null;

// rehash all keys in same cluster i = (i + 1) % m; while (keys[i] != null) { // delete keys[i] an vals[i] and reinsert Key keyToRehash = keys[i]; Value valToRehash = vals[i]; keys[i] = null; vals[i] = null; n--; put(keyToRehash, valToRehash); i = (i + 1) % m; }

n--;

// halves size of array if it's 12.5% full or less if (n > 0 && n <= m/8) resize(m/2);

assert check(); }

/** * Returns all keys in this symbol table as an {@code Iterable}. * To iterate over all of the keys in the symbol table named {@code st}, * use the foreach notation: {@code for (Key key : st.keys())}. * * @return all keys in this symbol table */ public Iterable keys() { Queue queue = new Queue(); for (int i = 0; i < m; i++) if (keys[i] != null) queue.enqueue(keys[i]); return queue; }

// integrity check - don't check after each put() because // integrity not maintained during a delete() private boolean check() {

// check that hash table is at most 50% full if (m < 2*n) { System.err.println("Hash table size m = " + m + "; array size n = " + n); return false; }

// check that each key in table can be found by get() for (int i = 0; i < m; i++) { if (keys[i] == null) continue; else if (get(keys[i]) != vals[i]) { System.err.println("get[" + keys[i] + "] = " + get(keys[i]) + "; vals[i] = " + vals[i]); return false; } } return true; }