/*@flow*/
/* jshint esversion: 6 */
/* global Buffer */
var Fs = require("fs");
var Path = require("path");
var nThen = require("nthen");
const ToPull = require('stream-to-pull-stream');
const Pull = require('pull-stream');

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

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 getChannelMetadata = function (Env, channelId, cb) {
    var path = mkPath(Env, channelId);
    getMetadataAtPath(Env, path, cb);
};

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); });
};

const NEWLINE_CHR = ('\n').charCodeAt(0);
const mkBufferSplit = () => {
    let remainder = null;
    return Pull((read) => {
        return (abort, cb) => {
            read(abort, function (end, data) {
                if (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), cb)
    );
};

var checkPath = function (path, callback) {
    // TODO check if we actually need to use stat at all
    Fs.stat(path, function (err) {
        if (!err) {
            callback(undefined, true);
            return;
        }
        if (err.code !== 'ENOENT') {
            callback(err);
            return;
        }
        // 511 -> octal 777
        Fs.mkdir(Path.dirname(path), 511, function (err) {
            if (err && err.code !== 'EEXIST') {
                callback(err);
                return;
            }
            callback(undefined, false);
        });
    });
};

var removeChannel = function (env, channelName, cb) {
    var filename = mkPath(env, channelName);
    Fs.unlink(filename, 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);
    });
};

/*::
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);
            if (!cb) { return; }
            //chan.messages.push(msg);
            chan.atime = +new Date();
            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',
        channels: { },
        channelExpirationMs: conf.channelExpirationMs || 30000,
        verbose: conf.verbose,
        openFiles: 0,
        openFileLimit: conf.openFileLimit || 2048,
    };
    // 0x1ff -> 777
    Fs.mkdir(env.root, 0x1ff, function (err) {
        if (err && err.code !== 'EEXIST') {
            // TODO: somehow return a nice error
            throw err;
        }
        cb({
            readMessagesBin: (channelName, start, asyncMsgHandler, cb) => {
                readMessagesBin(env, channelName, start, asyncMsgHandler, cb);
            },
            message: function (channelName, content, cb) {
                message(env, channelName, content, cb);
            },
            messageBin: (channelName, content, cb) => {
                messageBin(env, channelName, content, cb);
            },
            getMessages: function (channelName, msgHandler, cb) {
                getMessages(env, channelName, msgHandler, cb);
            },
            removeChannel: function (channelName, cb) {
                removeChannel(env, channelName, function (err) {
                    cb(err);
                });
            },
            closeChannel: function (channelName, cb) {
                closeChannel(env, channelName, cb);
            },
            flushUnusedChannels: function (cb) {
                flushUnusedChannels(env, cb);
            },
            getChannelSize: function (chanName, cb) {
                channelBytes(env, chanName, cb);
            },
            getChannelMetadata: function (channelName, cb) {
                getChannelMetadata(env, channelName, cb);
            },
            clearChannel: function (channelName, cb) {
                clearChannel(env, channelName, cb);
            },
        });
    });
    setInterval(function () {
        flushUnusedChannels(env, function () { });
    }, 5000);
};