/*

 * Copyright (c) 2014, 2018, Oracle and/or its affiliates. All rights reserved.

 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.

 *

 * This code is free software; you can redistribute it and/or modify it

 * under the terms of the GNU General Public License version 2 only, as

 * published by the Free Software Foundation.  Oracle designates this

 * particular file as subject to the "Classpath" exception as provided

 * by Oracle in the LICENSE file that accompanied this code.

 *

 * This code is distributed in the hope that it will be useful, but WITHOUT

 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or

 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License

 * version 2 for more details (a copy is included in the LICENSE file that

 * accompanied this code).

 *

 * You should have received a copy of the GNU General Public License version

 * 2 along with this work; if not, write to the Free Software Foundation,

 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.

 *

 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA

 * or visit www.oracle.com if you need additional information or have any

 * questions.

 */

import java.io.IOException;

import java.nio.ByteBuffer;

import java.util.Arrays;

import static java.lang.String.format;

final class IntegerReader {

    private static final int NEW             = 0;

    private static final int CONFIGURED      = 1;

    private static final int FIRST_BYTE_READ = 2;

    private static final int DONE            = 4;

    private int state = NEW;

    private int N;

    private int maxValue;

    private int value;

    private long r;

    private long b = 1;

    public IntegerReader configure(int N) {

        return configure(N, Integer.MAX_VALUE);

    }

    //

    // Why is it important to configure 'maxValue' here. After all we can wait

    // for the integer to be fully read and then check it. Can't we?

    //

    // Two reasons.

    //

    // 1. Value wraps around long won't be unnoticed.

    // 2. It can spit out an exception as soon as it becomes clear there's

    // an overflow. Therefore, no need to wait for the value to be fully read.

    //

    public IntegerReader configure(int N, int maxValue) {

        if (state != NEW) {

            throw new IllegalStateException("Already configured");

        }

        checkPrefix(N);

        if (maxValue < 0) {

            throw new IllegalArgumentException(

                    "maxValue >= 0: maxValue=" + maxValue);

        }

        this.maxValue = maxValue;

        this.N = N;

        state = CONFIGURED;

        return this;

    }

    public boolean read(ByteBuffer input) throws IOException {

        if (state == NEW) {

            throw new IllegalStateException("Configure first");

        }

        if (state == DONE) {

            return true;

        }

        if (!input.hasRemaining()) {

            return false;

        }

        if (state == CONFIGURED) {

            int max = (2 << (N - 1)) - 1;

            int n = input.get() & max;

            if (n != max) {

                value = n;

                state = DONE;

                return true;

            } else {

                r = max;

            }

            state = FIRST_BYTE_READ;

        }

        if (state == FIRST_BYTE_READ) {

            // variable-length quantity (VLQ)

            byte i;

            do {

                if (!input.hasRemaining()) {

                    return false;

                }

                i = input.get();

                long increment = b * (i & 127);

                if (r + increment > maxValue) {

                    throw new IOException(format(

                            "Integer overflow: maxValue=%,d, value=%,d",

                            maxValue, r + increment));

                }

                r += increment;

                b *= 128;

            } while ((128 & i) == 128);

            value = (int) r;

            state = DONE;

            return true;

        }

        throw new InternalError(Arrays.toString(

                new Object[]{state, N, maxValue, value, r, b}));

    }

    public int get() throws IllegalStateException {

        if (state != DONE) {

            throw new IllegalStateException("Has not been fully read yet");

        }

        return value;

    }

    private static void checkPrefix(int N) {

        if (N < 1 || N > 8) {

            throw new IllegalArgumentException("1 <= N <= 8: N= " + N);

        }

    }

    public IntegerReader reset() {

        b = 1;

        state = NEW;

        return this;

    }

}