/* ConstantFlowRegulator.java
 *
 * Copyright 2009-2012 Comcast Interactive Media, LLC.
 *
 * 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.
 */
package org.fishwife.jrugged;

import java.util.concurrent.Callable;

public class ConstantFlowRegulator implements ServiceWrapper {

    /**
     * By default a value of -1 allows all requests to go through unmolested.
     */
    private int requestPerSecondThreshold = -1;

    /**
     *
     */
    private long deltaWaitTimeMillis = 0;

    /**
     *
     */
    private long lastRequestOccurance = 0;

    ConstantFlowRegulatorExceptionMapper<? extends Exception> exceptionMapper;

    public ConstantFlowRegulator() {
    }

    public ConstantFlowRegulator(final int requestsPerSecond) {
        requestPerSecondThreshold = requestsPerSecond;
        calculateDeltaWaitTime();
    }

    public ConstantFlowRegulator(final int requestsPerSecond,
            final ConstantFlowRegulatorExceptionMapper<? extends Exception> mapper) {
        requestPerSecondThreshold = requestsPerSecond;
        calculateDeltaWaitTime();
        exceptionMapper = mapper;
    }

    /**
     * Wrap the given service call with the {@link ConstantFlowRegulator} protection
     * logic.
     *
     * @param c
     *            the {@link Callable} to attempt
     *
     * @return whatever c would return on success
     *
     * @throws FlowRateExceededException
     *             if the total requests per second through the flow regulator exceeds the
     *             configured value
     * @throws Exception
     *             if <code>c</code> throws one during execution
     */
    @Override
    public <T> T invoke(final Callable<T> c) throws Exception {
        if (canProceed()) {
            return c.call();
        }
        throw mapException(new FlowRateExceededException());
    }

    /**
     * Wrap the given service call with the {@link ConstantFlowRegulator} protection
     * logic.
     *
     * @param r
     *            the {@link Runnable} to attempt
     *
     * @throws FlowRateExceededException
     *             if the total requests per second through the flow regulator exceeds the
     *             configured value
     * @throws Exception
     *             if <code>c</code> throws one during execution
     */
    @Override
    public void invoke(final Runnable r) throws Exception {
        if (canProceed()) {
            r.run();
        } else {
            throw mapException(new FlowRateExceededException());
        }
    }

    /**
     * Wrap the given service call with the {@link ConstantFlowRegulator} protection
     * logic.
     *
     * @param r
     *            the {@link Runnable} to attempt
     * @param result
     *            what to return after <code>r</code> succeeds
     *
     * @return result
     *
     * @throws FlowRateExceededException
     *             if the total requests per second through the flow regulator exceeds the
     *             configured value
     * @throws Exception
     *             if <code>c</code> throws one during execution
     */
    @Override
    public <T> T invoke(final Runnable r, final T result) throws Exception {
        if (canProceed()) {
            r.run();
            return result;
        }
        throw mapException(new FlowRateExceededException());
    }

    protected synchronized boolean canProceed() {
        if (requestPerSecondThreshold == -1) {
            return true;
        }

        if (lastRequestOccurance == 0) {
            lastRequestOccurance = System.currentTimeMillis();
            return true;
        }

        if (System.currentTimeMillis() - lastRequestOccurance > deltaWaitTimeMillis) {
            lastRequestOccurance = System.currentTimeMillis();
            return true;
        }

        return false;
    }

    /**
     * Configures number of requests per second to allow through this flow regulator onto
     * a configured back end service.
     *
     * @param i
     *            the requests per second threshold for this flow regulator
     */
    public void setRequestPerSecondThreshold(final int i) {
        requestPerSecondThreshold = i;
        calculateDeltaWaitTime();
    }

    /**
     * Returns the currently configured number of requests per second to allow through
     * this flow regulator onto a configured back end service.
     *
     * @return int the currently configured requests per second threshold
     */
    public int getRequestPerSecondThreshold() {
        return requestPerSecondThreshold;
    }

    /**
     * Get the helper that converts {@link FlowRateExceededException}s into
     * application-specific exceptions.
     *
     * @return {@link ConstantFlowRegulatorExceptionMapper} my converter object, or
     *         <code>null</code> if one is not currently set.
     */
    public ConstantFlowRegulatorExceptionMapper<? extends Exception> getExceptionMapper() {
        return exceptionMapper;
    }

    /**
     * A helper that converts {@link FlowRateExceededException} into a known 'application'
     * exception.
     *
     * @param mapper
     *            my converter object
     */
    public void setExceptionMapper(
            final ConstantFlowRegulatorExceptionMapper<? extends Exception> mapper) {
        exceptionMapper = mapper;
    }

    private void calculateDeltaWaitTime() {
        if (requestPerSecondThreshold > 0) {
            deltaWaitTimeMillis = 1000L / requestPerSecondThreshold;
        }
    }

    private Exception mapException(final FlowRateExceededException e) {
        if (exceptionMapper == null) {
            return e;
        }

        return exceptionMapper.map(this, e);
    }
}