module.exports = function (args, opts) {

if (!opts) opts = {};

var flags = { bools : {}, strings : {}, unknownFn: null };

if (typeof opts['unknown'] === 'function') {
    flags.unknownFn = opts['unknown'];
}

if (typeof opts['boolean'] === 'boolean' && opts['boolean']) {
  flags.allBools = true;
} else {
  [].concat(opts['boolean']).filter(Boolean).forEach(function (key) {
      flags.bools[key] = true;
  });
}

var aliases = {};
Object.keys(opts.alias || {}).forEach(function (key) {
    aliases[key] = [].concat(opts.alias[key]);
    aliases[key].forEach(function (x) {
        aliases[x] = [key].concat(aliases[key].filter(function (y) {
            return x !== y;
        }));
    });
});

[].concat(opts.string).filter(Boolean).forEach(function (key) {
    flags.strings[key] = true;
    if (aliases[key]) {
        flags.strings[aliases[key]] = true;
    }
 });

var defaults = opts['default'] || {};

var argv = { _ : [] };
Object.keys(flags.bools).forEach(function (key) {
    setArg(key, defaults[key] === undefined ? false : defaults[key]);
});

var notFlags = [];

if (args.indexOf('--') !== -1) {
    notFlags = args.slice(args.indexOf('--')+1);
    args = args.slice(0, args.indexOf('--'));
}

function argDefined(key, arg) {
    return (flags.allBools && /^--[^=]+$/.test(arg)) ||
        flags.strings[key] || flags.bools[key] || aliases[key];
}

function setArg (key, val, arg) {
    if (arg && flags.unknownFn && !argDefined(key, arg)) {
        if (flags.unknownFn(arg) === false) return;
    }

    var value = !flags.strings[key] && isNumber(val)
        ? Number(val) : val
    ;
    setKey(argv, key.split('.'), value);

    (aliases[key] || []).forEach(function (x) {
        setKey(argv, x.split('.'), value);
    });
}

function setKey (obj, keys, value) {
    var o = obj;
    keys.slice(0,-1).forEach(function (key) {
        if (o[key] === undefined) o[key] = {};
        o = o[key];
    });

    var key = keys[keys.length - 1];
    if (o[key] === undefined || flags.bools[key] || typeof o[key] === 'boolean') {
        o[key] = value;
    }
    else if (Array.isArray(o[key])) {
        o[key].push(value);
    }
    else {
        o[key] = [ o[key], value ];
    }
}

function aliasIsBoolean(key) {
  return aliases[key].some(function (x) {
      return flags.bools[x];
  });
}

for (var i = 0; i < args.length; i++) {
    var arg = args[i];

    if (/^--.+=/.test(arg)) {
        // Using [\s\S] instead of . because js doesn't support the
        // 'dotall' regex modifier. See:
        // http://stackoverflow.com/a/1068308/13216
        var m = arg.match(/^--([^=]+)=([\s\S]*)$/);
        var key = m[1];
        var value = m[2];
        if (flags.bools[key]) {
            value = value !== 'false';
        }
        setArg(key, value, arg);
    }
    else if (/^--no-.+/.test(arg)) {
        var key = arg.match(/^--no-(.+)/)[1];
        setArg(key, false, arg);
    }
    else if (/^--.+/.test(arg)) {
        var key = arg.match(/^--(.+)/)[1];
        var next = args[i + 1];
        if (next !== undefined && !/^-/.test(next)
        && !flags.bools[key]
        && !flags.allBools
        && (aliases[key] ? !aliasIsBoolean(key) : true)) {
            setArg(key, next, arg);
            i++;
        }
        else if (/^(true|false)$/.test(next)) {
            setArg(key, next === 'true', arg);
            i++;
        }
        else {
            setArg(key, flags.strings[key] ? '' : true, arg);
        }
    }
    else if (/^-[^-]+/.test(arg)) {
        var letters = arg.slice(1,-1).split('');

        var broken = false;
        for (var j = 0; j < letters.length; j++) {
            var next = arg.slice(j+2);

            if (next === '-') {
                setArg(letters[j], next, arg)
                continue;
            }

            if (/[A-Za-z]/.test(letters[j]) && /=/.test(next)) {
                setArg(letters[j], next.split('=')[1], arg);
                broken = true;
                break;
            }

            if (/[A-Za-z]/.test(letters[j])
            && /-?\d+(\.\d*)?(e-?\d+)?$/.test(next)) {
                setArg(letters[j], next, arg);
                broken = true;
                break;
            }

            if (letters[j+1] && letters[j+1].match(/\W/)) {
                setArg(letters[j], arg.slice(j+2), arg);
                broken = true;
                break;
            }
            else {
                setArg(letters[j], flags.strings[letters[j]] ? '' : true, arg);
            }
        }

        var key = arg.slice(-1)[0];
        if (!broken && key !== '-') {
            if (args[i+1] && !/^(-|--)[^-]/.test(args[i+1])
            && !flags.bools[key]
            && (aliases[key] ? !aliasIsBoolean(key) : true)) {
                setArg(key, args[i+1], arg);
                i++;
            }
            else if (args[i+1] && /true|false/.test(args[i+1])) {
                setArg(key, args[i+1] === 'true', arg);
                i++;
            }
            else {
                setArg(key, flags.strings[key] ? '' : true, arg);
            }
        }
    }
    else {
        if (!flags.unknownFn || flags.unknownFn(arg) !== false) {
            argv._.push(
                flags.strings['_'] || !isNumber(arg) ? arg : Number(arg)
            );
        }
        if (opts.stopEarly) {
            argv._.push.apply(argv._, args.slice(i + 1));
            break;
        }
    }
}

Object.keys(defaults).forEach(function (key) {
    if (!hasKey(argv, key.split('.'))) {
        setKey(argv, key.split('.'), defaults[key]);

        (aliases[key] || []).forEach(function (x) {
            setKey(argv, x.split('.'), defaults[key]);
        });
    }
});

if (opts['--']) {
    argv['--'] = new Array();
    notFlags.forEach(function(key) {
        argv['--'].push(key);
    });
}
else {
    notFlags.forEach(function(key) {
        argv._.push(key);
    });
}

return argv;

};

function hasKey (obj, keys) {

var o = obj;
keys.slice(0,-1).forEach(function (key) {
    o = (o[key] || {});
});

var key = keys[keys.length - 1];
return key in o;

}

function isNumber (x) {

if (typeof x === 'number') return true;
if (/^0x[0-9a-f]+$/i.test(x)) return true;
return /^[-+]?(?:\d+(?:\.\d*)?|\.\d+)(e[-+]?\d+)?$/.test(x);

}