/*@flow*/
/* jshint esversion: 6 */
/* global Buffer */
var Fs = require("fs");
var Fse = require("fs-extra");
var Path = require("path");
var nThen = require("nthen");
var Semaphore = require("saferphore");
var Once = require("../lib/once");
const ToPull = require('stream-to-pull-stream');
const Pull = require('pull-stream');

const isValidChannelId = function (id) {
    return typeof(id) === 'string' &&
        id.length >= 32 && id.length < 50 &&
        /^[a-zA-Z0-9=+-]*$/.test(id);
};

// 511 -> octal 777
// read, write, execute permissions flag
const PERMISSIVE = 511;

var mkPath = function (env, channelId) {
    return Path.join(env.root, channelId.slice(0, 2), channelId) + '.ndjson';
};

var mkArchivePath = function (env, channelId) {
    return Path.join(env.archiveRoot, 'datastore', channelId.slice(0, 2), channelId) + '.ndjson';
};

var mkMetadataPath = function (env, channelId) {
    return Path.join(env.root, channelId.slice(0, 2), channelId) + '.metadata.ndjson';
};

var mkArchiveMetadataPath = function (env, channelId) {
    return Path.join(env.archiveRoot, 'datastore', channelId.slice(0, 2), channelId) + '.metadata.ndjson';
};

// pass in the path so we can reuse the same function for archived files
var channelExists = function (filepath, cb) {
    Fs.stat(filepath, function (err, stat) {
        if (err) {
            if (err.code === 'ENOENT') {
                // no, the file doesn't exist
                return void cb(void 0, false);
            }
            return void cb(err);
        }
        if (!stat.isFile()) { return void cb("E_NOT_FILE"); }
        return void cb(void 0, true);
    });
};

var getMetadataAtPath = function (Env, path, cb) {
    var remainder = '';
    var stream = Fs.createReadStream(path, { encoding: 'utf8' });
    var complete = function (err, data) {
        var _cb = cb;
        cb = undefined;
        if (_cb) { _cb(err, data); }
    };
    stream.on('data', function (chunk) {
        if (!/\n/.test(chunk)) {
            remainder += chunk;
            return;
        }
        stream.close();
        var metadata = chunk.split('\n')[0];

        var parsed = null;
        try {
            parsed = JSON.parse(metadata);
            complete(undefined, parsed);
        }
        catch (e) {
            console.log("getMetadataAtPath");
            console.error(e);
            complete('INVALID_METADATA');
        }
    });
    stream.on('end', function () {
        complete();
    });
    stream.on('error', function (e) { complete(e); });
};

var closeChannel = function (env, channelName, cb) {
    if (!env.channels[channelName]) { return void cb(); }
    try {
        env.channels[channelName].writeStream.close();
        delete env.channels[channelName];
        env.openFiles--;
        cb();
    } catch (err) {
        cb(err);
    }
};

var clearChannel = function (env, channelId, cb) {
    var path = mkPath(env, channelId);
    getMetadataAtPath(env, path, function (e, metadata) {
        if (e) { return cb(new Error(e)); }
        if (!metadata) {
            return void Fs.truncate(path, 0, function (err) {
                if (err) {
                    return cb(err);
                }
                cb(void 0);
            });
        }

        var len = JSON.stringify(metadata).length + 1;

        // as long as closeChannel is synchronous, this should not cause
        // any race conditions. truncate ought to return faster than a channel
        // can be opened and read by another user. if that turns out not to be
        // the case, we'll need to implement locking.
        closeChannel(env, channelId, function (err) {
            if (err) { cb(err); }
            Fs.truncate(path, len, function (err) {
                if (err) { return cb(err); }
                cb();
            });
        });
    });
};

var readMessages = function (path, msgHandler, cb) {
    var remainder = '';
    var stream = Fs.createReadStream(path, { encoding: 'utf8' });
    var complete = function (err) {
        var _cb = cb;
        cb = undefined;
        if (_cb) { _cb(err); }
    };
    stream.on('data', function (chunk) {
        var lines = chunk.split('\n');
        lines[0] = remainder + lines[0];
        remainder = lines.pop();
        lines.forEach(msgHandler);
    });
    stream.on('end', function () {
        msgHandler(remainder);
        complete();
    });
    stream.on('error', function (e) { complete(e); });
};

// FIXME METADATA
// XXX deprecate this everywhere in favour of the new method
var getChannelMetadata = function (Env, channelId, cb) {
    var path = mkPath(Env, channelId);

    // gets metadata embedded in a file
    getMetadataAtPath(Env, path, cb);
};

// low level method for getting just the dedicated metadata channel
var getDedicatedMetadata = function (env, channelId, handler, cb) {
    var metadataPath = mkMetadataPath(env, channelId);
    readMessages(metadataPath, function (line) {
        if (!line) { return; }
        try {
            var parsed = JSON.parse(line);
            handler(null, parsed);
        } catch (e) {
            handler(e, line);
        }
    }, function (err) {
        if (err) {
            // ENOENT => there is no metadata log
            if (err.code === 'ENOENT') { return void cb(); }
            // otherwise stream errors?
            return void cb(err);
        }
        cb();
    });
};

var readMetadata = function (env, channelId, handler, cb) {
/*

Possibilities

    1. there is no metadata because it's an old channel
    2. there is metadata in the first line of the channel, but nowhere else
    3. there is metadata in the first line of the channel as well as in a dedicated log
    4. there is no metadata in the first line of the channel. Everything is in the dedicated log

How to proceed

    1. load the first line of the channel and treat it as a metadata message if applicable
    2. load the dedicated log and treat it as an update

*/

    nThen(function (w) {
        // returns the first line of a channel, parsed...
        getChannelMetadata(env, channelId, w(function (err, data) {
            if (err) {
                // 'INVALID_METADATA' if it can't parse
                // stream errors if anything goes wrong at a lower level
                    // ENOENT (no channel here)
                return void handler(err);
            }
            // disregard anything that isn't a map
            if (!data || typeof(data) !== 'object' || Array.isArray(data)) { return; }

            // otherwise it's good.
            handler(null, data);
        }));
    }).nThen(function () {
        getDedicatedMetadata(env, channelId, handler, function (err) {
            if (err) {
                // stream errors?
                return void cb(err);
            }
            cb();
        });
    });
};

var writeMetadata = function (env, channelId, data, cb) {
    // XXX
    cb("NOT_IMPLEMENTED");
};

const NEWLINE_CHR = ('\n').charCodeAt(0);
const mkBufferSplit = () => {
    let remainder = null;
    return Pull((read) => {
        return (abort, cb) => {
            read(abort, function (end, data) {
                if (end) {
                    if (data) { console.log("mkBufferSplit() Data at the end"); }
                    cb(end, remainder ? [remainder, data] : [data]);
                    remainder = null;
                    return;
                }
                const queue = [];
                for (;;) {
                    const offset = data.indexOf(NEWLINE_CHR);
                    if (offset < 0) {
                        remainder = remainder ? Buffer.concat([remainder, data]) : data;
                        break;
                    }
                    let subArray = data.slice(0, offset);
                    if (remainder) {
                        subArray = Buffer.concat([remainder, subArray]);
                        remainder = null;
                    }
                    queue.push(subArray);
                    data = data.slice(offset + 1);
                }
                cb(end, queue);
            });
        };
    }, Pull.flatten());
};

const mkOffsetCounter = () => {
    let offset = 0;
    return Pull.map((buff) => {
        const out = { offset: offset, buff: buff };
        // +1 for the eaten newline
        offset += buff.length + 1;
        return out;
    });
};

const readMessagesBin = (env, id, start, msgHandler, cb) => {
    const stream = Fs.createReadStream(mkPath(env, id), { start: start });
    let keepReading = true;
    Pull(
        ToPull.read(stream),
        mkBufferSplit(),
        mkOffsetCounter(),
        Pull.asyncMap((data, moreCb) => {
            msgHandler(data, moreCb, () => { keepReading = false; moreCb(); });
        }),
        Pull.drain(() => (keepReading), (err) => {
            cb((keepReading) ? err : undefined);
        })
    );
};

var checkPath = function (path, callback) {
    Fs.stat(path, function (err) {
        if (!err) {
            callback(undefined, true);
            return;
        }
        if (err.code !== 'ENOENT') {
            callback(err);
            return;
        }
        Fse.mkdirp(Path.dirname(path), PERMISSIVE, function (err) {
            if (err && err.code !== 'EEXIST') {
                callback(err);
                return;
            }
            callback(undefined, false);
        });
    });
};

var labelError = function (label, err) {
    return label + (err.code ? "_" +  err.code: '');
};

var removeChannel = function (env, channelName, cb) {
    var channelPath = mkPath(env, channelName);
    var metadataPath = mkMetadataPath(env, channelName);

    var CB = Once(cb);

    nThen(function (w) {
        Fs.unlink(channelPath, w(function (err) {
            if (err) {
                w.abort();
                CB(labelError("E_CHANNEL_REMOVAL", err));
            }
        }));
        Fs.unlink(metadataPath, w(function (err) {
            if (err) {
                if (err.code === 'ENOENT') { return; } // proceed if there's no metadata to delete
                w.abort();
                CB(labelError("E_METADATA_REMOVAL", err));
            }
        }));
    }).nThen(function () {
        CB();
    });
};

var removeArchivedChannel = function (env, channelName, cb) {
    var channelPath = mkArchivePath(env, channelName);
    var metadataPath = mkArchiveMetadataPath(env, channelName);

    var CB = Once(cb);

    nThen(function (w) {
        Fs.unlink(channelPath, w(function (err) {
            if (err) {
                w.abort();
                CB(labelError("E_ARCHIVED_CHANNEL_REMOVAL", err));
            }
        }));
        Fs.unlink(metadataPath, w(function (err) {
            if (err) {
                if (err.code === "ENOENT") { return; }
                w.abort();
                CB(labelError("E_ARCHIVED_METADATA_REMOVAL", err));
            }
        }));
    }).nThen(function () {
        CB();
    });
};

// TODO implement a method of removing metadata that doesn't have a corresponding channel

// TODO confirm that we're ignoring metadata files
var listChannels = function (root, handler, cb) {
    // do twenty things at a time
    var sema = Semaphore.create(20);

    var dirList = [];

    nThen(function (w) {
        // the root of your datastore contains nested directories...
        Fs.readdir(root, w(function (err, list) {
            if (err) {
                w.abort();
                // TODO check if we normally return strings or errors
                return void cb(err);
            }
            dirList = list;
        }));
    }).nThen(function (w) {
        // search inside the nested directories
        // stream it so you don't put unnecessary data in memory
        var wait = w();
        dirList.forEach(function (dir) {
            sema.take(function (give) {
    // TODO modify the asynchronous bits here to keep less in memory at any given time
    // list a directory -> process its contents with semaphores until less than N jobs are running
    // then list the next directory...
                var nestedDirPath = Path.join(root, dir);
                Fs.readdir(nestedDirPath, w(give(function (err, list) {
                    if (err) { return void handler(err); } // Is this correct?

                    list.forEach(function (item) {
                        // ignore things that don't match the naming pattern
                        if (/^\./.test(item) || !/[0-9a-fA-F]{32}\.ndjson$/.test(item)) { return; }
                        var filepath = Path.join(nestedDirPath, item);
                        var channel = filepath.replace(/\.ndjson$/, '').replace(/.*\//, '');
                        if ([32, 34].indexOf(channel.length) === -1) { return; }

                        // otherwise throw it on the pile
                        sema.take(function (give) {
                            var next = w(give());
                            Fs.stat(filepath, w(function (err, stats) {
                                if (err) {
                                    return void handler(err);
                                }

                                handler(void 0, {
                                    channel: channel,
                                    atime: stats.atime,
                                    mtime: stats.mtime,
                                    ctime: stats.ctime,
                                    size: stats.size,
                                }, next);
                            }));
                        });
                    });
                })));
            });
        });
        wait();
    }).nThen(function () {
        cb();
    });
};

// move a channel's log file from its current location
// to an equivalent location in the cold storage directory
var archiveChannel = function (env, channelName, cb) {
    // TODO close channels before archiving them?
    if (!env.retainData) {
        return void cb("ARCHIVES_DISABLED");
    }

    // ctime is the most reliable indicator of when a file was archived
    // because it is used to indicate changes to the files metadata
    // and not its contents
    // if we find that this is not reliable in production, we can update it manually
    // https://nodejs.org/api/fs.html#fs_fs_utimes_path_atime_mtime_callback

    // check what the channel's path should be (in its current location)
    var currentPath = mkPath(env, channelName);

    // construct a parallel path in the new location
    var archivePath = mkArchivePath(env, channelName);

    // use Fse.move to move it, Fse makes paths to the directory when you use it.
    // https://github.com/jprichardson/node-fs-extra/blob/HEAD/docs/move.md
    nThen(function (w) {
        // move the channel log and abort if anything goes wrong
        Fse.move(currentPath, archivePath, { overwrite: true }, w(function (err) {
            if (!err) { return; }
            w.abort();
            cb(err);
        }));
    }).nThen(function (w) {
        // archive the dedicated metadata channel
        var metadataPath = mkMetadataPath(env, channelName);
        var archiveMetadataPath = mkArchiveMetadataPath(env, channelName);

        Fse.move(metadataPath, archiveMetadataPath, { overwrite: true, }, w(function (err) {
            // there's no metadata to archive, so you're done!
            if (err && err.code === "ENOENT") {
                return void cb();
            }

            // there was an error archiving the metadata
            if (err) {
                return void cb(labelError("E_METADATA_ARCHIVAL", err));
            }

            // it was archived successfully
            cb();
        }));
    });
};

var unarchiveChannel = function (env, channelName, cb) {
    // very much like 'archiveChannel' but in the opposite direction

    // the file is currently archived
    var channelPath = mkArchivePath(env, channelName);
    var metadataPath = mkMetadataPath(env, channelName);

    // don't call the callback multiple times
    var CB = Once(cb);

    // if a file exists in the unarchived path, you probably don't want to clobber its data
    // so unlike 'archiveChannel' we won't overwrite.
    // Fse.move will call back with EEXIST in such a situation

    nThen(function (w) {
        // if either metadata or a file exist in prod, abort
        channelExists(channelPath, w(function (err, exists) {
            if (err) {
                w.abort();
                return void CB(err);
            }
            if (exists) {
                w.abort();
                return CB('UNARCHIVE_CHANNEL_CONFLICT');
            }
        }));
        channelExists(metadataPath, w(function (err, exists) {
            if (err) {
                w.abort();
                return void CB(err);
            }
            if (exists) {
                w.abort();
                return CB("UNARCHIVE_METADATA_CONFLICT");
            }
        }));
    }).nThen(function (w) {
        // construct archive paths
        var archiveChannelPath = mkArchivePath(env, channelName);
        // restore the archived channel
        Fse.move(archiveChannelPath, channelPath, w(function (err) {
            if (err) {
                w.abort();
                return void CB(err);
            }
        }));
    }).nThen(function (w) {
        var archiveMetadataPath = mkArchiveMetadataPath(env, channelName);
        // TODO validate that it's ok to move metadata non-atomically

        // restore the metadata log
        Fse.move(archiveMetadataPath, metadataPath, w(function (err) {
            // if there's nothing to move, you're done.
            if (err && err.code === 'ENOENT') {
                return CB();
            }
            // call back with an error if something goes wrong
            if (err) {
                w.abort();
                return void CB(labelError("E_METADATA_RESTORATION", err));
            }
            // otherwise it was moved successfully
            CB();
        }));
    });
};

var flushUnusedChannels = function (env, cb, frame) {
    var currentTime = +new Date();

    var expiration = typeof(frame) === 'undefined'?  env.channelExpirationMs: frame;
    Object.keys(env.channels).forEach(function (chanId) {
        var chan = env.channels[chanId];
        if (typeof(chan.atime) !== 'number') { return; }
        if (currentTime >= expiration + chan.atime) {
            closeChannel(env, chanId, function (err) {
                if (err) {
                    console.error(err);
                    return;
                }
                if (env.verbose) {
                    console.log("Closed channel [%s]", chanId);
                }
            });
        }
    });
    cb();
};

var channelBytes = function (env, chanName, cb) {
    var path = mkPath(env, chanName);
    Fs.stat(path, function (err, stats) {
        if (err) { return void cb(err); }
        cb(undefined, stats.size);
    });
};

// implement metadata bytes as well?
/*::
export type ChainPadServer_ChannelInternal_t = {
    atime: number,
    writeStream: typeof(process.stdout),
    whenLoaded: ?Array<(err:?Error, chan:?ChainPadServer_ChannelInternal_t)=>void>,
    onError: Array<(?Error)=>void>,
    path: string
};
*/
var getChannel = function (
    env,
    id,
    callback /*:(err:?Error, chan:?ChainPadServer_ChannelInternal_t)=>void*/
) {
    if (env.channels[id]) {
        var chan = env.channels[id];
        chan.atime = +new Date();
        if (chan.whenLoaded) {
            chan.whenLoaded.push(callback);
        } else {
            callback(undefined, chan);
        }
        return;
    }

    if (env.openFiles >= env.openFileLimit) {
        // if you're running out of open files, asynchronously clean up expired files
        // do it on a shorter timeframe, though (half of normal)
        setTimeout(function () {
            flushUnusedChannels(env, function () {
                if (env.verbose) {
                    console.log("Approaching open file descriptor limit. Cleaning up");
                }
            }, env.channelExpirationMs / 2);
        });
    }
    var path = mkPath(env, id);
    var channel /*:ChainPadServer_ChannelInternal_t*/ = env.channels[id] = {
        atime: +new Date(),
        writeStream: (undefined /*:any*/),
        whenLoaded: [ callback ],
        onError: [ ],
        path: path
    };
    var complete = function (err) {
        var whenLoaded = channel.whenLoaded;
        // no guarantee stream.on('error') will not cause this to be called multiple times
        if (!whenLoaded) { return; }
        channel.whenLoaded = undefined;
        if (err) {
            delete env.channels[id];
        }
        if (!channel.writeStream) {
            throw new Error("getChannel() complete called without channel writeStream");
        }
        whenLoaded.forEach(function (wl) { wl(err, (err) ? undefined : channel); });
    };
    var fileExists;
    var errorState;
    nThen(function (waitFor) {
        checkPath(path, waitFor(function (err, exists) {
            if (err) {
                errorState = true;
                complete(err);
                return;
            }
            fileExists = exists;
        }));
    }).nThen(function (waitFor) {
        if (errorState) { return; }
        var stream = channel.writeStream = Fs.createWriteStream(path, { flags: 'a' });
        env.openFiles++;
        stream.on('open', waitFor());
        stream.on('error', function (err /*:?Error*/) {
            env.openFiles--;
            // this might be called after this nThen block closes.
            if (channel.whenLoaded) {
                complete(err);
            } else {
                channel.onError.forEach(function (handler) {
                    handler(err);
                });
            }
        });
    }).nThen(function () {
        if (errorState) { return; }
        complete();
    });
};

const messageBin = (env, chanName, msgBin, cb) => {
    getChannel(env, chanName, function (err, chan) {
        if (!chan) {
            cb(err);
            return;
        }
        let called = false;
        var complete = function (err) {
            if (called) { return; }
            called = true;
            cb(err);
        };
        chan.onError.push(complete);
        chan.writeStream.write(msgBin, function () {
            /*::if (!chan) { throw new Error("Flow unreachable"); }*/
            chan.onError.splice(chan.onError.indexOf(complete), 1);
            chan.atime = +new Date();
            if (!cb) { return; }
            complete();
        });
    });
};

var message = function (env, chanName, msg, cb) {
    messageBin(env, chanName, new Buffer(msg + '\n', 'utf8'), cb);
};

var getMessages = function (env, chanName, handler, cb) {
    getChannel(env, chanName, function (err, chan) {
        if (!chan) {
            cb(err);
            return;
        }
        var errorState = false;
        readMessages(chan.path, function (msg) {
            if (!msg || errorState) { return; }
            //console.log(msg);
            try {
                handler(msg);
            } catch (e) {
                errorState = true;
                return void cb(err);
            }
        }, function (err) {
            if (err) {
                errorState = true;
                return void cb(err);
            }
            if (!chan) { throw new Error("impossible, flow checking"); }
            chan.atime = +new Date();
            cb();
        });
    });
};

/*::
export type ChainPadServer_MessageObj_t = { buff: Buffer, offset: number };
export type ChainPadServer_Storage_t = {
    readMessagesBin: (
        channelName:string,
        start:number,
        asyncMsgHandler:(msg:ChainPadServer_MessageObj_t, moreCb:()=>void, abortCb:()=>void)=>void,
        cb:(err:?Error)=>void
    )=>void,
    message: (channelName:string, content:string, cb:(err:?Error)=>void)=>void,
    messageBin: (channelName:string, content:Buffer, cb:(err:?Error)=>void)=>void,
    getMessages: (channelName:string, msgHandler:(msg:string)=>void, cb:(err:?Error)=>void)=>void,
    removeChannel: (channelName:string, cb:(err:?Error)=>void)=>void,
    closeChannel: (channelName:string, cb:(err:?Error)=>void)=>void,
    flushUnusedChannels: (cb:()=>void)=>void,
    getChannelSize: (channelName:string, cb:(err:?Error, size:?number)=>void)=>void,
    getChannelMetadata: (channelName:string, cb:(err:?Error|string, data:?any)=>void)=>void,
    clearChannel: (channelName:string, (err:?Error)=>void)=>void
};
export type ChainPadServer_Config_t = {
    verbose?: boolean,
    filePath?: string,
    channelExpirationMs?: number,
    openFileLimit?: number
};
*/
module.exports.create = function (
    conf /*:ChainPadServer_Config_t*/,
    cb /*:(store:ChainPadServer_Storage_t)=>void*/
) {
    var env = {
        root: conf.filePath || './datastore',
        archiveRoot: conf.archivePath || './data/archive',
        retainData: conf.retainData,
        channels: { },
        channelExpirationMs: conf.channelExpirationMs || 30000,
        verbose: conf.verbose,
        openFiles: 0,
        openFileLimit: conf.openFileLimit || 2048,
    };
    var it;

    nThen(function (w) {
        // make sure the store's directory exists
        Fse.mkdirp(env.root, PERMISSIVE, w(function (err) {
            if (err && err.code !== 'EEXIST') {
                throw err;
            }
        }));
        // make sure the cold storage directory exists
        Fse.mkdirp(env.archiveRoot, PERMISSIVE, w(function (err) {
            if (err && err.code !== 'EEXIST') {
                throw err;
            }
        }));
    }).nThen(function () {
        cb({
        // OLDER METHODS
            // write a new message to a log
            message: function (channelName, content, cb) {
                if (!isValidChannelId(channelName)) { return void cb(new Error('EINVAL')); }
                message(env, channelName, content, cb);
            },
            // iterate over all the messages in a log
            getMessages: function (channelName, msgHandler, cb) {
                if (!isValidChannelId(channelName)) { return void cb(new Error('EINVAL')); }
                getMessages(env, channelName, msgHandler, cb);
            },

        // NEWER IMPLEMENTATIONS OF THE SAME THING
            // write a new message to a log
            messageBin: (channelName, content, cb) => {
                if (!isValidChannelId(channelName)) { return void cb(new Error('EINVAL')); }
                messageBin(env, channelName, content, cb);
            },
            // iterate over the messages in a log
            readMessagesBin: (channelName, start, asyncMsgHandler, cb) => {
                if (!isValidChannelId(channelName)) { return void cb(new Error('EINVAL')); }
                readMessagesBin(env, channelName, start, asyncMsgHandler, cb);
            },

        // METHODS for deleting data
            // remove a channel and its associated metadata log if present
            removeChannel: function (channelName, cb) {
                if (!isValidChannelId(channelName)) { return void cb(new Error('EINVAL')); }
                removeChannel(env, channelName, function (err) {
                    cb(err);
                });
            },
            // remove a channel and its associated metadata log from the archive directory
            removeArchivedChannel: function (channelName, cb) {
                if (!isValidChannelId(channelName)) { return void cb(new Error('EINVAL')); }
                removeArchivedChannel(env, channelName, cb);
            },
            // clear all data for a channel but preserve its metadata
            clearChannel: function (channelName, cb) {
                if (!isValidChannelId(channelName)) { return void cb(new Error('EINVAL')); }
                clearChannel(env, channelName, cb);
            },

            // check if a channel exists in the database
            isChannelAvailable: function (channelName, cb) {
                if (!isValidChannelId(channelName)) { return void cb(new Error('EINVAL')); }
                // construct the path
                var filepath = mkPath(env, channelName);
                channelExists(filepath, cb);
            },
            // check if a channel exists in the archive
            isChannelArchived: function (channelName, cb) {
                if (!isValidChannelId(channelName)) { return void cb(new Error('EINVAL')); }
                // construct the path
                var filepath = mkArchivePath(env, channelName);
                channelExists(filepath, cb);
            },
            // move a channel from the database to the archive, along with its metadata
            archiveChannel: function (channelName, cb) {
                if (!isValidChannelId(channelName)) { return void cb(new Error('EINVAL')); }
                archiveChannel(env, channelName, cb);
            },
            // restore a channel from the archive to the database, along with its metadata
            restoreArchivedChannel: function (channelName, cb) {
                if (!isValidChannelId(channelName)) { return void cb(new Error('EINVAL')); }
                unarchiveChannel(env, channelName, cb);
            },

        // METADATA METHODS
            // fetch the metadata for a channel
            getChannelMetadata: function (channelName, cb) {
                if (!isValidChannelId(channelName)) { return void cb(new Error('EINVAL')); }
                getChannelMetadata(env, channelName, cb);
            },
            // iterate over lines of metadata changes from a dedicated log
            readDedicatedMetadata: function (channelName, handler, cb) {
                if (!isValidChannelId(channelName)) { return void cb(new Error('EINVAL')); }
                getDedicatedMetadata(env, channelName, handler, cb);
            },

            // iterate over multiple lines of metadata changes
            readChannelMetadata: function (channelName, handler, cb) {
                if (!isValidChannelId(channelName)) { return void cb(new Error('EINVAL')); }
                readMetadata(env, channelName, handler, cb);
            },
            // write a new line to a metadata log
            writeMetadata: function (channelName, data, cb) {
                if (!isValidChannelId(channelName)) { return void cb(new Error('EINVAL')); }
                writeMetadata(env, channelName, data, cb);
            },

        // CHANNEL ITERATION
            listChannels: function (handler, cb) {
                listChannels(env.root, handler, cb);
            },
            listArchivedChannels: function (handler, cb) {
                listChannels(Path.join(env.archiveRoot, 'datastore'), handler, cb);
            },

            getChannelSize: function (channelName, cb) {
                if (!isValidChannelId(channelName)) { return void cb(new Error('EINVAL')); }
                channelBytes(env, channelName, cb);
            },
        // OTHER DATABASE FUNCTIONALITY
            // remove a particular channel from the cache
            closeChannel: function (channelName, cb) {
                if (!isValidChannelId(channelName)) { return void cb(new Error('EINVAL')); }
                closeChannel(env, channelName, cb);
            },
            // iterate over open channels and close any that are not active
            flushUnusedChannels: function (cb) {
                flushUnusedChannels(env, cb);
            },
            // write to a log file
            log: function (channelName, content, cb) {
                message(env, channelName, content, cb);
            },
            // shut down the database
            shutdown: function () {
                clearInterval(it);
            }
        });
    });
    it = setInterval(function () {
        flushUnusedChannels(env, function () { });
    }, 5000);
};