/*
 * Copyright (C) 2002-2017 Sebastiano Vigna
 *
 * 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 it.unimi.dsi.fastutil;

/**
 * NOTE: This source code was copied from the <a href="http://fastutil.di.unimi.it/">fastutil</a>
 * project, and has been modified.
 *
 * <p>Common code for all hash-based classes.
 *
 * @author Sebastiano Vigna
 * @see <a href="http://fastutil.di.unimi.it/">fastutil</a>
 */
public class HashCommon {

  /** 2<sup>32</sup> &middot; &phi;, &phi; = (&#x221A;5 &minus; 1)/2. */
  private static final int INT_PHI = 0x9E3779B9;
  /** The reciprocal of {@link #INT_PHI} modulo 2<sup>32</sup>. */
  private static final int INV_INT_PHI = 0x144cbc89;
  /** 2<sup>64</sup> &middot; &phi;, &phi; = (&#x221A;5 &minus; 1)/2. */
  private static final long LONG_PHI = 0x9E3779B97F4A7C15L;
  /** The reciprocal of {@link #LONG_PHI} modulo 2<sup>64</sup>. */
  private static final long INV_LONG_PHI = 0xf1de83e19937733dL;

  protected HashCommon() {}

  /**
   * Avalanches the bits of an integer by applying the finalisation step of MurmurHash3.
   *
   * <p>This method implements the finalisation step of Austin Appleby's <a
   * href="http://code.google.com/p/smhasher/">MurmurHash3</a>. Its purpose is to avalanche the bits
   * of the argument to within 0.25% bias.
   *
   * @param x an integer.
   * @return a hash value with good avalanching properties.
   */
  public static int murmurHash3(int x) {
    x ^= x >>> 16;
    x *= 0x85ebca6b;
    x ^= x >>> 13;
    x *= 0xc2b2ae35;
    x ^= x >>> 16;
    return x;
  }

  /**
   * Avalanches the bits of a long integer by applying the finalisation step of MurmurHash3.
   *
   * <p>This method implements the finalisation step of Austin Appleby's <a
   * href="http://code.google.com/p/smhasher/">MurmurHash3</a>. Its purpose is to avalanche the bits
   * of the argument to within 0.25% bias.
   *
   * @param x a long integer.
   * @return a hash value with good avalanching properties.
   */
  public static long murmurHash3(long x) {
    x ^= x >>> 33;
    x *= 0xff51afd7ed558ccdL;
    x ^= x >>> 33;
    x *= 0xc4ceb9fe1a85ec53L;
    x ^= x >>> 33;
    return x;
  }

  /**
   * Quickly mixes the bits of an integer.
   *
   * <p>This method mixes the bits of the argument by multiplying by the golden ratio and
   * xorshifting the result. It is borrowed from <a
   * href="https://github.com/OpenHFT/Koloboke">Koloboke</a>, and it has slightly worse behaviour
   * than {@link #murmurHash3(int)} (in open-addressing hash tables the average number of probes is
   * slightly larger), but it's much faster.
   *
   * @param x an integer.
   * @return a hash value obtained by mixing the bits of {@code x}.
   * @see #invMix(int)
   */
  public static int mix(final int x) {
    final int h = x * INT_PHI;
    return h ^ (h >>> 16);
  }

  /**
   * The inverse of {@link #mix(int)}. This method is mainly useful to create unit tests.
   *
   * @param x an integer.
   * @return a value that passed through {@link #mix(int)} would give {@code x}.
   */
  public static int invMix(final int x) {
    return (x ^ x >>> 16) * INV_INT_PHI;
  }

  /**
   * Quickly mixes the bits of a long integer.
   *
   * <p>This method mixes the bits of the argument by multiplying by the golden ratio and
   * xorshifting twice the result. It is borrowed from <a
   * href="https://github.com/OpenHFT/Koloboke">Koloboke</a>, and it has slightly worse behaviour
   * than {@link #murmurHash3(long)} (in open-addressing hash tables the average number of probes is
   * slightly larger), but it's much faster.
   *
   * @param x a long integer.
   * @return a hash value obtained by mixing the bits of {@code x}.
   */
  public static long mix(final long x) {
    long h = x * LONG_PHI;
    h ^= h >>> 32;
    return h ^ (h >>> 16);
  }

  /**
   * The inverse of {@link #mix(long)}. This method is mainly useful to create unit tests.
   *
   * @param x a long integer.
   * @return a value that passed through {@link #mix(long)} would give {@code x}.
   */
  public static long invMix(long x) {
    x ^= x >>> 32;
    x ^= x >>> 16;
    return (x ^ x >>> 32) * INV_LONG_PHI;
  }

  /**
   * Returns the hash code that would be returned by {@link Float#hashCode()}.
   *
   * @param f a float.
   * @return the same code as {@link Float#hashCode() new Float(f).hashCode()}.
   */
  public static int float2int(final float f) {
    return Float.floatToRawIntBits(f);
  }

  /**
   * Returns the hash code that would be returned by {@link Double#hashCode()}.
   *
   * @param d a double.
   * @return the same code as {@link Double#hashCode() new Double(f).hashCode()}.
   */
  public static int double2int(final double d) {
    final long l = Double.doubleToRawLongBits(d);
    return (int) (l ^ (l >>> 32));
  }

  /**
   * Returns the hash code that would be returned by {@link Long#hashCode()}.
   *
   * @param l a long.
   * @return the same code as {@link Long#hashCode() new Long(f).hashCode()}.
   */
  public static int long2int(final long l) {
    return (int) (l ^ (l >>> 32));
  }

  /**
   * Returns the least power of two greater than or equal to the specified value.
   *
   * <p>Note that this function will return 1 when the argument is 0.
   *
   * @param x an integer smaller than or equal to 2<sup>30</sup>.
   * @return the least power of two greater than or equal to the specified value.
   */
  public static int nextPowerOfTwo(int x) {
    if (x == 0) {
      return 1;
    }
    x--;
    x |= x >> 1;
    x |= x >> 2;
    x |= x >> 4;
    x |= x >> 8;
    return (x | x >> 16) + 1;
  }

  /**
   * Returns the least power of two greater than or equal to the specified value.
   *
   * <p>Note that this function will return 1 when the argument is 0.
   *
   * @param x a long integer smaller than or equal to 2<sup>62</sup>.
   * @return the least power of two greater than or equal to the specified value.
   */
  public static long nextPowerOfTwo(long x) {
    if (x == 0) {
      return 1;
    }
    x--;
    x |= x >> 1;
    x |= x >> 2;
    x |= x >> 4;
    x |= x >> 8;
    x |= x >> 16;
    return (x | x >> 32) + 1;
  }

  /**
   * Returns the maximum number of entries that can be filled before rehashing.
   *
   * @param n the size of the backing array.
   * @param f the load factor.
   * @return the maximum number of entries before rehashing.
   */
  public static int maxFill(final int n, final float f) {
    /* We must guarantee that there is always at least
     * one free entry (even with pathological load ffunctions). */
    return Math.min((int) Math.ceil(n * f), n - 1);
  }

  /**
   * Returns the maximum number of entries that can be filled before rehashing.
   *
   * @param n the size of the backing array.
   * @param f the load factor.
   * @return the maximum number of entries before rehashing.
   */
  public static long maxFill(final long n, final float f) {
    /* We must guarantee that there is always at least
     * one free entry (even with pathological load ffunctions). */
    return Math.min((long) Math.ceil(n * f), n - 1);
  }

  /**
   * Returns the least power of two smaller than or equal to 2<sup>30</sup> and larger than or equal
   * to {@code Math.ceil(expected / f)}.
   *
   * @param expected the expected number of elements in a hash table.
   * @param f the load factor.
   * @return the minimum possible size for a backing array.
   * @throws IllegalArgumentException if the necessary size is larger than 2<sup>30</sup>.
   */
  public static int arraySize(final int expected, final float f) {
    final long s = Math.max(2, nextPowerOfTwo((long) Math.ceil(expected / f)));
    if (s > (1 << 30)) {
      throw new IllegalArgumentException(
          "Too large (" + expected + " expected elements with load factor " + f + ")");
    }
    return (int) s;
  }

  /**
   * Returns the least power of two larger than or equal to {@code Math.ceil(expected / f)}.
   *
   * @param expected the expected number of elements in a hash table.
   * @param f the load factor.
   * @return the minimum possible size for a backing big array.
   */
  public static long bigArraySize(final long expected, final float f) {
    return nextPowerOfTwo((long) Math.ceil(expected / f));
  }
}