org.semanticweb.elk.util.collections.LinearProbing Maven / Gradle / Ivy
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package org.semanticweb.elk.util.collections;
/*
* #%L
* ELK Utilities Collections
* $Id:$
* $HeadURL:$
* %%
* Copyright (C) 2011 - 2014 Department of Computer Science, University of Oxford
* %%
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
* #L%
*/
/**
* A collection of utilities for creating collections (e.g., sets or maps)
* backed by arrays with linear probing as method for collision resolution: see
* [1] p.526.
*
* [1] Donald E. Knuth, The Art of Computer Programming, Volume 3, Sorting and
* Searching, Second Edition
*
* @author Yevgeny Kazakov
*
*/
class LinearProbing {
/**
* The default initial capacity - MUST be a power of two.
*/
static final int DEFAULT_INITIAL_CAPACITY = 16;
/**
* The maximum capacity, used if a higher value is implicitly specified by
* either of the constructors with arguments. MUST be a power of two <=
* 1<<30.
*/
static final int MAXIMUM_CAPACITY = 1 << 30;
/**
* Computes the initial capacity of the table for storing elements. The
* capacity is the largest power of two that does not exceed the given value
* or {@link #MAXIMUM_CAPACITY}.
*
* @param capacity
* the input value used as a guideline
* @return the largest power of two that does not exceed the given value or
* {@link #MAXIMUM_CAPACITY}
*/
static int getInitialCapacity(int capacity) {
if (capacity < 0)
throw new IllegalArgumentException("Illegal Capacity: " + capacity);
if (capacity > LinearProbing.MAXIMUM_CAPACITY)
capacity = LinearProbing.MAXIMUM_CAPACITY;
// Find a power of 2 >= initialCapacity
int result = 1;
while (result < capacity)
result <<= 1;
return result;
}
/**
* Computes a maximum size of the table for a given capacity after which the
* table should be expanded.
*
* @param capacity
* the capacity of the table.
* @return maximum size of the table for a given capacity after which to
* stretch the table.
*/
static int getUpperSize(int capacity) {
if (capacity > 64)
return (capacity >> 1) + (capacity >> 2); // max 75% filled
// else
return capacity;
}
/**
* Computes a minimum size of the table for a given capacity after which to
* shrink the table.
*
* @param capacity
* the capacity of the table.
* @return minimum size of the table for a given capacity after which to
* shrink the table
*/
static int getLowerSize(int capacity) {
return capacity >> 2;
}
/**
* Computes the index position for the object relative to the given length
* of the table. This is the position starting from which the object should
* be searched for using linear probing.
*
* @param o
* the object for which to compute the position
* @param length
* the length of the table in which the objects are located
* @return the first position in the table from which the object should be
* searched
*/
private static int getIndex(Object o, int length) {
return o.hashCode() & (length - 1);
}
static int getPosition(E[] d, Object o) {
int i = getIndex(o, d.length);
for (;;) {
Object probe = d[i];
if (probe == null || o.equals(probe))
return i;
if (++i == d.length)
i = 0;
}
}
/**
* Removes the element at the given position of the table shifting, if
* necessary, other elements so that all elements can be found by linear
* probing.
*
* @param d
* the array of the elements
* @param pos
* the position of data at which to delete the element
*/
static void remove(E[] d, int pos) {
for (;;) {
int next = getMovedPosition(d, pos);
E moved = d[pos] = d[next];
if (moved == null)
return;
// else
pos = next;
}
}
/**
* Removes the element at the given position of the table and the
* corresponding value at the same position, shifting, if necessary, other
* elements and values so that all elements can be found by linear probing.
*
* @param d
* the array of the elements
* @param pos
* the position of data at which to delete the element
*/
static void remove(K[] k, V[] v, int pos) {
for (;;) {
int next = getMovedPosition(k, pos);
K moved = k[pos] = k[next];
v[pos] = v[next];
if (moved == null)
return;
// else
pos = next;
}
}
/**
* Finds the position of the next element starting from the given position
* that would not be found by linear probing if the element at the given
* position are deleted. This should be the element whose index is smaller
* than this position.
*
* @param d
* @param del
* @return
*/
static int getMovedPosition(E[] d, int del) {
int j = del;
for (;;) {
if (++j == d.length)
j = 0;
// invariant: interval ]del, j] contains only non-null elements
// whose index is in ]del, j]
E test = d[j];
if (test == null)
return j;
int k = getIndex(test, d.length);
// check if k is in ]del, j] (this interval can wrap over)
if ((del < j) ? (del < k) && (k <= j) : (del < k) || (k <= j))
// the test element should not be shifted
continue;
// else it should be shifted
return j;
}
}
/**
* Tests if the set represented by given data array contains a given object.
*
* @param d
* the elements representing the set
* @param o
* the object to be tested on inclusion in the set
* @return {@code true} if the object occurs in the array and {@code false}
* otherwise
*/
static boolean contains(E[] d, Object o) {
int pos = getPosition(d, o);
if (d[pos] == null)
return false;
// else
return true;
}
/**
* Adds the element to the set represented by given data array, if it did
* not contain there already.
*
* @param d
* the elements of the set
* @param e
* the element to be added to the set
* @return {@code true} if the set has changed (the element is added),
* {@code false} otherwise
*/
static boolean add(E[] d, E e) {
int pos = getPosition(d, e);
if (d[pos] == null) {
d[pos] = e;
return true;
}
// else the element is already there
return false;
}
/**
* Removes the element from the set represented by given data array, if it
* occurs there.
*
* @param d
* the elements of the set
* @param e
* the element to be removed from the array
* @return {@code true} if the set has changed (the element is added),
* {@code false} otherwise
*/
static boolean remove(E[] d, Object o) {
int pos = getPosition(d, o);
if (d[pos] == null)
return false;
// else
remove(d, pos);
return true;
}
}
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