import abc
import base64
import collections
import hashlib
import json
import os
import shutil
import stat
import subprocess
import zipfile

import git
import portalocker
import pystache
import six
from dcos.api import constants, emitting, errors, subcommand, util

from six.moves import urllib

logger = util.get_logger(__name__)

emitter = emitting.FlatEmitter()


PACKAGE_METADATA_KEY = 'DCOS_PACKAGE_METADATA'
PACKAGE_NAME_KEY = 'DCOS_PACKAGE_NAME'
PACKAGE_VERSION_KEY = 'DCOS_PACKAGE_VERSION'
PACKAGE_SOURCE_KEY = 'DCOS_PACKAGE_SOURCE'
PACKAGE_FRAMEWORK_KEY = 'DCOS_PACKAGE_IS_FRAMEWORK'
PACKAGE_RELEASE_KEY = 'DCOS_PACKAGE_RELEASE'
PACKAGE_COMMAND_KEY = 'DCOS_PACKAGE_COMMAND'
PACKAGE_REGISTRY_VERSION_KEY = 'DCOS_PACKAGE_REGISTRY_VERSION'


def install_app(pkg, version, init_client, options, app_id):
    """Installs a package's application

    :param pkg: the package to install
    :type pkg: Package
    :param version: the package version to install
    :type version: str
    :param init_client: the program to use to run the package
    :type init_client: object
    :param options: package parameters
    :type options: dict
    :param app_id: app ID for installation of this package
    :type app_id: str
    :rtype: Error
    """

    # Insert option parameters into the init template
    init_desc, err = pkg.marathon_json(version, options)
    if err is not None:
        return err

    # Add package metadata
    package_labels, err = _make_package_labels(pkg, version, options)
    if err is not None:
        return err

    # Preserve existing labels
    labels = init_desc.get('labels', {})

    labels.update(package_labels)
    init_desc['labels'] = labels

    if app_id is not None:
        logger.debug('Setting app ID to "%s" (was "%s")',
                     app_id,
                     init_desc['id'])
        init_desc['id'] = app_id

    # Send the descriptor to init
    _, err = init_client.add_app(init_desc)
    return err


def _make_package_labels(pkg, version, options):
    """Returns Marathon app labels for a package.

    :param pkg: The package to install
    :type pkg: Package
    :param version: The package version to install
    :type version: str
    :param options: package parameters
    :type options: dict
    :returns: Marathon app labels
    :rtype: (dict, Error)
    """

    metadata, meta_error = pkg.package_json(version)
    if meta_error is not None:
        return (None, meta_error)

    encoded_metadata = _base64_encode(metadata)

    is_framework = metadata.get('framework')
    if not is_framework:
        is_framework = False

    package_registry_version, version_error = pkg.registry.get_version()

    if version_error is not None:
        return (None, version_error)

    package_labels = {
        PACKAGE_METADATA_KEY: encoded_metadata,
        PACKAGE_NAME_KEY: metadata['name'],
        PACKAGE_VERSION_KEY: metadata['version'],
        PACKAGE_SOURCE_KEY: pkg.registry.source.url,
        PACKAGE_FRAMEWORK_KEY: str(is_framework),
        PACKAGE_REGISTRY_VERSION_KEY: package_registry_version,
        PACKAGE_RELEASE_KEY: str(version)
    }

    if pkg.is_command_defined(version):
        command, cmd_error = pkg.command_json(version, options)
        if cmd_error is not None:
            return (None, cmd_error)

        package_labels[PACKAGE_COMMAND_KEY] = _base64_encode(command)

    return (package_labels, None)


def _base64_encode(dictionary):
    """Returns base64(json(dictionary)).

    :param dictionary: dict to encode
    :type dictionary: dict
    :returns: base64 encoding
    :rtype: str
    """

    json_str = json.dumps(dictionary, sort_keys=True)
    str_bytes = six.b(json_str)
    return base64.b64encode(str_bytes).decode('utf-8')


def uninstall(package_name, remove_all, app_id, init_client):
    """Uninstalls a package.

    :param package_name: The package to uninstall
    :type package_name: str
    :param remove_all: Whether to remove all instances of the named app
    :type remove_all: boolean
    :param app_id: App ID of the app instance to uninstall
    :type app_id: str
    :param init_client: The program to use to run the app
    :type init_client: object
    :rtype: None or Error
    """

    num_apps, err = uninstall_app(package_name,
                                  remove_all,
                                  app_id,
                                  init_client)
    if err:
        return err

    cmd_uninstalled = uninstall_subcommand(package_name)

    if num_apps == 0 and not cmd_uninstalled:
        msg = "Package [{}]".format(package_name)
        if app_id is not None:
            msg += " with id [{}]".format(app_id)
        msg += " is not installed."
        return Error(msg)


def uninstall_subcommand(distribution_name):
    """Uninstalls a subcommand.

    :param distribution_name: the name of the package
    :type distribution_name: str
    :returns: True if the subcommand was uninstalled
    :rtype: bool
    """

    return subcommand.uninstall(distribution_name, util.dcos_path())


def uninstall_app(app_name, remove_all, app_id, init_client):
    """Uninstalls an app.

    :param app_name: The app to uninstall
    :type app_name: str
    :param remove_all: Whether to remove all instances of the named app
    :type remove_all: boolean
    :param app_id: App ID of the app instance to uninstall
    :type app_id: str
    :param init_client: The program to use to run the app
    :type init_client: object
    :returns: (number of apps uninstalled, Error or None)
    :rtype: (int, Error or None)
    """

    apps, appsError = init_client.get_apps()

    if appsError is not None:
        return (0, appsError)

    def is_match(app):
        encoding = 'utf-8'  # We normalize encoding for byte-wise comparison
        name_label = app.get('labels', {}).get(PACKAGE_NAME_KEY, u'')
        name_label_enc = name_label.encode(encoding)
        app_name_enc = app_name.encode(encoding)
        name_matches = name_label_enc == app_name_enc

        if app_id is not None:
            pkg_app_id = app.get('id', '')
            normalized_app_id = init_client.normalize_app_id(app_id)
            return name_matches and pkg_app_id == normalized_app_id
        else:
            return name_matches

    matching_apps = [a for a in apps if is_match(a)]

    if not remove_all and len(matching_apps) > 1:
        app_ids = [a.get('id') for a in matching_apps]
        return (0, Error("""Multiple instances of app [{}] are installed. \
Please specify the app id of the instance to uninstall or uninstall all. \
The app ids of the installed package instances are: [{}].""".format(
            app_name, ', '.join(app_ids))))

    for app in matching_apps:
        init_client.remove_app(app['id'], force=True)

    return (len(matching_apps), None)


class InstalledPackage(object):
    """Represents an intalled DCOS package.  One of `app` and
    `subcommand` must be supplied.

    :param app: A dictionary representing a marathon app.  Of the
                format returned by `installed_apps()`
    :type app: dict
    :param subcommand: Installed subcommand
    :type subcommand: subcommand.InstalledSubcommand
    """

    def __init__(self, app=None, subcommand=None):
        assert app or subcommand
        self.app = app
        self.subcommand = subcommand

    def name(self):
        """
        :returns: The name of the package
        :rtype: str
        """
        if self.subcommand:
            return self.subcommand.name
        else:
            return self.app['name']

    def dict(self):
        """ A dictionary representation of the package.  Used by `dcos package
        list-installed`.

        :returns: A dictionary representation of the package.
        :rtype: (dict, None)
        """
        ret = {'name': self.name}

        if self.subcommand:
            ret['command'] = {'name': self.subcommand.name}

        if self.app:
            ret['app'] = {'appId': self.app['appId']}

        if self.subcommand:
            package_json, err = self.subcommand.package_json()
            if err is not None:
                return (None, err)

            ret.update(package_json)

            package_source, err = self.subcommand.package_source()
            if err is not None:
                return (None, err)

            ret['packageSource'] = package_source

            package_version, err = self.subcommand.package_version()
            if err is not None:
                return (None, err)

            ret['releaseVersion'] = package_version
        else:
            ret.update(self.app)
            ret.pop('appId')

        return (ret, None)


def installed_packages(init_client, endpoints):
    """Returns all installed packages in the format:

    [{
       'app': {
         'id': <id>
       },
       'command': {
         'name': <name>
       }
       ...<metadata>...
    }]

    :param init_client: The program to use to list packages
    :type init_client: object
    :param endpoints: Whether to include a list of
                      endpoints as port-host pairs
    :type endpoints: boolean
    :returns: A list of installed packages
    :rtype: ([InstalledPackage], Error)
    """

    apps, error = installed_apps(init_client, endpoints)
    if error is not None:
        return (None, error)

    subcommands, error = installed_subcommands()
    if error is not None:
        return (None, error)

    dicts = collections.defaultdict(lambda: {'app': None, 'command': None})

    for app in apps:
        key = (app['name'], app['releaseVersion'], app['packageSource'])
        dicts[key]['app'] = app

    for subcmd in subcommands:
        package_version, err = subcmd.package_version()
        if err is not None:
            return (None, err)

        package_source, err = subcmd.package_source()
        if err is not None:
            return (None, err)

        key = (subcmd.name, package_version, package_source)
        dicts[key]['command'] = subcmd

    pkgs = []

    for key, pkg in dicts.items():
        pkgs.append(InstalledPackage(pkg['app'], pkg['command']))

    return (pkgs, None)


def installed_subcommands():
    """Returns all installed subcommands.

    :returns: all installed subcommands
    :rtype: ([InstalledSubcommand], Error)
    """

    ret = [subcommand.InstalledSubcommand(name) for name in
           subcommand.distributions(util.dcos_path())]
    return (ret, None)


def installed_apps(init_client, endpoints=False):
    """
    Returns all installed apps.  An app is of the format:

    {
      'appId': <appId>,
      'packageSource': <source>,
      'registryVersion': <app_version>,
      'releaseVersion': <release_version>
      'endpoints' (optional): [{
        'host': <host>,
        'ports': <ports>,
      }]
      ..<package.json properties>..
    }

    :param init_client: The program to use to list packages
    :type init_client: object
    :param endpoints: Whether to include a list of
                      endpoints as port-host pairs
    :type endpoints: boolean
    :returns: all installed apps
    :rtype: (list of dict, Error)
    """

    apps, error = init_client.get_apps()
    if error is not None:
        return (None, error)

    encoded_apps = [(a['id'], a['labels'])
                    for a in apps
                    if a.get('labels', {}).get(PACKAGE_METADATA_KEY)]

    def decode_and_add_context(pair):
        app_id, labels = pair
        encoded = labels.get(PACKAGE_METADATA_KEY, {})
        source = labels.get(PACKAGE_SOURCE_KEY)
        release_version = labels.get(PACKAGE_RELEASE_KEY)

        decoded = base64.b64decode(six.b(encoded)).decode()
        decoded_json, error = util.load_jsons(decoded)
        if error is None:
            decoded_json['appId'] = app_id
            decoded_json['packageSource'] = source
            decoded_json['releaseVersion'] = release_version
        return (decoded_json, error)

    decoded_apps = [decode_and_add_context(encoded)
                    for encoded in encoded_apps]

    # Filter elements that failed to parse correctly as JSON,
    # or do not match the supplied predicate
    valid_apps = [pair[0] for pair in decoded_apps if pair[1] is None]

    if endpoints:
        for app in valid_apps:
            tasks, err = init_client.get_tasks(app["appId"])
            if err is not None:
                return (None, err)

            app['endpoints'] = [{"host": t["host"], "ports": t["ports"]}
                                for t in tasks]

    return (valid_apps, None)


def search(query, cfg):
    """Returns a list of index entry collections, one for each registry in
    the supplied config.

    :param query: The search term
    :type query: str
    :param cfg: Configuration dictionary
    :type cfg: dcos.api.config.Toml
    :rtype: (list of IndexEntries, Error)
    """

    threshold = 0.5  # Minimum rank required to appear in results
    results = []

    def clean_package_entry(entry):
        result = entry.copy()
        result.update({
            'versions': list(entry['versions'].keys())
        })
        return result

    for registry in registries(cfg):
        source_results = []
        index, error = registry.get_index()
        if error is not None:
            return (None, error)

        for pkg in index['packages']:
            rank = _search_rank(pkg, query)
            if rank >= threshold:
                source_results.append(clean_package_entry(pkg))

        entries = IndexEntries(registry.source, source_results)
        results.append(entries)

    return (results, None)


def _search_rank(pkg, query):
    """
    :param pkg: Index entry to rank for affinity with the search term
    :type pkg: object
    :param query: Search term
    :type query: str
    :rtype: float
    """

    q = query.lower()

    result = 0.0
    if q in pkg['name'].lower():
        result += 2.0

    for tag in pkg['tags']:
        if q in tag.lower():
            result += 1.0

    if q in pkg['description'].lower():
        result += 0.5

    return result


def _extract_default_values(config_schema):
    """
    :param config_schema: A json-schema describing configuration options.
    :type config_schema: (dict, Error)
    """

    properties = config_schema.get('properties')
    schema_type = config_schema.get('type')

    if schema_type != 'object' or properties is None:
        return ({}, None)

    defaults = [(p, properties[p]['default'])
                for p in properties
                if 'default' in properties[p]]

    return (dict(defaults), None)


def resolve_package(package_name, config):
    """Returns the first package with the supplied name found by looking at
    the configured sources in the order they are defined.

    :param package_name: The name of the package to resolve
    :type package_name: str
    :param config: Configuration dictionary
    :type config: dcos.api.config.Toml
    :returns: The named package, if found
    :rtype: Package or None
    """

    for registry in registries(config):
        package, error = registry.get_package(package_name)
        if package is not None:
            return package

    return None


def registries(config):
    """Returns configured cached package registries.

    :param config: Configuration dictionary
    :type config: dcos.api.config.Toml
    :returns: The list of registries, in resolution order
    :rtype: list of Registry
    """

    sources, errors = list_sources(config)
    return [Registry(source, source.local_cache(config)) for source in sources]


def list_sources(config):
    """List configured package sources.

    :param config: Configuration dictionary
    :type config: dcos.api.config.Toml
    :returns: The list of sources, in resolution order
    :rtype: (list of Source, list of Error)
    """

    source_uris = config.get('package.sources')

    if source_uris is None:
        config_error = Error('No configured value for [package.sources]')
        return (None, [config_error])

    results = [url_to_source(s) for s in config['package.sources']]
    sources = [source for (source, _) in results if source is not None]
    errors = [error for (_, error) in results if error is not None]
    return (sources, errors)


def url_to_source(url):
    """Creates a package source from the supplied URL.

    :param url: Location of the package source
    :type url: str
    :returns: A Source backed by the supplied URL
    :rtype: (Source, Error)
    """

    parse_result = urllib.parse.urlparse(url)
    scheme = parse_result.scheme

    if scheme == 'file':
        return (FileSource(url), None)

    elif scheme == 'http' or scheme == 'https':
        return (HttpSource(url), None)

    elif scheme == 'git':
        return (GitSource(url), None)

    else:
        err = Error("Source URL uses unsupported protocol [{}]".format(url))
        return (None, err)


def acquire_file_lock(lock_file_path):
    """Acquires an exclusive lock on the supplied file.

    :param lock_file_path: Path to the lock file
    :type lock_file_path: str
    :returns: Lock file descriptor
    :rtype: (file_descriptor, Error)
    """

    lock_fd = open(lock_file_path, 'w')
    acquire_mode = portalocker.LOCK_EX | portalocker.LOCK_NB

    try:
        portalocker.lock(lock_fd, acquire_mode)
        return (lock_fd, None)
    except portalocker.LockException:
        lock_fd.close()
        return (None, Error("Unable to acquire the package cache lock"))


def update_sources(config, validate=False):
    """Overwrites the local package cache with the latest source data.

    :param config: Configuration dictionary
    :type config: dcos.api.config.Toml
    :returns: Error, if any.
    :rtype: list of Error
    """

    errors = []

    # ensure the cache directory is properly configured
    cache_dir = config.get('package.cache')

    if cache_dir is None:
        config_error = Error("No configured value for [package.cache]")
        errors.append(config_error)
        return errors

    # ensure the cache directory exists
    if not os.path.exists(cache_dir):
        os.makedirs(cache_dir)

    if not os.path.isdir(cache_dir):
        err = Error('Cache directory does not exist! [{}]'.format(cache_dir))
        errors.append(err)
        return errors

    # obtain an exclusive file lock on $CACHE/.lock
    lock_path = os.path.join(cache_dir, '.lock')
    lock_fd, lock_error = acquire_file_lock(lock_path)

    if lock_error is not None:
        errors.append(lock_error)
        return errors

    with lock_fd:

        # list sources
        sources, list_errors = list_sources(config)

        if len(list_errors) > 0:
            errors = errors + list_errors
            return errors

        for source in sources:

            emitter.publish('Updating source [{}]'.format(source))

            # create a temporary staging directory
            with util.tempdir() as tmp_dir:

                stage_dir = os.path.join(tmp_dir, source.hash())

                # copy to the staging directory
                copy_err = source.copy_to_cache(stage_dir)
                if copy_err is not None:
                    errors.append(copy_err)
                    continue  # keep updating the other sources

                # validate content
                if validate:
                    validation_errors = Registry(source, stage_dir).validate()
                    if len(validation_errors) > 0:
                        errors += validation_errors
                        continue  # keep updating the other sources

                # remove the $CACHE/source.hash() directory
                target_dir = os.path.join(cache_dir, source.hash())
                try:
                    if os.path.exists(target_dir):
                        shutil.rmtree(target_dir, ignore_errors=False)
                except OSError:
                    err = Error(
                        'Could not remove directory [{}]'.format(target_dir))
                    errors.append(err)
                    continue  # keep updating the other sources

                # move the staging directory to $CACHE/source.hash()
                shutil.move(stage_dir, target_dir)

    return errors


class Source:
    """A source of DCOS packages."""

    @property
    @abc.abstractmethod
    def url(self):
        """
        :returns: Location of the package source
        :rtype: str
        """

        raise NotImplementedError

    def hash(self):
        """Returns a cryptographically secure hash derived from this source.

        :returns: a hexadecimal string
        :rtype: str
        """

        return hashlib.sha1(self.url.encode('utf-8')).hexdigest()

    def local_cache(self, config):
        """Returns the file system path to this source's local cache.

        :param config: Configuration dictionary
        :type config: dcos.api.config.Toml
        :returns: Path to this source's local cache on disk
        :rtype: str or None
        """

        cache_dir = config.get('package.cache')
        if cache_dir is None:
            return None

        return os.path.join(cache_dir, self.hash())

    def copy_to_cache(self, target_dir):
        """Copies the source content to the supplied local directory.

        :param target_dir: Path to the destination directory.
        :type target_dir: str
        :returns: The error, if one occurred
        :rtype: Error
        """

        raise NotImplementedError

    def __repr__(self):

        return self.url


class FileSource(Source):
    """A registry of DCOS packages.

    :param url: Location of the package source
    :type url: str
    """

    def __init__(self, url):
        self._url = url

    @property
    def url(self):
        """
        :returns: Location of the package source
        :rtype: str
        """

        return self._url

    def copy_to_cache(self, target_dir):
        """Copies the source content to the supplied local directory.

        :param target_dir: Path to the destination directory.
        :type target_dir: str
        :returns: The error, if one occurred
        :rtype: Error
        """

        # copy the source to the target_directory
        parse_result = urllib.parse.urlparse(self._url)
        source_dir = parse_result.path
        try:
            shutil.copytree(source_dir, target_dir)
            return None
        except OSError:
            return Error('Unable to fetch packages from [{}]'.format(self.url))


class HttpSource(Source):
    """A registry of DCOS packages.

    :param url: Location of the package source
    :type url: str
    """

    def __init__(self, url):
        self._url = url

    @property
    def url(self):
        """
        :returns: Location of the package source
        :rtype: str
        """

        return self._url

    def copy_to_cache(self, target_dir):
        """Copies the source content to the supplied local directory.

        :param target_dir: Path to the destination directory.
        :type target_dir: str
        :returns: The error, if one occurred
        :rtype: Error
        """

        try:
            with util.tempdir() as tmp_dir:

                tmp_file = os.path.join(tmp_dir, 'packages.zip')

                # Download the zip file.
                urllib.request.urlretrieve(self.url, tmp_file)

                # Unzip the downloaded file.
                packages_zip = zipfile.ZipFile(tmp_file, 'r')
                packages_zip.extractall(tmp_dir)

                # Move the enclosing directory to the target directory
                enclosing_dirs = [item
                                  for item in os.listdir(tmp_dir)
                                  if os.path.isdir(
                                      os.path.join(tmp_dir, item))]

                # There should only be one directory present after extracting.
                assert(len(enclosing_dirs) is 1)

                enclosing_dir = os.path.join(tmp_dir, enclosing_dirs[0])

                shutil.copytree(enclosing_dir, target_dir)

                # Set appropriate file permissions on the scripts.
                x_mode = (stat.S_IRUSR | stat.S_IWUSR | stat.S_IXUSR |
                          stat.S_IRGRP | stat.S_IWGRP | stat.S_IXGRP)

                scripts_dir = os.path.join(target_dir, 'scripts')
                scripts = os.listdir(scripts_dir)

                for script in scripts:
                    script_path = os.path.join(scripts_dir, script)
                    if os.path.isfile(script_path):
                        os.chmod(script_path, x_mode)

                return None

        except Exception:
            return Error('Unable to fetch packages from [{}]'.format(self.url))


class GitSource(Source):
    """A registry of DCOS packages.

    :param url: Location of the package source
    :type url: str
    """

    def __init__(self, url):
        self._url = url

    @property
    def url(self):
        """
        :returns: Location of the package source
        :rtype: str
        """

        return self._url

    def copy_to_cache(self, target_dir):
        """Copies the source content to the supplied local directory.

        :param target_dir: Path to the destination directory.
        :type target_dir: str
        :returns: The error, if one occurred
        :rtype: Error
        """

        try:
            # TODO(SS): add better url parsing

            # Ensure git is installed properly.
            git_program = util.which('git')
            if git_program is None:
                return Error("""Could not locate the git program.  Make sure \
it is installed and on the system search path.
PATH = {}""".format(os.environ[constants.PATH_ENV]))

            # Clone git repo into the supplied target directory.
            git.Repo.clone_from(self._url,
                                to_path=target_dir,
                                progress=None,
                                branch='master')

            # Remove .git directory to save space.
            shutil.rmtree(os.path.join(target_dir, ".git"))
            return None

        except git.exc.GitCommandError:
            return Error("Unable to fetch packages from [{}]".format(self.url))


class Error(errors.Error):
    """Class for describing errors during packaging operations.

    :param message: Error message
    :type message: str
    """

    def __init__(self, message):
        self._message = message

    def error(self):
        """Return error message

        :returns: The error message
        :rtype: str
        """

        return self._message


class Registry():
    """Represents a package registry on disk.

    :param base_path: Path to the registry
    :type base_path: str
    :param source: The associated package source
    :type source: Source
    """

    def __init__(self, source, base_path):
        self._base_path = base_path
        self._source = source

    def validate(self):
        """Validates a package registry.

        :returns: Validation errors
        :rtype: list of Error
        """

        # TODO(CD): implement these checks in pure Python?
        scripts_dir = os.path.join(self._base_path, 'scripts')
        validate_script = os.path.join(scripts_dir, '1-validate-packages.sh')
        errors = []
        result = subprocess.call(validate_script)
        if result is not 0:
            errors.append(
                Error('Source tree is not valid [{}]'.format(self._base_path)))

        return errors

    @property
    def source(self):
        """Returns the associated upstream package source for this registry.

        :rtype: Source
        """

        return self._source

    def get_version(self):
        """Returns the version of this registry.

        :rtype: (str, Error)
        """

        # The package version is found in $BASE/repo/meta/version.json
        index_path = os.path.join(
            self._base_path,
            'repo',
            'meta',
            'version.json')

        if not os.path.isfile(index_path):
            return (None, Error('Path [{}] is not a file'.format(index_path)))

        try:
            with open(index_path) as fd:
                version_json = json.load(fd)
                return (version_json.get('version'), None)
        except IOError:
            return (None, Error('Unable to open file [{}]'.format(index_path)))
        except ValueError:
            return (None, Error('Unable to parse [{}]'.format(index_path)))

    def get_index(self):
        """Returns the index of packages in this registry.

        :rtype: (object, Error)
        """

        # The package index is found in $BASE/repo/meta/index.json
        index_path = os.path.join(
            self._base_path,
            'repo',
            'meta',
            'index.json')

        if not os.path.isfile(index_path):
            return (None, Error('Path [{}] is not a file'.format(index_path)))

        try:
            with open(index_path) as fd:
                return (json.load(fd), None)
        except IOError:
            return (None, Error('Unable to open file [{}]'.format(index_path)))
        except ValueError:
            return (None, Error('Unable to parse [{}]'.format(index_path)))

    def get_package(self, package_name):
        """Returns the named package, if it exists.

        :param package_name: The name of the package to fetch
        :type package_name: str
        :returns: The requested package
        :rtype: (Package, Error)
        """

        if len(package_name) is 0:
            (None, Error('Package name must not be empty.'))

        # Packages are found in $BASE/repo/package/<first_character>/<pkg_name>
        first_character = package_name[0].title()

        package_path = os.path.join(
            self._base_path,
            'repo',
            'packages',
            first_character,
            package_name)

        if not os.path.isdir(package_path):
            return (None, Error("Package [{}] not found".format(package_name)))

        try:
            return (Package(self, package_path), None)

        except:
            error = Error('Could not read package [{}]'.format(package_name))
            return (None, error)


class Package():
    """Interface to a package on disk.

    :param registry: The containing registry for this package.
    :type registry: Registry
    :param path: Path to the package description on disk
    :type path: str
    """

    def __init__(self, registry, path):
        assert os.path.isdir(path)
        self._registry = registry
        self.path = path

    def name(self):
        """Returns the package name.

        :returns: The name of this package
        :rtype: str
        """

        return os.path.basename(self.path)

    def options(self, version, user_options):
        """Merges package options with user supplied options, validates, and
        returns the result.

        :param version: the package version to install
        :type version: str
        :param user_options: package parameters
        :type user_options: dict
        :returns: a dictionary with the user supplied options
        :rtype: (dict, dcos.api.errors.Error)
        """

        if user_options is None:
            user_options = {}

        config_schema, err = self.config_json(version)
        if err is not None:
            return (None, err)

        default_options, err = _extract_default_values(config_schema)
        if err is not None:
            return (None, err)

        # Merge option overrides
        options = dict(list(default_options.items()) +
                       list(user_options.items()))

        # Validate options with the config schema
        err = util.validate_json(options, config_schema)
        if err is not None:
            return (None, err)

        return (options, None)

    @property
    def registry(self):
        """Returns the containing registry for this package.

        :rtype: Registry
        """

        return self._registry

    def is_command_defined(self, version):
        """Returns true if the package defines a command; false otherwise.

        :param version: package version
        :type version: str
        :rtype: bool
        """

        return os.path.isfile(
            os.path.join(
                self.path,
                os.path.join(version, 'command.json')))

    def config_json(self, version):
        """Returns the JSON content of the config.json file.

        :returns: Package config schema
        :rtype: (dict, Error)
        """

        return self._json(os.path.join(version, 'config.json'))

    def package_json(self, version):
        """Returns the JSON content of the package.json file.

        :param version: the package version
        :type version: str
        :returns: Package data
        :rtype: (dict, Error)
        """

        return self._json(os.path.join(version, 'package.json'))

    def marathon_json(self, version, options):
        """Returns the JSON content of the marathon.json template, after
        rendering it with options.

        :param version: the package version
        :type version: str
        :param options: the template options to use in rendering
        :type options: dict
        :rtype: (dict, Error)
        """

        return self._render_template('marathon.json', version, options)

    def command_json(self, version, options):
        """Returns the JSON content of the comand.json template, after
        rendering it with options.

        :param version: the package version
        :type version: str
        :param options: the template options to use in rendering
        :type options: dict
        :returns: Package data
        :rtype: (dict, Error)
        """

        template, err = self._data(os.path.join(version, 'command.json'))
        if err is not None:
            return (None, err)

        rendered = pystache.render(template, options)
        return (json.loads(rendered), None)

    def _render_template(self, name, version, options):
        """Render a template.

        :param name: the file name of the template
        :type name: str
        :param version: the package version
        :type version: str
        :param options: the template options to use in rendering
        :type options: dict
        :rtype: (dict, Error)
        """

        template, err = self._data(os.path.join(version, name))
        if err is not None:
            return (None, err)

        json, err = util.render_mustache_json(template, options)
        if err is not None:
            return (None, err)

        return (json, None)

    def _json(self, path):
        """Returns the json content of the supplied file, relative to the
        base path.

        :param path: The relative path to the file to read
        :type path: str
        :rtype: (dict, Error)
        """

        data, error = self._data(path)
        if error is not None:
            return (None, error)

        return util.load_jsons(data)

    def _data(self, path):
        """Returns the content of the supplied file, relative to the base path.

        :param path: The relative path to the file to read
        :type path: str
        :returns: File content of the supplied path
        :rtype: (str, Error)
        """

        full_path = os.path.join(self.path, path)
        return util.read_file(full_path)

    def package_versions(self):
        """Returns all of the available package versions, most recent first.

        Note that the result does not describe versions of the package, not
        the software described by the package.

        :returns: Available versions of this package
        :rtype: list of str
        """

        vs = [f for f in os.listdir(self.path) if not f.startswith('.')]
        vs.reverse()
        return vs

    def software_versions(self):
        """Returns a mapping from the package version to the version of the
        software described by the package.

        :returns: Map from package versions to versions of the softwre.
        :rtype: (dict, Error)
        """

        software_package_map = collections.OrderedDict()
        for v in self.package_versions():
            pkg_json, error = self.package_json(v)
            if error is not None:
                return (None, error)
            software_package_map[v] = pkg_json['version']
        return (software_package_map, None)

    def latest_version(self):
        """Returns the latest package version.

        :returns: The latest version of this package
        :rtype: (str, Error)
        """

        pkg_versions = self.package_versions()

        if len(pkg_versions) is 0:
            return (None, Error(
                'No versions found for package [{}]'.format(self.name())))

        pkg_versions.sort()
        return (pkg_versions[-1], None)

    def __repr__(self):

        v, error = self.latest_version()
        if error is not None:
            return error.error()

        pkg_json, error = self.package_json(v)

        if error is not None:
            return error.error()

        return json.dumps(pkg_json)


class IndexEntries():
    """A collection of package index entries from a single source.
    Each entry is a dict as described by the JSON schema for the package index:
    https://github.com/mesosphere/universe/blob/master/repo/meta/schema/index-schema.json

    :param source: The source of these index entries
    :type source: Source
    :param packages: The index entries
    :type packages: list of dict
    """

    def __init__(self, source, packages):
        self._source = source
        self._packages = packages

    @property
    def source(self):
        """Returns the source of these index entries.

        :rtype: Source
        """

        return self._source

    @property
    def packages(self):
        """Returns the package index entries.

        :rtype: list of dict
        """

        return self._packages

    def as_dict(self):
        """
        :rtype: dict
        """

        return {'source': self.source.url, 'packages': self.packages}