DigestVisitor.java

/*
 * Copyright 2011 Robert W. Vawter III <bob@vawter.org>
 * 
 * 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 org.jsonddl.impl;

import java.nio.ByteBuffer;
import java.nio.charset.Charset;
import java.security.MessageDigest;
import java.security.NoSuchAlgorithmException;
import java.util.ArrayList;
import java.util.List;
import java.util.Map;

import org.jsonddl.JsonDdlObject;
import org.jsonddl.JsonDdlVisitor.PropertyVisitor;
import org.jsonddl.model.Kind;

/**
 * Computes a MessageDigest.
 */
public class DigestVisitor implements PropertyVisitor {
  private static final char[] HEX_DIGITS = { '0', '1', '2', '3', '4', '5', '6', '7', '8', '9', 'a',
      'b', 'c', 'd', 'e', 'f' };
  private static final Charset UTF8 = Charset.forName("UTF8");
  private final MessageDigest sha;
  private final ByteBuffer temp = ByteBuffer.allocate(8);

  public DigestVisitor() {
    try {
      sha = MessageDigest.getInstance("SHA1");
    } catch (NoSuchAlgorithmException e) {
      throw new RuntimeException("SHA1 unsupported", e);
    }
  }

  public DigestVisitor(String digestName) throws NoSuchAlgorithmException {
    sha = MessageDigest.getInstance(digestName);
  }

  public void endVisit(JsonDdlObject<?> x) {
    sha.update(x.getDdlObjectType().getName().getBytes(UTF8));
  }

  @Override
  public <T> void endVisitProperty(T value, Context<T> ctx) {
    update(ctx.getProperty());
    update(value, ctx.getKind(), ctx.getNestedKinds());
  }

  /**
   * Returns the computed digest and resets the state of the visitor.
   */
  public byte[] getDigest() {
    return sha.digest();
  }

  /**
   * Returns the computed digest as a hexadecimal-encoded string and resets the state of the
   * visitor.
   */
  public String getDigestHex() {
    StringBuilder sb = new StringBuilder();
    for (byte b : getDigest()) {
      char lsc = HEX_DIGITS[b & 0xF];
      char msc = HEX_DIGITS[b >>> 4];
      sb.append(msc).append(lsc);
    }
    return sb.toString();
  }

  @Override
  public <T> boolean visitProperty(T value, Context<T> ctx) {
    return false;
  }

  private void update(double d) {
    temp.rewind();
    temp.putDouble(d);
    temp.rewind();
    temp.limit(8);
    sha.update(temp);
  }

  private void update(int i) {
    temp.rewind();
    temp.putInt(i);
    temp.rewind();
    temp.limit(4);
    sha.update(temp);
  }

  private void update(Object value, Kind kind, List<Kind> nestedKinds) {
    if (value == null) {
      return;
    }

    switch (kind) {
      case BOOLEAN:
        sha.update(((Boolean) value).booleanValue() ? (byte) 1 : 0);
        break;
      case DDL:
        sha.update(((Digested) value).computeDigest());
        break;
      case DOUBLE:
        update((Double) value);
        break;
      case ENUM:
        update(((Enum<?>) value).name());
        break;
      case INTEGER:
        update((Integer) value);
        break;
      case STRING:
        update(value.toString());
        break;
      case LIST: {
        List<?> list = (List<?>) value;
        update(list.size());
        List<Kind> newKinds = new ArrayList<Kind>(nestedKinds);
        Kind newKind = newKinds.remove(0);
        for (Object o : list) {
          update(o, newKind, newKinds);
        }
        break;
      }
      case MAP: {
        Map<?, ?> map = (Map<?, ?>) value;
        update(map.size());
        List<Kind> newKinds = new ArrayList<Kind>(nestedKinds);
        if (!Kind.STRING.equals(newKinds.remove(0))) {
          throw new RuntimeException("Did not find expected String kind");
        }
        Kind newKind = newKinds.remove(0);
        for (Map.Entry<?, ?> entry : map.entrySet()) {
          update(entry.getKey().toString());
          update(entry.getValue(), newKind, newKinds);
        }
        break;
      }
      case EXTERNAL:
        if (value instanceof Digested) {
          sha.update(((Digested) value).computeDigest());
        } else {
          // Fake it.
          update(value.getClass().getName());
          update(value.hashCode());
          update(value.toString());
        }
        break;
      default:
        throw new RuntimeException("Unhandled kind " + kind);
    }
  }

  private void update(String s) {
    sha.update(s.getBytes(UTF8));
  }
}