function filter (xs, fn) {
    var res = [];
    for (var i = 0; i < xs.length; i++) {
        if (fn(xs[i], i, xs)) res.push(xs[i]);
    }
    return res;
}

// resolves . and .. elements in a path array with directory names there
// must be no slashes, empty elements, or device names (c:\) in the array
// (so also no leading and trailing slashes - it does not distinguish
// relative and absolute paths)
function normalizeArray(parts, allowAboveRoot) {
  // if the path tries to go above the root, `up` ends up > 0
  var up = 0;
  for (var i = parts.length; i >= 0; i--) {
    var last = parts[i];
    if (last == '.') {
      parts.splice(i, 1);
    } else if (last === '..') {
      parts.splice(i, 1);
      up++;
    } else if (up) {
      parts.splice(i, 1);
      up--;
    }
  }

  // if the path is allowed to go above the root, restore leading ..s
  if (allowAboveRoot) {
    for (; up--; up) {
      parts.unshift('..');
    }
  }

  return parts;
}

// Regex to split a filename into [*, dir, basename, ext]
// posix version
var splitPathRe = /^(.+\/(?!$)|\/)?((?:.+?)?(\.[^.]*)?)$/;

// path.resolve([from ...], to)
// posix version
exports.resolve = function() {
var resolvedPath = '',
    resolvedAbsolute = false;

for (var i = arguments.length; i >= -1 && !resolvedAbsolute; i--) {
  var path = (i >= 0)
      ? arguments[i]
      : process.cwd();

  // Skip empty and invalid entries
  if (typeof path !== 'string' || !path) {
    continue;
  }

  resolvedPath = path + '/' + resolvedPath;
  resolvedAbsolute = path.charAt(0) === '/';
}

// At this point the path should be resolved to a full absolute path, but
// handle relative paths to be safe (might happen when process.cwd() fails)

// Normalize the path
resolvedPath = normalizeArray(filter(resolvedPath.split('/'), function(p) {
    return !!p;
  }), !resolvedAbsolute).join('/');

  return ((resolvedAbsolute ? '/' : '') + resolvedPath) || '.';
};

// path.normalize(path)
// posix version
exports.normalize = function(path) {
var isAbsolute = path.charAt(0) === '/',
    trailingSlash = path.slice(-1) === '/';

// Normalize the path
path = normalizeArray(filter(path.split('/'), function(p) {
    return !!p;
  }), !isAbsolute).join('/');

  if (!path && !isAbsolute) {
    path = '.';
  }
  if (path && trailingSlash) {
    path += '/';
  }
  
  return (isAbsolute ? '/' : '') + path;
};


// posix version
exports.join = function() {
  var paths = Array.prototype.slice.call(arguments, 0);
  return exports.normalize(filter(paths, function(p, index) {
    return p && typeof p === 'string';
  }).join('/'));
};


exports.dirname = function(path) {
  var dir = splitPathRe.exec(path)[1] || '';
  var isWindows = false;
  if (!dir) {
    // No dirname
    return '.';
  } else if (dir.length === 1 ||
      (isWindows && dir.length <= 3 && dir.charAt(1) === ':')) {
    // It is just a slash or a drive letter with a slash
    return dir;
  } else {
    // It is a full dirname, strip trailing slash
    return dir.substring(0, dir.length - 1);
  }
};


exports.basename = function(path, ext) {
  var f = splitPathRe.exec(path)[2] || '';
  // TODO: make this comparison case-insensitive on windows?
  if (ext && f.substr(-1 * ext.length) === ext) {
    f = f.substr(0, f.length - ext.length);
  }
  return f;
};


exports.extname = function(path) {
  return splitPathRe.exec(path)[3] || '';
};

exports.relative = function(from, to) {
  from = exports.resolve(from).substr(1);
  to = exports.resolve(to).substr(1);

  function trim(arr) {
    var start = 0;
    for (; start < arr.length; start++) {
      if (arr[start] !== '') break;
    }

    var end = arr.length - 1;
    for (; end >= 0; end--) {
      if (arr[end] !== '') break;
    }

    if (start > end) return [];
    return arr.slice(start, end - start + 1);
  }

  var fromParts = trim(from.split('/'));
  var toParts = trim(to.split('/'));

  var length = Math.min(fromParts.length, toParts.length);
  var samePartsLength = length;
  for (var i = 0; i < length; i++) {
    if (fromParts[i] !== toParts[i]) {
      samePartsLength = i;
      break;
    }
  }

  var outputParts = [];
  for (var i = samePartsLength; i < fromParts.length; i++) {
    outputParts.push('..');
  }

  outputParts = outputParts.concat(toParts.slice(samePartsLength));

  return outputParts.join('/');
};