/*
 * Copyright 2012-present Facebook, Inc.
 *
 * 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.facebook.buck.rules;

import com.facebook.buck.log.views.JsonViews;
import com.fasterxml.jackson.annotation.JsonProperty;
import com.fasterxml.jackson.annotation.JsonView;
import com.google.common.base.Objects;
import com.google.common.hash.HashCode;
import javax.annotation.Nullable;

/**
 * RuleKey encapsulates regimented computation of SHA-1 keys that incorporate all BuildRule state
 * relevant to idempotency. The RuleKey.Builder API conceptually implements the construction of an
 * ordered map, and the key/val pairs are digested using an internal serialization that guarantees a
 * 1:1 mapping for each distinct vector of keys <header,k1,...,kn> in
 * RuleKey.builder(header).set(k1, v1) ... .set(kn, vn).build().
 *
 * <p>Note carefully that in order to reliably avoid accidental collisions, each RuleKey schema, as
 * defined by the key vector, must have a distinct header. Otherwise it is possible (if unlikely)
 * for serialized value data to alias serialized key data, with the result being identical RuleKeys
 * for differing input. In practical terms this means that each BuildRule implementer should specify
 * a distinct header, and that for all RuleKeys built with a particular header, the sequence of
 * set() calls should be identical, even if values are missing. The set() methods specifically
 * handle null values to accommodate this regime.
 */
public class RuleKey {

  private final HashCode hashCode;

  public RuleKey(HashCode hashCode) {
    this.hashCode = hashCode;
  }

  /**
   * @param hashString string that conforms to the contract of the return value of {@link
   *     com.google.common.hash.HashCode#toString()}.
   */
  public RuleKey(String hashString) {
    this(HashCode.fromString(hashString));
  }

  public HashCode getHashCode() {
    return hashCode;
  }

  /** @return the {@code toString()} of the hash code that underlies this RuleKey. */
  @Override
  @JsonProperty("hashCode")
  @JsonView(JsonViews.MachineReadableLog.class)
  public String toString() {
    return getHashCode().toString();
  }

  @Override
  public boolean equals(@Nullable Object obj) {
    if (!(obj instanceof RuleKey)) {
      return false;
    }
    RuleKey that = (RuleKey) obj;
    return Objects.equal(this.getHashCode(), that.getHashCode());
  }

  @Override
  public int hashCode() {
    return this.getHashCode().hashCode();
  }
}