// Copyright 2014 The Bazel Authors. 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.devtools.build.lib.vfs;

import com.google.common.base.Function;
import com.google.common.base.Predicate;
import com.google.common.collect.ImmutableSet;
import com.google.common.collect.Iterables;
import com.google.devtools.build.lib.concurrent.ThreadSafety.Immutable;
import com.google.devtools.build.lib.concurrent.ThreadSafety.ThreadSafe;
import com.google.devtools.build.lib.util.OS;
import com.google.devtools.build.lib.util.Preconditions;
import com.google.devtools.build.lib.util.StringCanonicalizer;

import java.io.File;
import java.io.InvalidObjectException;
import java.io.ObjectInputStream;
import java.io.Serializable;
import java.util.Arrays;
import java.util.Set;

/**
 * This class represents an immutable UNIX filesystem path, which may be absolute or relative. The
 * path is maintained as a simple ordered list of path segment strings.
 *
 * <p>This class is independent from other VFS classes, especially anything requiring native code.
 * It is safe to use in places that need simple segmented string path functionality.
 *
 * <p>There is some limited support for Windows-style paths. Most importantly, drive identifiers
 * in front of a path (c:/abc) are supported and such paths are correctly recognized as absolute.
 * However, Windows-style backslash separators (C:\\foo\\bar) are explicitly not supported, same
 * with advanced features like \\\\network\\paths and \\\\?\\unc\\paths.
 */
@Immutable @ThreadSafe
public final class  PathFragment implements Comparable<PathFragment>, Serializable {

  public static final int INVALID_SEGMENT = -1;

  public static final char SEPARATOR_CHAR = '/';

  public static final char EXTRA_SEPARATOR_CHAR =
      (OS.getCurrent() == OS.WINDOWS) ? '\\' : '/';

  public static final String ROOT_DIR = "/";

  /** An empty path fragment. */
  public static final PathFragment EMPTY_FRAGMENT = new PathFragment("");

  public static final Function<String, PathFragment> TO_PATH_FRAGMENT =
      new Function<String, PathFragment>() {
        @Override
        public PathFragment apply(String str) {
          return new PathFragment(str);
        }
      };

  public static final Predicate<PathFragment> IS_ABSOLUTE =
      new Predicate<PathFragment>() {
        @Override
        public boolean apply(PathFragment input) {
          return input.isAbsolute();
        }
      };

  private static final Function<PathFragment, String> TO_SAFE_PATH_STRING =
      new Function<PathFragment, String>() {
        @Override
        public String apply(PathFragment path) {
          return path.getSafePathString();
        }
      };

  /** Lower-level API. Create a PathFragment, interning segments. */
  public static PathFragment create(char driveLetter, boolean isAbsolute, String[] segments) {
    String[] internedSegments = new String[segments.length];
    for (int i = 0; i < segments.length; i++) {
      internedSegments[i] = StringCanonicalizer.intern(segments[i]);
    }
    return new PathFragment(driveLetter, isAbsolute, segments);
  }

  // We have 3 word-sized fields (segments, hashCode and path), and 2
  // byte-sized ones, which fits in 16 bytes. Object sizes are rounded
  // to 16 bytes.  Medium sized builds can easily hold millions of
  // live PathFragments, so do not add further fields on a whim.

  // The individual path components.
  private final String[] segments;

  // True both for UNIX-style absolute paths ("/foo") and Windows-style ("C:/foo").
  private final boolean isAbsolute;

  // Upper case windows drive letter, or '\0' if none. While a volumeName string is more
  // general, we create a lot of these objects, so space is at a premium.
  private final char driveLetter;

  // hashCode and path are lazily initialized but semantically immutable.
  private int hashCode;
  private String path;

  /**
   * Construct a PathFragment from a string, which is an absolute or relative UNIX or Windows path.
   */
  public PathFragment(String path) {
    this.driveLetter = getWindowsDriveLetter(path);
    if (driveLetter != '\0') {
      path = path.substring(2);
      // TODO(bazel-team): Decide what to do about non-absolute paths with a volume name, e.g. C:x.
    }
    this.isAbsolute = path.length() > 0 && isSeparator(path.charAt(0));
    this.segments = segment(path, isAbsolute ? 1 : 0);
  }

  private static boolean isSeparator(char c) {
    return c == SEPARATOR_CHAR || c == EXTRA_SEPARATOR_CHAR;
  }

  /**
   * Construct a PathFragment from a java.io.File, which is an absolute or
   * relative UNIX path.  Does not support Windows-style Files.
   */
  public PathFragment(File path) {
    this(path.getPath());
  }

  /**
   * Constructs a PathFragment, taking ownership of segments. Package-private,
   * because it does not perform a defensive clone of the segments array. Used
   * here in PathFragment, and by Path.asFragment() and Path.relativeTo().
   */
  PathFragment(char driveLetter, boolean isAbsolute, String[] segments) {
    this.driveLetter = driveLetter;
    this.isAbsolute = isAbsolute;
    this.segments = segments;
  }

  /**
   * Construct a PathFragment from a sequence of other PathFragments. The new
   * fragment will be absolute iff the first fragment was absolute.
   */
  public PathFragment(PathFragment first, PathFragment second, PathFragment... more) {
    // TODO(bazel-team): The handling of absolute path fragments in this constructor is unexpected.
    this.segments = new String[sumLengths(first, second, more)];
    int offset = 0;
    offset += addSegments(offset, first);
    offset += addSegments(offset, second);
    for (PathFragment fragment : more) {
      offset += addSegments(offset, fragment);
    }
    this.isAbsolute = first.isAbsolute;
    this.driveLetter = first.driveLetter;
  }

  private int addSegments(int offset, PathFragment fragment) {
    int count = fragment.segmentCount();
    System.arraycopy(fragment.segments, 0, this.segments, offset, count);
    return count;
  }

  private static int sumLengths(PathFragment first, PathFragment second, PathFragment[] more) {
    int total = first.segmentCount() + second.segmentCount();
    for (PathFragment fragment : more) {
      total += fragment.segmentCount();
    }
    return total;
  }

  /**
   * Segments the string passed in as argument and returns an array of strings.
   * The split is performed along occurrences of (sequences of) the slash
   * character.
   *
   * @param toSegment the string to segment
   * @param offset how many characters from the start of the string to ignore.
   */
  private static String[] segment(String toSegment, int offset) {
    int length = toSegment.length();

    // Handle "/" and "" quickly.
    if (length == offset) {
      return new String[0];
    }

    // We make two passes through the array of characters: count & alloc,
    // because simply using ArrayList was a bottleneck showing up during profiling.
    int seg = 0;
    int start = offset;
    for (int i = offset; i < length; i++) {
      if (isSeparator(toSegment.charAt(i))) {
        if (i > start) {  // to skip repeated separators
          seg++;
        }
        start = i + 1;
      }
    }
    if (start < length) {
      seg++;
    }
    String[] result = new String[seg];
    seg = 0;
    start = offset;
    for (int i = offset; i < length; i++) {
      if (isSeparator(toSegment.charAt(i))) {
        if (i > start) {  // to skip repeated separators
          result[seg] = StringCanonicalizer.intern(toSegment.substring(start,  i));
          seg++;
        }
        start = i + 1;
      }
    }
    if (start < length) {
      result[seg] = StringCanonicalizer.intern(toSegment.substring(start, length));
    }
    return result;
  }

  private Object writeReplace() {
    return new PathFragmentSerializationProxy(toString());
  }

  private void readObject(ObjectInputStream stream) throws InvalidObjectException {
    throw new InvalidObjectException("Serialization is allowed only by proxy");
  }

  /**
   * Returns the path string using '/' as the name-separator character.  Returns "" if the path
   * is both relative and empty.
   */
  public String getPathString() {
    // Double-checked locking works, even without volatile, because path is a String, according to:
    // http://www.cs.umd.edu/~pugh/java/memoryModel/DoubleCheckedLocking.html
    if (path == null) {
      synchronized (this) {
        if (path == null) {
          path = StringCanonicalizer.intern(joinSegments(SEPARATOR_CHAR));
        }
      }
    }
    return path;
  }

  /**
   * Returns "." if the path fragment is both relative and empty, or {@link
   * #getPathString} otherwise.
   */
  // TODO(bazel-team): Change getPathString to do this - this behavior makes more sense.
  public String getSafePathString() {
    return (!isAbsolute && (segmentCount() == 0)) ? "." : getPathString();
  }

  /**
   * Returns a sequence consisting of the {@link #getSafePathString()} return of each item in
   * {@code fragments}.
   */
  public static Iterable<String> safePathStrings(Iterable<PathFragment> fragments) {
    return Iterables.transform(fragments, TO_SAFE_PATH_STRING);
  }

  /** Returns the subset of {@code paths} that start with {@code startingWithPath}. */
  public static ImmutableSet<PathFragment> filterPathsStartingWith(Set<PathFragment> paths,
      PathFragment startingWithPath) {
    return ImmutableSet.copyOf(Iterables.filter(paths, startsWithPredicate(startingWithPath)));
  }

  public static Predicate<PathFragment> startsWithPredicate(final PathFragment prefix) {
    return new Predicate<PathFragment>() {
      @Override
      public boolean apply(PathFragment pathFragment) {
        return pathFragment.startsWith(prefix);
      }
    };
  }

  /**
  * Throws {@link IllegalArgumentException} if {@code paths} contains any paths that
  * are equal to {@code startingWithPath} or that are not beneath {@code startingWithPath}.
  */
  public static void checkAllPathsAreUnder(Iterable<PathFragment> paths,
      PathFragment startingWithPath) {
    for (PathFragment path : paths) {
      Preconditions.checkArgument(
          !path.equals(startingWithPath) && path.startsWith(startingWithPath),
              "%s is not beneath %s", path, startingWithPath);
    }
  }

  private String joinSegments(char separatorChar) {
    if (segments.length == 0 && isAbsolute) {
      return windowsVolume() + ROOT_DIR;
    }

    // Profile driven optimization:
    // Preallocate a size determined by the number of segments, so that
    // we do not have to expand the capacity of the StringBuilder.
    // Heuristically, this estimate is right for about 99% of the time.
    int estimateSize =
        ((driveLetter != '\0') ? 2 : 0)
        + ((segments.length == 0) ? 0 : (segments.length + 1) * 20);
    StringBuilder result = new StringBuilder(estimateSize);
    result.append(windowsVolume());
    boolean initialSegment = true;
    for (String segment : segments) {
      if (!initialSegment || isAbsolute) {
        result.append(separatorChar);
      }
      initialSegment = false;
      result.append(segment);
    }
    return result.toString();
  }

  /**
   * Return true iff none of the segments are either "." or "..".
   */
  public boolean isNormalized() {
    for (String segment : segments) {
      if (segment.equals(".") || segment.equals("..")) {
        return false;
      }
    }
    return true;
  }

  /**
   * Normalizes the path fragment: removes "." and ".." segments if possible
   * (if there are too many ".." segments, the resulting PathFragment will still
   * start with "..").
   */
  public PathFragment normalize() {
    String[] scratchSegments = new String[segments.length];
    int segmentCount = 0;

    for (String segment : segments) {
      switch (segment) {
        case ".":
          // Just discard it
          break;
        case "..":
          if (segmentCount > 0 && !scratchSegments[segmentCount - 1].equals("..")) {
            // Remove the last segment, if there is one and it is not "..". This
            // means that the resulting PathFragment can still contain ".."
            // segments at the beginning.
            segmentCount--;
          } else {
            scratchSegments[segmentCount++] = segment;
          }
          break;
        default:
          scratchSegments[segmentCount++] = segment;
      }
    }

    if (segmentCount == segments.length) {
      // Optimization, no new PathFragment needs to be created.
      return this;
    }

    return new PathFragment(driveLetter, isAbsolute,
        subarray(scratchSegments, 0, segmentCount));
  }

  /**
   * Returns the path formed by appending the relative or absolute path fragment
   * {@code otherFragment} to this path.
   *
   * <p>If {@code otherFragment} is absolute, the current path will be ignored;
   * otherwise, they will be concatenated. This is a purely syntactic operation,
   * with no path normalization or I/O performed.
   */
  public PathFragment getRelative(PathFragment otherFragment) {
    if (otherFragment == EMPTY_FRAGMENT) {
      return this;
    }
    return otherFragment.isAbsolute()
        ? otherFragment
        : new PathFragment(this, otherFragment);
  }

  /**
   * Returns the path formed by appending the relative or absolute string
   * {@code path} to this path.
   *
   * <p>If the given path string is absolute, the current path will be ignored;
   * otherwise, they will be concatenated. This is a purely syntactic operation,
   * with no path normalization or I/O performed.
   */
  public PathFragment getRelative(String path) {
    return getRelative(new PathFragment(path));
  }

  /**
   * Returns the path formed by appending the single non-special segment
   * "baseName" to this path.
   *
   * <p>You should almost always use {@link #getRelative} instead, which has
   * the same performance characteristics if the given name is a valid base
   * name, and which also works for '.', '..', and strings containing '/'.
   *
   * @throws IllegalArgumentException if {@code baseName} is not a valid base
   *     name according to {@link FileSystemUtils#checkBaseName}
   */
  public PathFragment getChild(String baseName) {
    FileSystemUtils.checkBaseName(baseName);
    baseName = StringCanonicalizer.intern(baseName);
    String[] newSegments = Arrays.copyOf(segments, segments.length + 1);
    newSegments[newSegments.length - 1] = baseName;
    return new PathFragment(driveLetter, isAbsolute, newSegments);
  }

  /**
   * Returns the last segment of this path, or "" for the empty fragment.
   */
  public String getBaseName() {
    return (segments.length == 0) ? "" : segments[segments.length - 1];
  }

  /**
   * Returns a relative path fragment to this path, relative to
   * {@code ancestorDirectory}.
   * <p>
   * <code>x.relativeTo(z) == y</code> implies
   * <code>z.getRelative(y) == x</code>.
   * <p>
   * For example, <code>"foo/bar/wiz".relativeTo("foo")</code>
   * returns <code>"bar/wiz"</code>.
   */
  public PathFragment relativeTo(PathFragment ancestorDirectory) {
    String[] ancestorSegments = ancestorDirectory.segments();
    int ancestorLength = ancestorSegments.length;

    if (isAbsolute != ancestorDirectory.isAbsolute()
        || segments.length < ancestorLength) {
      throw new IllegalArgumentException("PathFragment " + this
          + " is not beneath " + ancestorDirectory);
    }

    for (int index = 0; index < ancestorLength; index++) {
      if (!segments[index].equals(ancestorSegments[index])) {
        throw new IllegalArgumentException("PathFragment " + this
            + " is not beneath " + ancestorDirectory);
      }
    }

    int length = segments.length - ancestorLength;
    String[] resultSegments = subarray(segments, ancestorLength, length);
    return new PathFragment('\0', false, resultSegments);
  }

  /**
   * Returns a relative path fragment to this path, relative to {@code path}.
   */
  public PathFragment relativeTo(String path) {
    return relativeTo(new PathFragment(path));
  }

  /**
   * Returns a new PathFragment formed by appending {@code newName} to the
   * parent directory. Null is returned iff this method is called on a
   * PathFragment with zero segments.  If {@code newName} designates an absolute path,
   * the value of {@code this} will be ignored and a PathFragment corresponding to
   * {@code newName} will be returned.  This behavior is consistent with the behavior of
   * {@link #getRelative(String)}.
   */
  public PathFragment replaceName(String newName) {
    return segments.length == 0 ? null : getParentDirectory().getRelative(newName);
  }

  /**
   * Returns a path representing the parent directory of this path,
   * or null iff this Path represents the root of the filesystem.
   *
   * <p>Note: This method DOES NOT normalize ".."  and "." path segments.
   */
  public PathFragment getParentDirectory() {
    return segments.length == 0 ? null : subFragment(0, segments.length - 1);
  }

  /**
   * Returns true iff {@code prefix}, considered as a list of path segments, is
   * a prefix of {@code this}, and that they are both relative or both
   * absolute.
   *
   * <p>This is a reflexive, transitive, anti-symmetric relation (i.e. a partial
   * order)
   */
  public boolean startsWith(PathFragment prefix) {
    if (this.isAbsolute != prefix.isAbsolute ||
        this.segments.length < prefix.segments.length ||
        this.driveLetter != prefix.driveLetter) {
      return false;
    }
    for (int i = 0, len = prefix.segments.length; i < len; i++) {
      if (!this.segments[i].equals(prefix.segments[i])) {
        return false;
      }
    }
    return true;
  }

  /**
   * Returns true iff {@code suffix}, considered as a list of path segments, is
   * relative and a suffix of {@code this}, or both are absolute and equal.
   *
   * <p>This is a reflexive, transitive, anti-symmetric relation (i.e. a partial
   * order)
   */
  public boolean endsWith(PathFragment suffix) {
    if ((suffix.isAbsolute && !suffix.equals(this)) ||
        this.segments.length < suffix.segments.length) {
      return false;
    }
    int offset = this.segments.length - suffix.segments.length;
    for (int i = 0; i < suffix.segments.length; i++) {
      if (!this.segments[offset + i].equals(suffix.segments[i])) {
        return false;
      }
    }
    return true;
  }

  private static String[] subarray(String[] array, int start, int length) {
    String[] subarray = new String[length];
    System.arraycopy(array, start, subarray, 0, length);
    return subarray;
  }

  /**
   * Returns a new path fragment that is a sub fragment of this one.
   * The sub fragment begins at the specified <code>beginIndex</code> segment
   * and ends at the segment at index <code>endIndex - 1</code>. Thus the number
   * of segments in the new PathFragment is <code>endIndex - beginIndex</code>.
   *
   * @param      beginIndex   the beginning index, inclusive.
   * @param      endIndex     the ending index, exclusive.
   * @return     the specified sub fragment, never null.
   * @exception  IndexOutOfBoundsException  if the
   *             <code>beginIndex</code> is negative, or
   *             <code>endIndex</code> is larger than the length of
   *             this <code>String</code> object, or
   *             <code>beginIndex</code> is larger than
   *             <code>endIndex</code>.
   */
  public PathFragment subFragment(int beginIndex, int endIndex) {
    int count = segments.length;
    if ((beginIndex < 0) || (beginIndex > endIndex) || (endIndex > count)) {
      throw new IndexOutOfBoundsException(String.format("path: %s, beginIndex: %d endIndex: %d",
          toString(), beginIndex, endIndex));
    }
    boolean isAbsolute = (beginIndex == 0) && this.isAbsolute;
    return ((beginIndex == 0) && (endIndex == count)) ? this :
        new PathFragment(driveLetter, isAbsolute,
            subarray(segments, beginIndex, endIndex - beginIndex));
  }

  /**
   * Returns true iff the path represented by this object is absolute.
   */
  public boolean isAbsolute() {
    return isAbsolute;
  }

  /**
   * Returns the segments of this path fragment. This array should not be
   * modified.
   */
  String[] segments() {
    return segments;
  }

  public String windowsVolume() {
    if (OS.getCurrent() != OS.WINDOWS) {
      return "";
    }
    return (driveLetter != '\0') ? driveLetter + ":" : "";
  }

  /** Return the drive letter or '\0' if not applicable. */
  public char getDriveLetter() {
    return driveLetter;
  }

  /**
   * Returns the number of segments in this path.
   */
  public int segmentCount() {
    return segments.length;
  }

  /**
   * Returns the specified segment of this path; index must be positive and
   * less than numSegments().
   */
  public String getSegment(int index) {
    return segments[index];
  }

  /**
   * Returns the index of the first segment which equals one of the input values
   * or {@link PathFragment#INVALID_SEGMENT} if none of the segments match.
   */
  public int getFirstSegment(Set<String> values) {
    for (int i = 0; i < segments.length; i++) {
      if (values.contains(segments[i])) {
        return i;
      }
    }
    return INVALID_SEGMENT;
  }

  /**
   * Returns true iff this path contains uplevel references "..".
   */
  public boolean containsUplevelReferences() {
    for (String segment : segments) {
      if (segment.equals("..")) {
        return true;
      }
    }
    return false;
  }

  /**
   * Returns a relative PathFragment created from this absolute PathFragment using the
   * same segments and drive letter.
   */
  public PathFragment toRelative() {
    Preconditions.checkArgument(isAbsolute);
    return new PathFragment(driveLetter, false, segments);
  }

  /**
   * Given a path, returns the Windows drive letter ('X'), or an null character if no volume
   * name was specified.
   */
  static char getWindowsDriveLetter(String path) {
    if (OS.getCurrent() == OS.WINDOWS
        && path.length() >= 2 && path.charAt(1) == ':' && Character.isLetter(path.charAt(0))) {
      return Character.toUpperCase(path.charAt(0));
    }
    return '\0';
  }

  @Override
  public int hashCode() {
    // We use the hash code caching strategy employed by java.lang.String. There are three subtle
    // things going on here:
    //
    // (1) We use a value of 0 to indicate that the hash code hasn't been computed and cached yet.
    // Yes, this means that if the hash code is really 0 then we will "recompute" it each time. But
    // this isn't a problem in practice since a hash code of 0 is rare.
    //
    // (2) Since we have no synchronization, multiple threads can race here thinking there are the
    // first one to compute and cache the hash code.
    //
    // (3) Moreover, since 'hashCode' is non-volatile, the cached hash code value written from one
    // thread may not be visible by another.
    //
    // All three of these issues are benign from a correctness perspective; in the end we have no
    // overhead from synchronization, at the cost of potentially computing the hash code more than
    // once.
    int h = hashCode;
    if (h == 0) {
      h = isAbsolute ? 1 : 0;
      for (String segment : segments) {
        h = h * 31 + segment.hashCode();
      }
      hashCode = h;
    }
    return h;
  }

  @Override
  public boolean equals(Object other) {
    if (this == other) {
      return true;
    }
    if (!(other instanceof PathFragment)) {
      return false;
    }
    PathFragment otherPath = (PathFragment) other;
    return isAbsolute == otherPath.isAbsolute &&
        Arrays.equals(otherPath.segments, segments);
  }

  /**
   * Compares two PathFragments using the lexicographical order.
   */
  @Override
  public int compareTo(PathFragment p2) {
    if (isAbsolute != p2.isAbsolute) {
      return isAbsolute ? -1 : 1;
    }
    PathFragment p1 = this;
    String[] segments1 = p1.segments;
    String[] segments2 = p2.segments;
    int len1 = segments1.length;
    int len2 = segments2.length;
    int n = Math.min(len1, len2);
    for (int i = 0; i < n; i++) {
      String segment1 = segments1[i];
      String segment2 = segments2[i];
      if (!segment1.equals(segment2)) {
       return segment1.compareTo(segment2);
      }
    }
    return len1 - len2;
  }

  @Override
  public String toString() {
    return getPathString();
  }
}