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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 ConsString or String. Copying of characters to a proper string is delayed until it becomes necessary.
/** * This class represents a string composed of two parts which may themselves be * instances of <code>ConsString</code> 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 CharSequence instances that, concatenated, forms this ConsString
Params:
  • left – left char sequence
  • right – right char sequence
/** * 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 CharSequence array with two elements. The elements will be either Strings or other ConsStrings.
Returns:CharSequence array of length 2
/** * 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; } }