ConsString.java

/*
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 * 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
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package jdk.nashorn.internal.runtime;

import static jdk.nashorn.internal.runtime.JSType.isString;

import java.util.ArrayDeque;
import java.util.Deque;

/**
 * This class represents a string composed of two parts which may themselves be
 * instances of <tt>ConsString</tt> or {@link String}. Copying of characters to
 * a proper string is delayed until it becomes necessary.
 */
public final class ConsString implements CharSequence {

    private CharSequence left, right;
    private final int length;
    private volatile int state = STATE_NEW;

    private final static int STATE_NEW       =  0;
    private final static int STATE_THRESHOLD =  2;
    private final static int STATE_FLATTENED = -1;

    /**
     * Constructor
     *
     * Takes two {@link CharSequence} instances that, concatenated, forms this {@code ConsString}
     *
     * @param left  left char sequence
     * @param right right char sequence
     */
    public ConsString(final CharSequence left, final CharSequence right) {
        assert isString(left);
        assert isString(right);
        this.left = left;
        this.right = right;
        length = left.length() + right.length();
        if (length < 0) {
            throw new IllegalArgumentException("too big concatenated String");
        }
    }

    @Override
    public String toString() {
        return (String) flattened(true);
    }

    @Override
    public int length() {
        return length;
    }

    @Override
    public char charAt(final int index) {
        return flattened(true).charAt(index);
    }

    @Override
    public CharSequence subSequence(final int start, final int end) {
        return flattened(true).subSequence(start, end);
    }

    /**
     * Returns the components of this ConsString as a {@code CharSequence} array with two elements.
     * The elements will be either {@code Strings} or other {@code ConsStrings}.
     * @return CharSequence array of length 2
     */
    public synchronized CharSequence[] getComponents() {
        return new CharSequence[] { left, right };
    }

    private CharSequence flattened(final boolean flattenNested) {
        if (state != STATE_FLATTENED) {
            flatten(flattenNested);
        }
        return left;
    }

    private synchronized void flatten(final boolean flattenNested) {
        // We use iterative traversal as recursion may exceed the stack size limit.
        final char[] chars = new char[length];
        int pos = length;
        // Strings are most often composed by appending to the end, which causes ConsStrings
        // to be very unbalanced, with mostly single string elements on the right and a long
        // linear list on the left. Traversing from right to left helps to keep the stack small
        // in this scenario.
        final Deque<CharSequence> stack = new ArrayDeque<>();
        stack.addFirst(left);
        CharSequence cs = right;

        do {
            if (cs instanceof ConsString) {
                final ConsString cons = (ConsString) cs;
                // Count the times a cons-string is traversed as part of other cons-strings being flattened.
                // If it crosses a threshold we flatten the nested cons-string internally.
                if (cons.state == STATE_FLATTENED || (flattenNested && ++cons.state >= STATE_THRESHOLD)) {
                    cs = cons.flattened(false);
                } else {
                    stack.addFirst(cons.left);
                    cs = cons.right;
                }
            } else {
                final String str = (String) cs;
                pos -= str.length();
                str.getChars(0, str.length(), chars, pos);
                cs = stack.isEmpty() ? null : stack.pollFirst();
            }
        } while (cs != null);

        left = new String(chars);
        right = "";
        state = STATE_FLATTENED;
    }

}