/*
 * Copyright (C) 2012 The Guava Authors
 *
 * 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 com.google.common.base;

import com.google.common.annotations.GwtIncompatible;
import com.google.common.annotations.VisibleForTesting;
import com.google.common.base.CharMatcher.NamedFastMatcher;
import java.util.BitSet;

An immutable version of CharMatcher for smallish sets of characters that uses a hash table with
linear probing to check for matches.
Author:
Christopher Swenson/**
 * An immutable version of CharMatcher for smallish sets of characters that uses a hash table with
 * linear probing to check for matches.
 *
 * @author Christopher Swenson
 */
@GwtIncompatible // no precomputation is done in GWT
final class SmallCharMatcher extends NamedFastMatcher {
  static final int MAX_SIZE = 1023;
  private final char[] table;
  private final boolean containsZero;
  private final long filter;

  private SmallCharMatcher(char[] table, long filter, boolean containsZero, String description) {
    super(description);
    this.table = table;
    this.filter = filter;
    this.containsZero = containsZero;
  }

  private static final int C1 = 0xcc9e2d51;
  private static final int C2 = 0x1b873593;

  /*
   * This method was rewritten in Java from an intermediate step of the Murmur hash function in
   * http://code.google.com/p/smhasher/source/browse/trunk/MurmurHash3.cpp, which contained the
   * following header:
   *
   * MurmurHash3 was written by Austin Appleby, and is placed in the public domain. The author
   * hereby disclaims copyright to this source code.
   */
  static int smear(int hashCode) {
    return C2 * Integer.rotateLeft(hashCode * C1, 15);
  }

  private boolean checkFilter(int c) {
    return 1 == (1 & (filter >> c));
  }

  // This is all essentially copied from ImmutableSet, but we have to duplicate because
  // of dependencies.

  // Represents how tightly we can pack things, as a maximum.
  private static final double DESIRED_LOAD_FACTOR = 0.5;

  
Returns an array size suitable for the backing array of a hash table that uses open addressing
with linear probing in its implementation. The returned size is the smallest power of two that
can hold setSize elements with the desired load factor.
/**
   * Returns an array size suitable for the backing array of a hash table that uses open addressing
   * with linear probing in its implementation. The returned size is the smallest power of two that
   * can hold setSize elements with the desired load factor.
   */
  @VisibleForTesting
  static int chooseTableSize(int setSize) {
    if (setSize == 1) {
      return 2;
    }
    // Correct the size for open addressing to match desired load factor.
    // Round up to the next highest power of 2.
    int tableSize = Integer.highestOneBit(setSize - 1) << 1;
    while (tableSize * DESIRED_LOAD_FACTOR < setSize) {
      tableSize <<= 1;
    }
    return tableSize;
  }

  static CharMatcher from(BitSet chars, String description) {
    // Compute the filter.
    long filter = 0;
    int size = chars.cardinality();
    boolean containsZero = chars.get(0);
    // Compute the hash table.
    char[] table = new char[chooseTableSize(size)];
    int mask = table.length - 1;
    for (int c = chars.nextSetBit(0); c != -1; c = chars.nextSetBit(c + 1)) {
      // Compute the filter at the same time.
      filter |= 1L << c;
      int index = smear(c) & mask;
      while (true) {
        // Check for empty.
        if (table[index] == 0) {
          table[index] = (char) c;
          break;
        }
        // Linear probing.
        index = (index + 1) & mask;
      }
    }
    return new SmallCharMatcher(table, filter, containsZero, description);
  }

  @Override
  public boolean matches(char c) {
    if (c == 0) {
      return containsZero;
    }
    if (!checkFilter(c)) {
      return false;
    }
    int mask = table.length - 1;
    int startingIndex = smear(c) & mask;
    int index = startingIndex;
    do {
      if (table[index] == 0) { // Check for empty.
        return false;
      } else if (table[index] == c) { // Check for match.
        return true;
      } else { // Linear probing.
        index = (index + 1) & mask;
      }
      // Check to see if we wrapped around the whole table.
    } while (index != startingIndex);
    return false;
  }

  @Override
  void setBits(BitSet table) {
    if (containsZero) {
      table.set(0);
    }
    for (char c : this.table) {
      if (c != 0) {
        table.set(c);
      }
    }
  }
}