pkgs/path/lib/src/context.dart

// Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
// for details. All rights reserved. Use of this source code is governed by a
// BSD-style license that can be found in the LICENSE file.

import 'dart:math' as math;

import '../path.dart' as p;
import 'characters.dart' as chars;
import 'internal_style.dart';
import 'parsed_path.dart';
import 'path_exception.dart';
import 'style.dart';

Context createInternal() => Context._internal();

/// An instantiable class for manipulating paths. Unlike the top-level
/// functions, this lets you explicitly select what platform the paths will use.
class Context {
  /// Creates a new path context for the given style and current directory.
  ///
  /// If [style] is omitted, it uses the host operating system's path style. If
  /// only [current] is omitted, it defaults ".". If *both* [style] and
  /// [current] are omitted, [current] defaults to the real current working
  /// directory.
  ///
  /// On the browser, [style] defaults to [Style.url] and [current] defaults to
  /// the current URL.
  factory Context({Style? style, String? current}) {
    if (current == null) {
      if (style == null) {
        current = p.current;
      } else {
        current = '.';
      }
    }

    if (style == null) {
      style = Style.platform;
    } else if (style is! InternalStyle) {
      throw ArgumentError(
        'Only styles defined by the path package are '
        'allowed.',
      );
    }

    return Context._(style as InternalStyle, current);
  }

  /// Create a [Context] to be used internally within path.
  Context._internal()
      : style = Style.platform as InternalStyle,
        _current = null;

  Context._(this.style, this._current);

  /// The style of path that this context works with.
  final InternalStyle style;

  /// The current directory given when Context was created. If null, current
  /// directory is evaluated from 'p.current'.
  final String? _current;

  /// The current directory that relative paths are relative to.
  String get current => _current ?? p.current;

  /// Gets the path separator for the context's [style]. On Mac and Linux,
  /// this is `/`. On Windows, it's `\`.
  String get separator => style.separator;

  /// Returns a new path with the given path parts appended to [current].
  ///
  /// Equivalent to [join()] with [current] as the first argument. Example:
  ///
  ///     var context = Context(current: '/root');
  ///     context.absolute('path', 'to', 'foo'); // -> '/root/path/to/foo'
  ///
  /// If [current] isn't absolute, this won't return an absolute path. Does not
  /// [normalize] or [canonicalize] paths.
  String absolute(
    String part1, [
    String? part2,
    String? part3,
    String? part4,
    String? part5,
    String? part6,
    String? part7,
    String? part8,
    String? part9,
    String? part10,
    String? part11,
    String? part12,
    String? part13,
    String? part14,
    String? part15,
  ]) {
    // If there's a single absolute path, just return it. This is a lot faster
    // for the common case of `p.absolute(path)`.
    if (part2 == null &&
        isAbsolute(part1) &&
        !isRootRelative(part1) &&
        part3 == null &&
        part4 == null &&
        part5 == null &&
        part6 == null &&
        part7 == null &&
        part8 == null &&
        part9 == null &&
        part10 == null &&
        part11 == null &&
        part12 == null &&
        part13 == null &&
        part14 == null &&
        part15 == null) {
      return part1;
    }

    final args = _validateArgs(
      'absolute',
      current,
      part1,
      part2,
      part3,
      part4,
      part5,
      part6,
      part7,
      part8,
      part9,
      part10,
      part11,
      part12,
      part13,
      part14,
      part15,
    );
    return joinAll(args);
  }

  /// Gets the part of [path] after the last separator on the context's
  /// platform.
  ///
  ///     context.basename('path/to/foo.dart'); // -> 'foo.dart'
  ///     context.basename('path/to');          // -> 'to'
  ///
  /// Trailing separators are ignored.
  ///
  ///     context.basename('path/to/'); // -> 'to'
  String basename(String path) => _parse(path).basename;

  /// Gets the part of [path] after the last separator on the context's
  /// platform, and without any trailing file extension.
  ///
  ///     context.basenameWithoutExtension('path/to/foo.dart'); // -> 'foo'
  ///
  /// Trailing separators are ignored.
  ///
  ///     context.basenameWithoutExtension('path/to/foo.dart/'); // -> 'foo'
  String basenameWithoutExtension(String path) =>
      _parse(path).basenameWithoutExtension;

  /// Gets the part of [path] before the last separator.
  ///
  ///     context.dirname('path/to/foo.dart'); // -> 'path/to'
  ///     context.dirname('path/to');          // -> 'path'
  ///
  /// Trailing separators are ignored.
  ///
  ///     context.dirname('path/to/'); // -> 'path'
  String dirname(String path) {
    final parsed = _parse(path);
    parsed.removeTrailingSeparators();
    if (parsed.parts.isEmpty) return parsed.root ?? '.';
    if (parsed.parts.length == 1) return parsed.root ?? '.';
    parsed.parts.removeLast();
    parsed.separators.removeLast();
    parsed.removeTrailingSeparators();
    return parsed.toString();
  }

  /// Gets the file extension of [path]: the portion of [basename] from the last
  /// `.` to the end (including the `.` itself).
  ///
  ///     context.extension('path/to/foo.dart'); // -> '.dart'
  ///     context.extension('path/to/foo'); // -> ''
  ///     context.extension('path.to/foo'); // -> ''
  ///     context.extension('path/to/foo.dart.js'); // -> '.js'
  ///
  /// If the file name starts with a `.`, then it is not considered an
  /// extension:
  ///
  ///     context.extension('~/.bashrc');    // -> ''
  ///     context.extension('~/.notes.txt'); // -> '.txt'
  ///
  /// Takes an optional parameter `level` which makes possible to return
  /// multiple extensions having `level` number of dots. If `level` exceeds the
  /// number of dots, the full extension is returned. The value of `level` must
  /// be greater than 0, else `RangeError` is thrown.
  ///
  ///     context.extension('foo.bar.dart.js', 2);   // -> '.dart.js
  ///     context.extension('foo.bar.dart.js', 3);   // -> '.bar.dart.js'
  ///     context.extension('foo.bar.dart.js', 10);  // -> '.bar.dart.js'
  ///     context.extension('path/to/foo.bar.dart.js', 2);  // -> '.dart.js'
  String extension(String path, [int level = 1]) =>
      _parse(path).extension(level);

  /// Returns the root of [path] if it's absolute, or an empty string if it's
  /// relative.
  ///
  ///     // Unix
  ///     context.rootPrefix('path/to/foo'); // -> ''
  ///     context.rootPrefix('/path/to/foo'); // -> '/'
  ///
  ///     // Windows
  ///     context.rootPrefix(r'path\to\foo'); // -> ''
  ///     context.rootPrefix(r'C:\path\to\foo'); // -> r'C:\'
  ///     context.rootPrefix(r'\\server\share\a\b'); // -> r'\\server\share'
  ///
  ///     // URL
  ///     context.rootPrefix('path/to/foo'); // -> ''
  ///     context.rootPrefix('https://dart.dev/path/to/foo');
  ///       // -> 'https://dart.dev'
  String rootPrefix(String path) => path.substring(0, style.rootLength(path));

  /// Returns `true` if [path] is an absolute path and `false` if it is a
  /// relative path.
  ///
  /// On POSIX systems, absolute paths start with a `/` (forward slash). On
  /// Windows, an absolute path starts with `\\`, or a drive letter followed by
  /// `:/` or `:\`. For URLs, absolute paths either start with a protocol and
  /// optional hostname (e.g. `https://dart.dev`, `file://`) or with a `/`.
  ///
  /// URLs that start with `/` are known as "root-relative", since they're
  /// relative to the root of the current URL. Since root-relative paths are
  /// still absolute in every other sense, [isAbsolute] will return true for
  /// them. They can be detected using [isRootRelative].
  bool isAbsolute(String path) => style.rootLength(path) > 0;

  /// Returns `true` if [path] is a relative path and `false` if it is absolute.
  /// On POSIX systems, absolute paths start with a `/` (forward slash). On
  /// Windows, an absolute path starts with `\\`, or a drive letter followed by
  /// `:/` or `:\`.
  bool isRelative(String path) => !isAbsolute(path);

  /// Returns `true` if [path] is a root-relative path and `false` if it's not.
  ///
  /// URLs that start with `/` are known as "root-relative", since they're
  /// relative to the root of the current URL. Since root-relative paths are
  /// still absolute in every other sense, [isAbsolute] will return true for
  /// them. They can be detected using [isRootRelative].
  ///
  /// No POSIX and Windows paths are root-relative.
  bool isRootRelative(String path) => style.isRootRelative(path);

  /// Joins the given path parts into a single path. Example:
  ///
  ///     context.join('path', 'to', 'foo'); // -> 'path/to/foo'
  ///
  /// If any part ends in a path separator, then a redundant separator will not
  /// be added:
  ///
  ///     context.join('path/', 'to', 'foo'); // -> 'path/to/foo'
  ///
  /// If a part is an absolute path, then anything before that will be ignored:
  ///
  ///     context.join('path', '/to', 'foo'); // -> '/to/foo'
  ///
  String join(
    String part1, [
    String? part2,
    String? part3,
    String? part4,
    String? part5,
    String? part6,
    String? part7,
    String? part8,
    String? part9,
    String? part10,
    String? part11,
    String? part12,
    String? part13,
    String? part14,
    String? part15,
    String? part16,
  ]) {
    final args = _validateArgs(
      'join',
      part1,
      part2,
      part3,
      part4,
      part5,
      part6,
      part7,
      part8,
      part9,
      part10,
      part11,
      part12,
      part13,
      part14,
      part15,
      part16,
    );
    return joinAll(args);
  }

  /// Joins the given path parts into a single path. Example:
  ///
  ///     context.joinAll(['path', 'to', 'foo']); // -> 'path/to/foo'
  ///
  /// If any part ends in a path separator, then a redundant separator will not
  /// be added:
  ///
  ///     context.joinAll(['path/', 'to', 'foo']); // -> 'path/to/foo'
  ///
  /// If a part is an absolute path, then anything before that will be ignored:
  ///
  ///     context.joinAll(['path', '/to', 'foo']); // -> '/to/foo'
  ///
  /// For a fixed number of parts, [join] is usually terser.
  String joinAll(Iterable<String> parts) {
    final buffer = StringBuffer();
    var needsSeparator = false;
    var isAbsoluteAndNotRootRelative = false;

    for (var part in parts.where((part) => part != '')) {
      if (isRootRelative(part) && isAbsoluteAndNotRootRelative) {
        // If the new part is root-relative, it preserves the previous root but
        // replaces the path after it.
        final parsed = _parse(part);
        final path = buffer.toString();
        parsed.root = path.substring(
          0,
          style.rootLength(path, withDrive: true),
        );
        if (style.needsSeparator(parsed.root!)) {
          parsed.separators[0] = style.separator;
        }
        buffer.clear();
        buffer.write(parsed.toString());
      } else if (isAbsolute(part)) {
        isAbsoluteAndNotRootRelative = !isRootRelative(part);
        // An absolute path discards everything before it.
        buffer.clear();
        buffer.write(part);
      } else {
        if (part.isNotEmpty && style.containsSeparator(part[0])) {
          // The part starts with a separator, so we don't need to add one.
        } else if (needsSeparator) {
          buffer.write(separator);
        }

        buffer.write(part);
      }

      // Unless this part ends with a separator, we'll need to add one before
      // the next part.
      needsSeparator = style.needsSeparator(part);
    }

    return buffer.toString();
  }

  /// Splits [path] into its components using the current platform's
  /// [separator]. Example:
  ///
  ///     context.split('path/to/foo'); // -> ['path', 'to', 'foo']
  ///
  /// The path will *not* be normalized before splitting.
  ///
  ///     context.split('path/../foo'); // -> ['path', '..', 'foo']
  ///
  /// If [path] is absolute, the root directory will be the first element in the
  /// array. Example:
  ///
  ///     // Unix
  ///     context.split('/path/to/foo'); // -> ['/', 'path', 'to', 'foo']
  ///
  ///     // Windows
  ///     context.split(r'C:\path\to\foo'); // -> [r'C:\', 'path', 'to', 'foo']
  ///     context.split(r'\\server\share\path\to\foo');
  ///       // -> [r'\\server\share', 'path', 'to', 'foo']
  ///
  ///     // Browser
  ///     context.split('https://dart.dev/path/to/foo');
  ///       // -> ['https://dart.dev', 'path', 'to', 'foo']
  List<String> split(String path) {
    final parsed = _parse(path);
    // Filter out empty parts that exist due to multiple separators in a row.
    parsed.parts = parsed.parts.where((part) => part.isNotEmpty).toList();
    if (parsed.root != null) parsed.parts.insert(0, parsed.root!);
    return parsed.parts;
  }

  /// Canonicalizes [path].
  ///
  /// This is guaranteed to return the same path for two different input paths
  /// only if both input paths point to the same location. Unlike
  /// [normalize], it returns absolute paths when possible and canonicalizes
  /// ASCII case on Windows, and scheme and authority case for URLs (but does
  /// not normalize or canonicalize `%`-escapes.)
  ///
  /// Note that this does not resolve symlinks.
  ///
  /// If you want a map that uses path keys, it's probably more efficient to use
  /// a Map with [equals] and [hash] specified as the callbacks to use for keys
  /// than it is to canonicalize every key.
  ///
  String canonicalize(String path) {
    path = absolute(path);
    // Windows and URL styles need to case-canonicalize, even if it doesn't
    // need to normalize anything.
    if (style == Style.posix && !_needsNormalization(path)) return path;

    final parsed = _parse(path);
    parsed.normalize(canonicalize: true);
    return parsed.toString();
  }

  /// Normalizes [path], simplifying it by handling `..`, and `.`, and
  /// removing redundant path separators whenever possible.
  ///
  /// Note that this is *not* guaranteed to return the same result for two
  /// equivalent input paths. For that, see [canonicalize]. Or, if you're using
  /// paths as map keys, use [equals] and [hash] as the key callbacks.
  ///
  ///     context.normalize('path/./to/..//file.text'); // -> 'path/file.txt'
  String normalize(String path) {
    if (!_needsNormalization(path)) return path;

    final parsed = _parse(path);
    parsed.normalize();
    return parsed.toString();
  }

  /// Returns whether [path] needs to be normalized.
  bool _needsNormalization(String path) {
    // Empty paths are normalized to ".".
    if (path.isEmpty) return true;

    // At start, no previous separator.
    const stateStart = 0;

    // Previous character was a separator.
    const stateSeparator = 1;

    // Added to state for each `.` seen.
    const stateDotCount = 2;

    // Path segment that contains anything other than nothing, `.` or `..`.
    //
    // Includes any value at or above this one.
    const stateNotDots = 6;

    // Current state of the last few characters.
    //
    // Seeing a separator resets to [stateSeparator].
    // Seeing a `.` adds [stateDotCount].
    // Seeing any non-separator or more than two dots will
    // bring the value above [stateNotDots].
    // (The separator may be optional at the start, seeing one is fine,
    // and seeing dots will start counting.)
    // (That is, `/` has value 1, `/.` value 3, ``/..` value 5, and anything
    // else is 6 or above, except at the very start where empty path, `.`
    // and `..` have values 0, 2 and 4.)
    var state = stateStart;

    // Skip past the root before we start looking for snippets that need
    // normalization. We want to normalize "//", but not when it's part of
    // "http://".
    final start = style.rootLength(path);
    if (start != 0) {
      if (style.isSeparator(path.codeUnitAt(start - 1))) {
        state = stateSeparator;
      }
      // On Windows, the root still needs to be normalized if it contains a
      // forward slash.
      if (style == Style.windows) {
        for (var i = 0; i < start; i++) {
          if (path.codeUnitAt(i) == chars.slash) return true;
        }
      }
    }

    for (var i = start; i < path.length; i++) {
      final codeUnit = path.codeUnitAt(i);
      if (style.isSeparator(codeUnit)) {
        // If ending empty, `.` or `..` path segment.
        if (state >= stateSeparator && state < stateNotDots) return true;
        // Forward slashes in Windows paths are normalized to backslashes.
        if (style == Style.windows && codeUnit == chars.slash) return true;
        state = stateSeparator;
      } else if (codeUnit == chars.period) {
        state += stateDotCount;
      } else {
        state = stateNotDots;
        if (style == Style.url &&
            (codeUnit == chars.question || codeUnit == chars.hash)) {
          // Normalize away `?` query parts and `#` fragment parts in URL
          // styled paths.
          return true;
        }
      }
    }

    // Otherwise only normalize if there are separators and single/double dots.
    return state >= stateSeparator && state < stateNotDots;
  }

  /// Attempts to convert [path] to an equivalent relative path relative to
  /// [current].
  ///
  ///     var context = Context(current: '/root/path');
  ///     context.relative('/root/path/a/b.dart'); // -> 'a/b.dart'
  ///     context.relative('/root/other.dart'); // -> '../other.dart'
  ///
  /// If the [from] argument is passed, [path] is made relative to that instead.
  ///
  ///     context.relative('/root/path/a/b.dart',
  ///         from: '/root/path'); // -> 'a/b.dart'
  ///     context.relative('/root/other.dart',
  ///         from: '/root/path'); // -> '../other.dart'
  ///
  /// If [path] and/or [from] are relative paths, they are assumed to be
  /// relative to [current].
  ///
  /// Since there is no relative path from one drive letter to another on
  /// Windows, this will return an absolute path in that case.
  ///
  ///     context.relative(r'D:\other', from: r'C:\other'); // -> 'D:\other'
  ///
  /// This will also return an absolute path if an absolute [path] is passed to
  /// a context with a relative path for [current].
  ///
  ///     var context = Context(r'some/relative/path');
  ///     context.relative(r'/absolute/path'); // -> '/absolute/path'
  ///
  /// If [current] is relative, it may be impossible to determine a path from
  /// [from] to [path]. For example, if [current] and [path] are "." and [from]
  /// is "/", no path can be determined. In this case, a [PathException] will be
  /// thrown.
  String relative(String path, {String? from}) {
    // Avoid expensive computation if the path is already relative.
    if (from == null && isRelative(path)) return normalize(path);

    from = from == null ? current : absolute(from);

    // We can't determine the path from a relative path to an absolute path.
    if (isRelative(from) && isAbsolute(path)) {
      return normalize(path);
    }

    // If the given path is relative, resolve it relative to the context's
    // current directory.
    if (isRelative(path) || isRootRelative(path)) {
      path = absolute(path);
    }

    // If the path is still relative and `from` is absolute, we're unable to
    // find a path from `from` to `path`.
    if (isRelative(path) && isAbsolute(from)) {
      throw PathException('Unable to find a path to "$path" from "$from".');
    }

    final fromParsed = _parse(from)..normalize();
    final pathParsed = _parse(path)..normalize();

    if (fromParsed.parts.isNotEmpty && fromParsed.parts[0] == '.') {
      return pathParsed.toString();
    }

    // If the root prefixes don't match (for example, different drive letters
    // on Windows), then there is no relative path, so just return the absolute
    // one. In Windows, drive letters are case-insenstive and we allow
    // calculation of relative paths, even if a path has not been normalized.
    if (fromParsed.root != pathParsed.root &&
        ((fromParsed.root == null || pathParsed.root == null) ||
            !style.pathsEqual(fromParsed.root!, pathParsed.root!))) {
      return pathParsed.toString();
    }

    // Strip off their common prefix.
    while (fromParsed.parts.isNotEmpty &&
        pathParsed.parts.isNotEmpty &&
        style.pathsEqual(fromParsed.parts[0], pathParsed.parts[0])) {
      fromParsed.parts.removeAt(0);
      fromParsed.separators.removeAt(1);
      pathParsed.parts.removeAt(0);
      pathParsed.separators.removeAt(1);
    }

    // If there are any directories left in the from path, we need to walk up
    // out of them. If a directory left in the from path is '..', it cannot
    // be cancelled by adding a '..'.
    if (fromParsed.parts.isNotEmpty && fromParsed.parts[0] == '..') {
      throw PathException('Unable to find a path to "$path" from "$from".');
    }
    pathParsed.parts.insertAll(0, List.filled(fromParsed.parts.length, '..'));
    pathParsed.separators[0] = '';
    pathParsed.separators.insertAll(
      1,
      List.filled(fromParsed.parts.length, style.separator),
    );

    // Corner case: the paths completely collapsed.
    if (pathParsed.parts.isEmpty) return '.';

    // Corner case: path was '.' and some '..' directories were added in front.
    // Don't add a final '/.' in that case.
    if (pathParsed.parts.length > 1 && pathParsed.parts.last == '.') {
      pathParsed.parts.removeLast();
      pathParsed.separators
        ..removeLast()
        ..removeLast()
        ..add('');
    }

    // Make it relative.
    pathParsed.root = '';
    pathParsed.removeTrailingSeparators();

    return pathParsed.toString();
  }

  /// Returns `true` if [child] is a path beneath `parent`, and `false`
  /// otherwise.
  ///
  ///     path.isWithin('/root/path', '/root/path/a'); // -> true
  ///     path.isWithin('/root/path', '/root/other'); // -> false
  ///     path.isWithin('/root/path', '/root/path'); // -> false
  bool isWithin(String parent, String child) =>
      _isWithinOrEquals(parent, child) == _PathRelation.within;

  /// Returns `true` if [path1] points to the same location as [path2], and
  /// `false` otherwise.
  ///
  /// The [hash] function returns a hash code that matches these equality
  /// semantics.
  bool equals(String path1, String path2) =>
      _isWithinOrEquals(path1, path2) == _PathRelation.equal;

  /// Compares two paths and returns an enum value indicating their relationship
  /// to one another.
  ///
  /// This never returns [_PathRelation.inconclusive].
  _PathRelation _isWithinOrEquals(String parent, String child) {
    // Make both paths the same level of relative. We're only able to do the
    // quick comparison if both paths are in the same format, and making a path
    // absolute is faster than making it relative.
    final parentIsAbsolute = isAbsolute(parent);
    final childIsAbsolute = isAbsolute(child);
    if (parentIsAbsolute && !childIsAbsolute) {
      child = absolute(child);
      if (style.isRootRelative(parent)) parent = absolute(parent);
    } else if (childIsAbsolute && !parentIsAbsolute) {
      parent = absolute(parent);
      if (style.isRootRelative(child)) child = absolute(child);
    } else if (childIsAbsolute && parentIsAbsolute) {
      final childIsRootRelative = style.isRootRelative(child);
      final parentIsRootRelative = style.isRootRelative(parent);

      if (childIsRootRelative && !parentIsRootRelative) {
        child = absolute(child);
      } else if (parentIsRootRelative && !childIsRootRelative) {
        parent = absolute(parent);
      }
    }

    final result = _isWithinOrEqualsFast(parent, child);
    if (result != _PathRelation.inconclusive) return result;

    String relative;
    try {
      relative = this.relative(child, from: parent);
    } on PathException catch (_) {
      // If no relative path from [parent] to [child] is found, [child]
      // definitely isn't a child of [parent].
      return _PathRelation.different;
    }

    if (!isRelative(relative)) return _PathRelation.different;
    if (relative == '.') return _PathRelation.equal;
    if (relative == '..') return _PathRelation.different;
    return (relative.length >= 3 &&
            relative.startsWith('..') &&
            style.isSeparator(relative.codeUnitAt(2)))
        ? _PathRelation.different
        : _PathRelation.within;
  }

  /// An optimized implementation of [_isWithinOrEquals] that doesn't handle a
  /// few complex cases.
  _PathRelation _isWithinOrEqualsFast(String parent, String child) {
    // Normally we just bail when we see "." path components, but we can handle
    // a single dot easily enough.
    if (parent == '.') parent = '';

    final parentRootLength = style.rootLength(parent);
    final childRootLength = style.rootLength(child);

    // If the roots aren't the same length, we know both paths are absolute or
    // both are root-relative, and thus that the roots are meaningfully
    // different.
    //
    //     isWithin("C:/bar", "//foo/bar/baz") //=> false
    //     isWithin("http://example.com/", "http://google.com/bar") //=> false
    if (parentRootLength != childRootLength) return _PathRelation.different;

    // Make sure that the roots are textually the same as well.
    //
    //     isWithin("C:/bar", "D:/bar/baz") //=> false
    //     isWithin("http://example.com/", "http://example.org/bar") //=> false
    for (var i = 0; i < parentRootLength; i++) {
      final parentCodeUnit = parent.codeUnitAt(i);
      final childCodeUnit = child.codeUnitAt(i);
      if (!style.codeUnitsEqual(parentCodeUnit, childCodeUnit)) {
        return _PathRelation.different;
      }
    }

    // Start by considering the last code unit as a separator, since
    // semantically we're starting at a new path component even if we're
    // comparing relative paths.
    var lastCodeUnit = chars.slash;

    /// The index of the last separator in [parent].
    int? lastParentSeparator;

    // Iterate through both paths as long as they're semantically identical.
    var parentIndex = parentRootLength;
    var childIndex = childRootLength;
    while (parentIndex < parent.length && childIndex < child.length) {
      var parentCodeUnit = parent.codeUnitAt(parentIndex);
      var childCodeUnit = child.codeUnitAt(childIndex);
      if (style.codeUnitsEqual(parentCodeUnit, childCodeUnit)) {
        if (style.isSeparator(parentCodeUnit)) {
          lastParentSeparator = parentIndex;
        }

        lastCodeUnit = parentCodeUnit;
        parentIndex++;
        childIndex++;
        continue;
      }

      // Ignore multiple separators in a row.
      if (style.isSeparator(parentCodeUnit) &&
          style.isSeparator(lastCodeUnit)) {
        lastParentSeparator = parentIndex;
        parentIndex++;
        continue;
      } else if (style.isSeparator(childCodeUnit) &&
          style.isSeparator(lastCodeUnit)) {
        childIndex++;
        continue;
      }

      // If a dot comes after a separator, it may be a directory traversal
      // operator. To check that, we need to know if it's followed by either
      // "/" or "./". Otherwise, it's just a normal non-matching character.
      //
      //     isWithin("foo/./bar", "foo/bar/baz") //=> true
      //     isWithin("foo/bar/../baz", "foo/bar/.foo") //=> false
      if (parentCodeUnit == chars.period && style.isSeparator(lastCodeUnit)) {
        parentIndex++;

        // We've hit "/." at the end of the parent path, which we can ignore,
        // since the paths were equivalent up to this point.
        if (parentIndex == parent.length) break;
        parentCodeUnit = parent.codeUnitAt(parentIndex);

        // We've hit "/./", which we can ignore.
        if (style.isSeparator(parentCodeUnit)) {
          lastParentSeparator = parentIndex;
          parentIndex++;
          continue;
        }

        // We've hit "/..", which may be a directory traversal operator that
        // we can't handle on the fast track.
        if (parentCodeUnit == chars.period) {
          parentIndex++;
          if (parentIndex == parent.length ||
              style.isSeparator(parent.codeUnitAt(parentIndex))) {
            return _PathRelation.inconclusive;
          }
        }

        // If this isn't a directory traversal, fall through so we hit the
        // normal handling for mismatched paths.
      }

      // This is the same logic as above, but for the child path instead of the
      // parent.
      if (childCodeUnit == chars.period && style.isSeparator(lastCodeUnit)) {
        childIndex++;
        if (childIndex == child.length) break;
        childCodeUnit = child.codeUnitAt(childIndex);

        if (style.isSeparator(childCodeUnit)) {
          childIndex++;
          continue;
        }

        if (childCodeUnit == chars.period) {
          childIndex++;
          if (childIndex == child.length ||
              style.isSeparator(child.codeUnitAt(childIndex))) {
            return _PathRelation.inconclusive;
          }
        }
      }

      // If we're here, we've hit two non-matching, non-significant characters.
      // As long as the remainders of the two paths don't have any unresolved
      // ".." components, we can be confident that [child] is not within
      // [parent].
      final childDirection = _pathDirection(child, childIndex);
      if (childDirection != _PathDirection.belowRoot) {
        return _PathRelation.inconclusive;
      }

      final parentDirection = _pathDirection(parent, parentIndex);
      if (parentDirection != _PathDirection.belowRoot) {
        return _PathRelation.inconclusive;
      }

      return _PathRelation.different;
    }

    // If the child is shorter than the parent, it's probably not within the
    // parent. The only exception is if the parent has some weird ".." stuff
    // going on, in which case we do the slow check.
    //
    //     isWithin("foo/bar/baz", "foo/bar") //=> false
    //     isWithin("foo/bar/baz/../..", "foo/bar") //=> true
    if (childIndex == child.length) {
      if (parentIndex == parent.length ||
          style.isSeparator(parent.codeUnitAt(parentIndex))) {
        lastParentSeparator = parentIndex;
      } else {
        lastParentSeparator ??= math.max(0, parentRootLength - 1);
      }

      final direction = _pathDirection(parent, lastParentSeparator);
      if (direction == _PathDirection.atRoot) return _PathRelation.equal;
      return direction == _PathDirection.aboveRoot
          ? _PathRelation.inconclusive
          : _PathRelation.different;
    }

    // We've reached the end of the parent path, which means it's time to make a
    // decision. Before we do, though, we'll check the rest of the child to see
    // what that tells us.
    final direction = _pathDirection(child, childIndex);

    // If there are no more components in the child, then it's the same as
    // the parent.
    //
    //     isWithin("foo/bar", "foo/bar") //=> false
    //     isWithin("foo/bar", "foo/bar//") //=> false
    //     equals("foo/bar", "foo/bar") //=> true
    //     equals("foo/bar", "foo/bar//") //=> true
    if (direction == _PathDirection.atRoot) return _PathRelation.equal;

    // If there are unresolved ".." components in the child, no decision we make
    // will be valid. We'll abort and do the slow check instead.
    //
    //     isWithin("foo/bar", "foo/bar/..") //=> false
    //     isWithin("foo/bar", "foo/bar/baz/bang/../../..") //=> false
    //     isWithin("foo/bar", "foo/bar/baz/bang/../../../bar/baz") //=> true
    if (direction == _PathDirection.aboveRoot) {
      return _PathRelation.inconclusive;
    }

    // The child is within the parent if and only if we're on a separator
    // boundary.
    //
    //     isWithin("foo/bar", "foo/bar/baz") //=> true
    //     isWithin("foo/bar/", "foo/bar/baz") //=> true
    //     isWithin("foo/bar", "foo/barbaz") //=> false
    return (style.isSeparator(child.codeUnitAt(childIndex)) ||
            style.isSeparator(lastCodeUnit))
        ? _PathRelation.within
        : _PathRelation.different;
  }

  // Returns a [_PathDirection] describing the path represented by [codeUnits]
  // starting at [index].
  //
  // This ignores leading separators.
  //
  //     pathDirection("foo") //=> below root
  //     pathDirection("foo/bar/../baz") //=> below root
  //     pathDirection("//foo/bar/baz") //=> below root
  //     pathDirection("/") //=> at root
  //     pathDirection("foo/..") //=> at root
  //     pathDirection("foo/../baz") //=> reaches root
  //     pathDirection("foo/../..") //=> above root
  //     pathDirection("foo/../../foo/bar/baz") //=> above root
  _PathDirection _pathDirection(String path, int index) {
    var depth = 0;
    var reachedRoot = false;
    var i = index;
    while (i < path.length) {
      // Ignore initial separators or doubled separators.
      while (i < path.length && style.isSeparator(path.codeUnitAt(i))) {
        i++;
      }

      // If we're at the end, stop.
      if (i == path.length) break;

      // Move through the path component to the next separator.
      final start = i;
      while (i < path.length && !style.isSeparator(path.codeUnitAt(i))) {
        i++;
      }

      // See if the path component is ".", "..", or a name.
      if (i - start == 1 && path.codeUnitAt(start) == chars.period) {
        // Don't change the depth.
      } else if (i - start == 2 &&
          path.codeUnitAt(start) == chars.period &&
          path.codeUnitAt(start + 1) == chars.period) {
        // ".." backs out a directory.
        depth--;

        // If we work back beyond the root, stop.
        if (depth < 0) break;

        // Record that we reached the root so we don't return
        // [_PathDirection.belowRoot].
        if (depth == 0) reachedRoot = true;
      } else {
        // Step inside a directory.
        depth++;
      }

      // If we're at the end, stop.
      if (i == path.length) break;

      // Move past the separator.
      i++;
    }

    if (depth < 0) return _PathDirection.aboveRoot;
    if (depth == 0) return _PathDirection.atRoot;
    if (reachedRoot) return _PathDirection.reachesRoot;
    return _PathDirection.belowRoot;
  }

  /// Returns a hash code for [path] that matches the semantics of [equals].
  ///
  /// Note that the same path may have different hash codes in different
  /// [Context]s.
  int hash(String path) {
    // Make [path] absolute to ensure that equivalent relative and absolute
    // paths have the same hash code.
    path = absolute(path);

    final result = _hashFast(path);
    if (result != null) return result;

    final parsed = _parse(path);
    parsed.normalize();
    return _hashFast(parsed.toString())!;
  }

  /// An optimized implementation of [hash] that doesn't handle internal `..`
  /// components.
  ///
  /// This will handle `..` components that appear at the beginning of the path.
  int? _hashFast(String path) {
    var hash = 4603;
    var beginning = true;
    var wasSeparator = true;
    for (var i = 0; i < path.length; i++) {
      final codeUnit = style.canonicalizeCodeUnit(path.codeUnitAt(i));

      // Take advantage of the fact that collisions are allowed to ignore
      // separators entirely. This lets us avoid worrying about cases like
      // multiple trailing slashes.
      if (style.isSeparator(codeUnit)) {
        wasSeparator = true;
        continue;
      }

      if (codeUnit == chars.period && wasSeparator) {
        // If a dot comes after a separator, it may be a directory traversal
        // operator. To check that, we need to know if it's followed by either
        // "/" or "./". Otherwise, it's just a normal character.
        //
        //     hash("foo/./bar") == hash("foo/bar")

        // We've hit "/." at the end of the path, which we can ignore.
        if (i + 1 == path.length) break;

        final next = path.codeUnitAt(i + 1);

        // We can just ignore "/./", since they don't affect the semantics of
        // the path.
        if (style.isSeparator(next)) continue;

        // If the path ends with "/.." or contains "/../", we need to
        // canonicalize it before we can hash it. We make an exception for ".."s
        // at the beginning of the path, since those may appear even in a
        // canonicalized path.
        if (!beginning &&
            next == chars.period &&
            (i + 2 == path.length ||
                style.isSeparator(path.codeUnitAt(i + 2)))) {
          return null;
        }
      }

      // Make sure [hash] stays under 32 bits even after multiplication.
      hash &= 0x3FFFFFF;
      hash *= 33;
      hash ^= codeUnit;
      wasSeparator = false;
      beginning = false;
    }
    return hash;
  }

  /// Removes a trailing extension from the last part of [path].
  ///
  ///     context.withoutExtension('path/to/foo.dart'); // -> 'path/to/foo'
  String withoutExtension(String path) {
    final parsed = _parse(path);

    for (var i = parsed.parts.length - 1; i >= 0; i--) {
      if (parsed.parts[i].isNotEmpty) {
        parsed.parts[i] = parsed.basenameWithoutExtension;
        break;
      }
    }

    return parsed.toString();
  }

  /// Returns [path] with the trailing extension set to [extension].
  ///
  /// If [path] doesn't have a trailing extension, this just adds [extension] to
  /// the end.
  ///
  ///     context.setExtension('path/to/foo.dart', '.js')
  ///       // -> 'path/to/foo.js'
  ///     context.setExtension('path/to/foo.dart.js', '.map')
  ///       // -> 'path/to/foo.dart.map'
  ///     context.setExtension('path/to/foo', '.js')
  ///       // -> 'path/to/foo.js'
  String setExtension(String path, String extension) =>
      withoutExtension(path) + extension;

  /// Returns the path represented by [uri], which may be a [String] or a [Uri].
  ///
  /// For POSIX and Windows styles, [uri] must be a `file:` URI. For the URL
  /// style, this will just convert [uri] to a string, but if the input was
  /// a string, it will be parsed and normalized as a [Uri] first.
  ///
  ///
  ///     // POSIX
  ///     context.fromUri('file:///path/to/foo')
  ///       // -> '/path/to/foo'
  ///
  ///     // Windows
  ///     context.fromUri('file:///C:/path/to/foo')
  ///       // -> r'C:\path\to\foo'
  ///
  ///     // URL
  ///     context.fromUri('https://dart.dev/path/to/foo')
  ///       // -> 'https://dart.dev/path/to/foo'
  ///
  /// If [uri] is relative, a relative path will be returned.
  ///
  ///     // POSIX
  ///     path.fromUri('path/to/foo'); // -> 'path/to/foo'
  ///
  ///     // Windows
  ///     path.fromUri('/C:/foo'); // -> r'C:\foo`
  String fromUri(Object? uri) => style.pathFromUri(_parseUri(uri!));

  /// Returns the URI that represents [path].
  ///
  /// For POSIX and Windows styles, this will return a `file:` URI. For the URL
  /// style, this will just convert [path] to a [Uri].
  ///
  ///     // POSIX
  ///     context.toUri('/path/to/foo')
  ///       // -> Uri.parse('file:///path/to/foo')
  ///
  ///     // Windows
  ///     context.toUri(r'C:\path\to\foo')
  ///       // -> Uri.parse('file:///C:/path/to/foo')
  ///
  ///     // URL
  ///     context.toUri('https://dart.dev/path/to/foo')
  ///       // -> Uri.parse('https://dart.dev/path/to/foo')
  Uri toUri(String path) {
    if (isRelative(path)) {
      return style.relativePathToUri(path);
    } else {
      return style.absolutePathToUri(join(current, path));
    }
  }

  /// Returns a terse, human-readable representation of [uri].
  ///
  /// [uri] can be a [String] or a [Uri]. If it can be made relative to the
  /// current working directory, that's done. Otherwise, it's returned as-is.
  /// This gracefully handles non-`file:` URIs for [Style.posix] and
  /// [Style.windows].
  ///
  /// The returned value is meant for human consumption, and may be either URI-
  /// or path-formatted.
  ///
  ///     // POSIX
  ///     var context = Context(current: '/root/path');
  ///     context.prettyUri('file:///root/path/a/b.dart'); // -> 'a/b.dart'
  ///     context.prettyUri('https://dart.dev/'); // -> 'https://dart.dev'
  ///
  ///     // Windows
  ///     var context = Context(current: r'C:\root\path');
  ///     context.prettyUri('file:///C:/root/path/a/b.dart'); // -> r'a\b.dart'
  ///     context.prettyUri('https://dart.dev/'); // -> 'https://dart.dev'
  ///
  ///     // URL
  ///     var context = Context(current: 'https://dart.dev/root/path');
  ///     context.prettyUri('https://dart.dev/root/path/a/b.dart');
  ///         // -> r'a/b.dart'
  ///     context.prettyUri('file:///root/path'); // -> 'file:///root/path'
  String prettyUri(Object? uri) {
    final typedUri = _parseUri(uri!);
    if (typedUri.scheme == 'file' && style == Style.url) {
      return typedUri.toString();
    } else if (typedUri.scheme != 'file' &&
        typedUri.scheme != '' &&
        style != Style.url) {
      return typedUri.toString();
    }

    final path = normalize(fromUri(typedUri));
    final rel = relative(path);

    // Only return a relative path if it's actually shorter than the absolute
    // path. This avoids ugly things like long "../" chains to get to the root
    // and then go back down.
    return split(rel).length > split(path).length ? path : rel;
  }

  ParsedPath _parse(String path) => ParsedPath.parse(path, style);
}

/// Parses argument if it's a [String] or returns it intact if it's a [Uri].
///
/// Throws an [ArgumentError] otherwise.
Uri _parseUri(Object uri) {
  if (uri is String) return Uri.parse(uri);
  if (uri is Uri) return uri;
  throw ArgumentError.value(uri, 'uri', 'Value must be a String or a Uri');
}

/// Validates that there are no non-null arguments following a null one and
/// throws an appropriate [ArgumentError] on failure.
///
/// This function operates on individual arguments rather than a list for better
/// performance by avoiding list allocation.
///
/// Returns a list of only the non-null arguments.
List<String> _validateArgs(
  String method,
  String p0,
  String? p1,
  String? p2,
  String? p3,
  String? p4,
  String? p5,
  String? p6,
  String? p7,
  String? p8,
  String? p9,
  String? p10,
  String? p11,
  String? p12,
  String? p13,
  String? p14,
  String? p15,
) {
  // Helper to create the list of args for error messages.
  List<String?> allArgs() => [
        p0,
        p1,
        p2,
        p3,
        p4,
        p5,
        p6,
        p7,
        p8,
        p9,
        p10,
        p11,
        p12,
        p13,
        p14,
        p15,
      ];

  // p0 is always non-null (enforced by caller), so we start checking from p1.
  // Once we see a null, all subsequent arguments must also be null.
  var seenNull = false;
  final args = <String>[p0];

  if (p1 == null) {
    seenNull = true;
  } else {
    args.add(p1);
  }

  if (p2 == null) {
    seenNull = true;
  } else if (seenNull) {
    _throwArgError(method, 2, allArgs());
  } else {
    args.add(p2);
  }

  if (p3 == null) {
    seenNull = true;
  } else if (seenNull) {
    _throwArgError(method, 3, allArgs());
  } else {
    args.add(p3);
  }

  if (p4 == null) {
    seenNull = true;
  } else if (seenNull) {
    _throwArgError(method, 4, allArgs());
  } else {
    args.add(p4);
  }

  if (p5 == null) {
    seenNull = true;
  } else if (seenNull) {
    _throwArgError(method, 5, allArgs());
  } else {
    args.add(p5);
  }

  if (p6 == null) {
    seenNull = true;
  } else if (seenNull) {
    _throwArgError(method, 6, allArgs());
  } else {
    args.add(p6);
  }

  if (p7 == null) {
    seenNull = true;
  } else if (seenNull) {
    _throwArgError(method, 7, allArgs());
  } else {
    args.add(p7);
  }

  if (p8 == null) {
    seenNull = true;
  } else if (seenNull) {
    _throwArgError(method, 8, allArgs());
  } else {
    args.add(p8);
  }

  if (p9 == null) {
    seenNull = true;
  } else if (seenNull) {
    _throwArgError(method, 9, allArgs());
  } else {
    args.add(p9);
  }

  if (p10 == null) {
    seenNull = true;
  } else if (seenNull) {
    _throwArgError(method, 10, allArgs());
  } else {
    args.add(p10);
  }

  if (p11 == null) {
    seenNull = true;
  } else if (seenNull) {
    _throwArgError(method, 11, allArgs());
  } else {
    args.add(p11);
  }

  if (p12 == null) {
    seenNull = true;
  } else if (seenNull) {
    _throwArgError(method, 12, allArgs());
  } else {
    args.add(p12);
  }

  if (p13 == null) {
    seenNull = true;
  } else if (seenNull) {
    _throwArgError(method, 13, allArgs());
  } else {
    args.add(p13);
  }

  if (p14 == null) {
    seenNull = true;
  } else if (seenNull) {
    _throwArgError(method, 14, allArgs());
  } else {
    args.add(p14);
  }

  if (p15 == null) {
    // No more arguments to check after p15, so this is valid.
  } else if (seenNull) {
    _throwArgError(method, 15, allArgs());
  } else {
    args.add(p15);
  }

  return args;
}

/// Throws an [ArgumentError] for invalid argument ordering.
///
/// [nonNullIndex] is the index of the non-null argument that follows a null.
Never _throwArgError(String method, int nonNullIndex, List<String?> args) {
  int numArgs;
  for (numArgs = args.length; numArgs >= 1; numArgs--) {
    if (args[numArgs - 1] != null) break;
  }

  final message = StringBuffer();
  message.write('$method(');
  message.write(
    args.take(numArgs).map((arg) => arg == null ? 'null' : '"$arg"').join(', '),
  );
  message.write(
    '): part ${nonNullIndex - 1} was null, but part $nonNullIndex was not.',
  );
  throw ArgumentError(message.toString());
}

/// An enum of possible return values for [Context._pathDirection].
class _PathDirection {
  /// The path contains enough ".." components that at some point it reaches
  /// above its original root.
  ///
  /// Note that this applies even if the path ends beneath its original root. It
  /// takes precendence over any other return values that may apple.
  static const aboveRoot = _PathDirection('above root');

  /// The path contains enough ".." components that it ends at its original
  /// root.
  static const atRoot = _PathDirection('at root');

  /// The path contains enough ".." components that at some point it reaches its
  /// original root, but it ends beneath that root.
  static const reachesRoot = _PathDirection('reaches root');

  /// The path never reaches to or above its original root.
  static const belowRoot = _PathDirection('below root');

  final String name;

  const _PathDirection(this.name);

  @override
  String toString() => name;
}

/// An enum of possible return values for [Context._isWithinOrEquals].
class _PathRelation {
  /// The first path is a proper parent of the second.
  ///
  /// For example, `foo` is a proper parent of `foo/bar`, but not of `foo`.
  static const within = _PathRelation('within');

  /// The two paths are equivalent.
  ///
  /// For example, `foo//bar` is equivalent to `foo/bar`.
  static const equal = _PathRelation('equal');

  /// The first path is neither a parent of nor equal to the second.
  static const different = _PathRelation('different');

  /// We couldn't quickly determine any information about the paths'
  /// relationship to each other.
  ///
  /// Only returned by [Context._isWithinOrEqualsFast].
  static const inconclusive = _PathRelation('inconclusive');

  final String name;

  const _PathRelation(this.name);

  @override
  String toString() => name;
}