// Copyright 2016 Google Inc. All rights reserved.
//
// 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.archivepatcher.generator.bsdiff;

import java.io.IOException;

/**
 * A Java implementation of the "bsdiff" algorithm based on the BSD-2 licensed source code available
 * here: https://github.com/mendsley/bsdiff.
 * <p>
 * A canonical description of the bsdiff algorithm can be found at the following URL:
 * http://www.daemonology.net/bsdiff/
 * <p>
 * Since Java only supports "int" for array indexing, the maximum size of files that this
 * implementation can handle is 2^31, or 2 gibibytes.
 */
class BsDiff {

  /**
   * Search the specified arrays for a contiguous sequence of identical bytes, starting at the
   * specified "start" offsets and scanning as far ahead as possible till one or the other of the
   * arrays ends or a non-matching byte is found. Returns the length of the matching sequence of
   * bytes, which may be zero.
   *
   * @param oldData the old data to scan
   * @param oldStart the position in the old data at which to start the scan
   * @param newData the new data to scan
   * @param newStart the position in the new data at which to start the scan
   * @return the number of matching bytes in the two arrays starting at the specified indices; zero
   * if the first byte fails to match
   */
  // Visible for testing only
  static int lengthOfMatch(
      final RandomAccessObject oldData,
      final int oldStart,
      final RandomAccessObject newData,
      final int newStart)
      throws IOException {
    final int max = Math.min((int) oldData.length() - oldStart, (int) newData.length() - newStart);
    if (max > 0) {
      // If max is 0, it's sometimes possible for this seek to seek to length + 1 and throw an
      // exception unnecessarily.
      oldData.seek(oldStart);
      newData.seek(newStart);
      for (int offset = 0; offset < max; offset++) {
        if (oldData.readByte() != newData.readByte()) {
          return offset;
        }
      }
    }

    return max;
  }

  // Visible for testing only
  static Match searchForMatchBaseCase(
      final RandomAccessObject groupArray,
      final RandomAccessObject oldData,
      final RandomAccessObject newData,
      final int newStart,
      final int oldDataRangeStartA,
      final int oldDataRangeStartB)
      throws IOException {
    // Located the start of a matching range (no further search required) or the size of the range
    // has shrunk to one byte (no further search possible).
    groupArray.seekToIntAligned(oldDataRangeStartA);
    final int groupArrayOldDataRangeStartA = groupArray.readInt();
    final int lengthOfMatchA =
        lengthOfMatch(oldData, groupArrayOldDataRangeStartA, newData, newStart);
    groupArray.seekToIntAligned(oldDataRangeStartB);
    final int groupArrayOldDataRangeStartB = groupArray.readInt();
    final int lengthOfMatchB =
        lengthOfMatch(oldData, groupArrayOldDataRangeStartB, newData, newStart);

    if (lengthOfMatchA > lengthOfMatchB) {
      return Match.of(groupArrayOldDataRangeStartA, lengthOfMatchA);
    }

    return Match.of(groupArrayOldDataRangeStartB, lengthOfMatchB);
  }

  /**
   * Locates the run of bytes in |oldData| which matches the longest prefix of
   * newData[newStart ... newData.length - 1].
   * @param groupArray
   * @param oldData the old data to scan
   * @param newData the new data to scan
   * @param newStart the position of the first byte in newData to consider
   * @param oldDataRangeStartA
   * @param oldDataRangeStartB
   * @return a Match containing the length of the matching range, and the position at which the
   * matching range begins.
   */
  // Visible for testing only
  static Match searchForMatch(
      final RandomAccessObject groupArray,
      final RandomAccessObject oldData,
      final RandomAccessObject newData,
      final int newStart,
      final int oldDataRangeStartA,
      final int oldDataRangeStartB)
      throws IOException {
    if (oldDataRangeStartB - oldDataRangeStartA < 2) {
      return searchForMatchBaseCase(
          groupArray, oldData, newData, newStart, oldDataRangeStartA, oldDataRangeStartB);
    }

    // Cut range in half and search again
    final int rangeLength = oldDataRangeStartB - oldDataRangeStartA;
    final int pivot = oldDataRangeStartA + (rangeLength / 2);
    groupArray.seekToIntAligned(pivot);
    final int groupArrayPivot = groupArray.readInt();
    if (BsUtil.lexicographicalCompare(
            oldData,
            groupArrayPivot,
            (int) oldData.length() - groupArrayPivot,
            newData,
            newStart,
            (int) newData.length() - newStart)
        < 0) {
      return searchForMatch(groupArray, oldData, newData, newStart, pivot, oldDataRangeStartB);
    }
    return searchForMatch(groupArray, oldData, newData, newStart, oldDataRangeStartA, pivot);
  }

  static class Match {
    final int start;
    final int length;

    static Match of(int start, int length) {
      return new Match(start, length);
    }

    private Match(int start, int length) {
      this.start = start;
      this.length = length;
    }
  }
}