/**
 * Licensed to the Apache Software Foundation (ASF) under one or more
 * contributor license agreements.  See the NOTICE file distributed with
 * this work for additional information regarding copyright ownership.
 * The ASF licenses this file to You 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.
 */

package com.nearinfinity.bloomfilter;

/**
 * Provides some useful methods for estimating memory usage, optimal hashes, etc.
 * @author Aaron McCurry (amccurry@nearinfinity.com)
 */
public abstract class BloomFilterFormulas {
	
	/**
	 * k = (m / n) ln 2 from wikipedia. 
	 * @param elementSize the number of elements expected.
	 * @param numberOfBytes the number of bytes allowed.
	 * @return the best number of hashes.
	 */
	public static int getOptimalNumberOfHashesByBytes(long elementSize, long numberOfBytes) {
		return getOptimalNumberOfHashesByBits(elementSize, numberOfBytes *8);
	}
	
	/**
	 * k = (m / n) ln 2 from wikipedia. 
	 * @param elementSize the number of elements expected.
	 * @param numberOfBits the number of bytes allowed.
	 * @return the best number of hashes.
	 */
	public static int getOptimalNumberOfHashesByBits(long elementSize, long numberOfBits) {
		return (int) Math.ceil(Math.log(2) * ((double) numberOfBits / elementSize));
	}
	
	/**
	 * Calculate the number of bytes needed to produce the provided probability of false 
	 * positives with the given element size.
	 * @param probabilityOfFalsePositives the probability of false positives for the given number of elements. 
	 * @param elementSize the estimated element size.
	 * @return the number of bytes.
	 */
	public static int getNumberOfBytes(double probabilityOfFalsePositives, long elementSize) {
		return getNumberOfBits(probabilityOfFalsePositives,elementSize) / 8;
	}
	
	/**
	 * Calculate the number of bits needed to produce the provided probability of false 
	 * positives with the given element size.
	 * @param probabilityOfFalsePositives the probability of false positives. 
	 * @param elementSize the estimated element size.
	 * @return the number of bytes.
	 */
	public static int getNumberOfBits(double probabilityOfFalsePositives, long elementSize) {
		return (int) (Math.abs(elementSize * Math.log(probabilityOfFalsePositives)) / (Math.pow(Math.log(2),2)));
	}
	
}