/*
 * Copyright (C) 2011 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.hash;

import static com.google.common.base.Preconditions.checkArgument;
import static com.google.common.base.Preconditions.checkNotNull;
import static com.google.common.base.Preconditions.checkState;

import com.google.common.annotations.Beta;
import com.google.common.base.Preconditions;
import com.google.common.primitives.Ints;
import com.google.common.primitives.UnsignedInts;
import com.google.errorprone.annotations.CanIgnoreReturnValue;

import java.io.Serializable;

import javax.annotation.Nullable;

/**
 * An immutable hash code of arbitrary bit length.
 *
 * @author Dimitris Andreou
 * @author Kurt Alfred Kluever
 * @since 11.0
 */
@Beta
public abstract class HashCode {
  HashCode() {}

  /**
   * Returns the number of bits in this hash code; a positive multiple of 8.
   */
  public abstract int bits();

  /**
   * Returns the first four bytes of {@linkplain #asBytes() this hashcode's bytes}, converted to an
   * {@code int} value in little-endian order.
   *
   * @throws IllegalStateException if {@code bits() < 32}
   */
  public abstract int asInt();

  /**
   * Returns the first eight bytes of {@linkplain #asBytes() this hashcode's bytes}, converted to a
   * {@code long} value in little-endian order.
   *
   * @throws IllegalStateException if {@code bits() < 64}
   */
  public abstract long asLong();

  /**
   * If this hashcode has enough bits, returns {@code asLong()}, otherwise returns a {@code long}
   * value with {@code asBytes()} as the least-significant bytes and {@code 0x00} as the remaining
   * most-significant bytes.
   *
   * @since 14.0 (since 11.0 as {@code Hashing.padToLong(HashCode)})
   */
  public abstract long padToLong();

  /**
   * Returns the value of this hash code as a byte array. The caller may modify the byte array;
   * changes to it will <i>not</i> be reflected in this {@code HashCode} object or any other arrays
   * returned by this method.
   */
  // TODO(user): consider ByteString here, when that is available
  public abstract byte[] asBytes();

  /**
   * Copies bytes from this hash code into {@code dest}.
   *
   * @param dest the byte array into which the hash code will be written
   * @param offset the start offset in the data
   * @param maxLength the maximum number of bytes to write
   * @return the number of bytes written to {@code dest}
   * @throws IndexOutOfBoundsException if there is not enough room in {@code dest}
   */
  @CanIgnoreReturnValue
  public int writeBytesTo(byte[] dest, int offset, int maxLength) {
    maxLength = Ints.min(maxLength, bits() / 8);
    Preconditions.checkPositionIndexes(offset, offset + maxLength, dest.length);
    writeBytesToImpl(dest, offset, maxLength);
    return maxLength;
  }

  abstract void writeBytesToImpl(byte[] dest, int offset, int maxLength);

  /**
   * Returns a mutable view of the underlying bytes for the given {@code HashCode} if it is a
   * byte-based hashcode. Otherwise it returns {@link HashCode#asBytes}. Do <i>not</i> mutate this
   * array or else you will break the immutability contract of {@code HashCode}.
   */
  byte[] getBytesInternal() {
    return asBytes();
  }

  /**
   * Returns whether this {@code HashCode} and that {@code HashCode} have the same value, given that
   * they have the same number of bits.
   */
  abstract boolean equalsSameBits(HashCode that);

  /**
   * Creates a 32-bit {@code HashCode} representation of the given int value. The underlying bytes
   * are interpreted in little endian order.
   *
   * @since 15.0 (since 12.0 in HashCodes)
   */
  public static HashCode fromInt(int hash) {
    return new IntHashCode(hash);
  }

  private static final class IntHashCode extends HashCode implements Serializable {
    final int hash;

    IntHashCode(int hash) {
      this.hash = hash;
    }

    @Override
    public int bits() {
      return 32;
    }

    @Override
    public byte[] asBytes() {
      return new byte[] {
        (byte) hash,
        (byte) (hash >> 8),
        (byte) (hash >> 16),
        (byte) (hash >> 24)
      };
    }

    @Override
    public int asInt() {
      return hash;
    }

    @Override
    public long asLong() {
      throw new IllegalStateException("this HashCode only has 32 bits; cannot create a long");
    }

    @Override
    public long padToLong() {
      return UnsignedInts.toLong(hash);
    }

    @Override
    void writeBytesToImpl(byte[] dest, int offset, int maxLength) {
      for (int i = 0; i < maxLength; i++) {
        dest[offset + i] = (byte) (hash >> (i * 8));
      }
    }

    @Override
    boolean equalsSameBits(HashCode that) {
      return hash == that.asInt();
    }

    private static final long serialVersionUID = 0;
  }

  /**
   * Creates a 64-bit {@code HashCode} representation of the given long value. The underlying bytes
   * are interpreted in little endian order.
   *
   * @since 15.0 (since 12.0 in HashCodes)
   */
  public static HashCode fromLong(long hash) {
    return new LongHashCode(hash);
  }

  private static final class LongHashCode extends HashCode implements Serializable {
    final long hash;

    LongHashCode(long hash) {
      this.hash = hash;
    }

    @Override
    public int bits() {
      return 64;
    }

    @Override
    public byte[] asBytes() {
      return new byte[] {
        (byte) hash,
        (byte) (hash >> 8),
        (byte) (hash >> 16),
        (byte) (hash >> 24),
        (byte) (hash >> 32),
        (byte) (hash >> 40),
        (byte) (hash >> 48),
        (byte) (hash >> 56)
      };
    }

    @Override
    public int asInt() {
      return (int) hash;
    }

    @Override
    public long asLong() {
      return hash;
    }

    @Override
    public long padToLong() {
      return hash;
    }

    @Override
    void writeBytesToImpl(byte[] dest, int offset, int maxLength) {
      for (int i = 0; i < maxLength; i++) {
        dest[offset + i] = (byte) (hash >> (i * 8));
      }
    }

    @Override
    boolean equalsSameBits(HashCode that) {
      return hash == that.asLong();
    }

    private static final long serialVersionUID = 0;
  }

  /**
   * Creates a {@code HashCode} from a byte array. The array is defensively copied to preserve the
   * immutability contract of {@code HashCode}. The array cannot be empty.
   *
   * @since 15.0 (since 12.0 in HashCodes)
   */
  public static HashCode fromBytes(byte[] bytes) {
    checkArgument(bytes.length >= 1, "A HashCode must contain at least 1 byte.");
    return fromBytesNoCopy(bytes.clone());
  }

  /**
   * Creates a {@code HashCode} from a byte array. The array is <i>not</i> copied defensively, so it
   * must be handed-off so as to preserve the immutability contract of {@code HashCode}.
   */
  static HashCode fromBytesNoCopy(byte[] bytes) {
    return new BytesHashCode(bytes);
  }

  private static final class BytesHashCode extends HashCode implements Serializable {
    final byte[] bytes;

    BytesHashCode(byte[] bytes) {
      this.bytes = checkNotNull(bytes);
    }

    @Override
    public int bits() {
      return bytes.length * 8;
    }

    @Override
    public byte[] asBytes() {
      return bytes.clone();
    }

    @Override
    public int asInt() {
      checkState(
          bytes.length >= 4,
          "HashCode#asInt() requires >= 4 bytes (it only has %s bytes).",
          bytes.length);
      return (bytes[0] & 0xFF)
          | ((bytes[1] & 0xFF) << 8)
          | ((bytes[2] & 0xFF) << 16)
          | ((bytes[3] & 0xFF) << 24);
    }

    @Override
    public long asLong() {
      checkState(
          bytes.length >= 8,
          "HashCode#asLong() requires >= 8 bytes (it only has %s bytes).",
          bytes.length);
      return padToLong();
    }

    @Override
    public long padToLong() {
      long retVal = (bytes[0] & 0xFF);
      for (int i = 1; i < Math.min(bytes.length, 8); i++) {
        retVal |= (bytes[i] & 0xFFL) << (i * 8);
      }
      return retVal;
    }

    @Override
    void writeBytesToImpl(byte[] dest, int offset, int maxLength) {
      System.arraycopy(bytes, 0, dest, offset, maxLength);
    }

    @Override
    byte[] getBytesInternal() {
      return bytes;
    }

    @Override
    boolean equalsSameBits(HashCode that) {
      // We don't use MessageDigest.isEqual() here because its contract does not guarantee
      // constant-time evaluation (no short-circuiting).
      if (this.bytes.length != that.getBytesInternal().length) {
        return false;
      }

      boolean areEqual = true;
      for (int i = 0; i < this.bytes.length; i++) {
        areEqual &= (this.bytes[i] == that.getBytesInternal()[i]);
      }
      return areEqual;
    }

    private static final long serialVersionUID = 0;
  }

  /**
   * Creates a {@code HashCode} from a hexadecimal ({@code base 16}) encoded string. The string must
   * be at least 2 characters long, and contain only valid, lower-cased hexadecimal characters.
   *
   * <p>This method accepts the exact format generated by {@link #toString}. If you require more
   * lenient {@code base 16} decoding, please use {@link com.google.common.io.BaseEncoding#decode}
   * (and pass the result to {@link #fromBytes}).
   *
   * @since 15.0
   */
  public static HashCode fromString(String string) {
    checkArgument(
        string.length() >= 2, "input string (%s) must have at least 2 characters", string);
    checkArgument(
        string.length() % 2 == 0,
        "input string (%s) must have an even number of characters",
        string);

    byte[] bytes = new byte[string.length() / 2];
    for (int i = 0; i < string.length(); i += 2) {
      int ch1 = decode(string.charAt(i)) << 4;
      int ch2 = decode(string.charAt(i + 1));
      bytes[i / 2] = (byte) (ch1 + ch2);
    }
    return fromBytesNoCopy(bytes);
  }

  private static int decode(char ch) {
    if (ch >= '0' && ch <= '9') {
      return ch - '0';
    }
    if (ch >= 'a' && ch <= 'f') {
      return ch - 'a' + 10;
    }
    throw new IllegalArgumentException("Illegal hexadecimal character: " + ch);
  }

  /**
   * Returns {@code true} if {@code object} is a {@link HashCode} instance with the identical byte
   * representation to this hash code.
   *
   * <p><b>Security note:</b> this method uses a constant-time (not short-circuiting) implementation
   * to protect against <a href="http://en.wikipedia.org/wiki/Timing_attack">timing attacks</a>.
   */
  @Override
  public final boolean equals(@Nullable Object object) {
    if (object instanceof HashCode) {
      HashCode that = (HashCode) object;
      return bits() == that.bits() && equalsSameBits(that);
    }
    return false;
  }

  /**
   * Returns a "Java hash code" for this {@code HashCode} instance; this is well-defined (so, for
   * example, you can safely put {@code HashCode} instances into a {@code
   * HashSet}) but is otherwise probably not what you want to use.
   */
  @Override
  public final int hashCode() {
    // If we have at least 4 bytes (32 bits), just take the first 4 bytes. Since this is
    // already a (presumably) high-quality hash code, any four bytes of it will do.
    if (bits() >= 32) {
      return asInt();
    }
    // If we have less than 4 bytes, use them all.
    byte[] bytes = getBytesInternal();
    int val = (bytes[0] & 0xFF);
    for (int i = 1; i < bytes.length; i++) {
      val |= ((bytes[i] & 0xFF) << (i * 8));
    }
    return val;
  }

  /**
   * Returns a string containing each byte of {@link #asBytes}, in order, as a two-digit unsigned
   * hexadecimal number in lower case.
   *
   * <p>Note that if the output is considered to be a single hexadecimal number, this hash code's
   * bytes are the <i>big-endian</i> representation of that number. This may be surprising since
   * everything else in the hashing API uniformly treats multibyte values as little-endian. But this
   * format conveniently matches that of utilities such as the UNIX {@code md5sum} command.
   *
   * <p>To create a {@code HashCode} from its string representation, see {@link #fromString}.
   */
  @Override
  public final String toString() {
    byte[] bytes = getBytesInternal();
    StringBuilder sb = new StringBuilder(2 * bytes.length);
    for (byte b : bytes) {
      sb.append(hexDigits[(b >> 4) & 0xf]).append(hexDigits[b & 0xf]);
    }
    return sb.toString();
  }

  private static final char[] hexDigits = "0123456789abcdef".toCharArray();
}
