import psycopg2
from contextlib import closing

class Currency:
	""" Currency
	
	Each account has a currency (1 Kč, 1 Club Maté, ...), pairs of
	currencies have (asymmetric) exchange rates. """
	def __init__(self, db, id, name):
		self.db = db
		self.id = id
		self.name = name

	@classmethod
	def default(cls, db):
		""" Default wallet currency """
		return cls.load(db, name = "Kč")

	@classmethod
	def load(cls, db, id = None, name = None):
		""" Constructor for existing currency """
		if id is not None:
			with closing(db.cursor()) as cur:
				cur.execute("SELECT name FROM currencies WHERE id = %s", [id])
				name = cur.fetchone()[0]
		elif name is not None:
			with closing(db.cursor()) as cur:
				cur.execute("SELECT id FROM currencies WHERE name = %s", [name])
				id = cur.fetchone()[0]
		else:
			raise NameError("Currency.load(): Specify either id or name")
		return cls(db, name = name, id = id)

	@classmethod
	def create(cls, db, name):
		""" Constructor for new currency """
		with closing(db.cursor()) as cur:
			cur.execute("INSERT INTO currencies (name) VALUES (?) RETURNING id", [name])
			id = cur.fetchone()[0]
		return cls.new(db, name = name, id = id)

	def rates(self, other):
		""" Return tuple ($buy, $sell) of rates of $self in relation to $other (brmbar.Currency):
		$buy is the price of $self in means of $other when buying it (into brmbar)
		$sell is the price of $self in means of $other when selling it (from brmbar) """
		with closing(self.db.cursor()) as cur:

			cur.execute("SELECT rate, rate_dir FROM exchange_rates WHERE target = %s AND source = %s", [self.id, other.id])
			res = cur.fetchone()
			if res is None:
				raise NameError("Currency.rate(): Unknown conversion " + other.name() + " to " + self.name())
			buy_rate, buy_rate_dir = res
			buy = buy_rate if buy_rate_dir == "target_to_source" else 1/buy_rate

			cur.execute("SELECT rate, rate_dir FROM exchange_rates WHERE target = %s AND source = %s", [other.id, self.id])
			res = cur.fetchone()
			if res is None:
				raise NameError("Currency.rate(): Unknown conversion " + self.name() + " to " + other.name())
			sell_rate, sell_rate_dir = res
			sell = sell_rate if sell_rate_dir == "source_to_target" else 1/sell_rate

			return (buy, sell)

	def convert(self, amount, target):
		with closing(self.db.cursor()) as cur:
			cur.execute("SELECT rate, rate_dir FROM exchange_rates WHERE target = %s AND source = %s AND valid_since <= NOW() ORDER BY valid_since ASC LIMIT 1", [target.id, self.id])
			res = cur.fetchone()
			if res is None:
				raise NameError("Currency.convert(): Unknown conversion " + self.name() + " to " + other.name())
			rate, rate_dir = res
			if rate_dir == "source_to_target":
				resamount = amount * rate
			else:
				resamount = amount / rate
			return resamount

	def str(self, amount):
		return "{:.2f} {}".format(amount, self.name)