import pint


class ConverterError(Exception):
    pass


class Converter(object):
    def __init__(self):
        self.ureg = pint.UnitRegistry(autoconvert_offset_to_baseunit=True)
        self.ureg.define('degreeC = kelvin; offset: 273.15 = °C = C')
        self.ureg.define('degreeF = 5 / 9 * kelvin; offset: 255.372222 = °F = F')
        self.default_units = {
            self.ureg.degC: self.ureg.degreeF,
            self.ureg.degreeC: self.ureg.degreeF,
            self.ureg.degF: self.ureg.degreeC,
            self.ureg.degreeF: self.ureg.degreeC,
            self.ureg.kilometer: self.ureg.mile,
            self.ureg.meter: self.ureg.foot,
            self.ureg.centimeter: self.ureg.inch,
            self.ureg.millimeter: self.ureg.inch,
            self.ureg.mile: self.ureg.kilometer,
            self.ureg.foot: self.ureg.meter,
            self.ureg.inch: self.ureg.millimeter,
            self.ureg.kilogram: self.ureg.pound,
            self.ureg.pound: self.ureg.kilogram,
            self.ureg.ounce: self.ureg.kilogram,
            self.ureg.liter: self.ureg.gallon,
            self.ureg.milliliter: self.ureg.floz,
            self.ureg.gallon: self.ureg.liter,
            self.ureg.floz: self.ureg.milliliter,
        }

    def update_currencies(self, base, rates):
        self.ureg.define('{0} = [currency]'.format(base))
        for currency, rate in rates.items():
            self.ureg.define('{0} = {1} / {2}'.format(currency, base, rate))

    def convert(self, expression, unit=None):
        try:
            q = self.ureg.parse_expression(expression)
        except pint.errors.UndefinedUnitError:
            raise ConverterError('unknown units')
        try:
            q.units
        except AttributeError:
            if unit is not None:
                q = self.ureg.parse_expression(expression + unit)
                unit = None
            else:
                raise ConverterError('unknown units')
        try:
            q.units
        except AttributeError:
            raise ConverterError('unknown units')
        if unit is not None:
            try:
                u = self.ureg.parse_units(unit)
            except pint.errors.UndefinedUnitError:
                raise ConverterError('unknown units')
        else:
            try:
                u = self.default_units[q.units]
            except KeyError:
                raise ConverterError('no target units specified')
        try:
            result = q.to(u)
        except pint.errors.DimensionalityError:
            raise ConverterError('invalid conversion')
        return '{:0.5g~P}'.format(result)