# GNU GPLv3, see LICENSE

import os
import re
import base64
import binascii

import gf256


class SException(Exception): pass
class InvalidParams(SException): pass
class DetectionException(SException): pass
class DecodingException(SException): pass
class MalformedShare(SException): pass


def _shareByte(secretB,k,n):
	if not k<=n<255:
		raise InvalidParams("Failed k<=n<255, k={0}, n={1}".format(k,n))
	# we might be concerned with zero coefficients degenerating our polynomial, but there's no reason - we still need k shares to determine it is the case
	coefs=[int(secretB)]+[int(b) for b in os.urandom(k-1)]
	points=[gf256.evaluate(coefs,i) for i in range(1,n+1)]
	return points


def generateRaw(secret,k,n):
	"""Splits secret into shares.

	:param secret: (bytes)
	:param k: number of shares necessary for secret recovery. 1 <= k <= n
	:param n: (int) number of shares generated. 1 <= n < 255
	:return: [(i, (bytes) share), ...]"""
	shares=[_shareByte(b,k,n) for b in secret]
	return [(i+1, bytes([s[i] for s in shares])) for i in range(n)]


def reconstructRaw(*shares):
	"""Tries to recover the secret from its shares.

	:param shares: ((i, (bytes) share), ...)
	:return: (bytes) reconstructed secret. Too few shares returns garbage."""
	secretLen=len(shares[0][1])
	res=[None]*secretLen
	for i in range(secretLen):
		points=[(x,s[i]) for (x,s) in shares]
		res[i]=(gf256.getConstantCoef(*points))
	return bytes(res)


def generate(secret,k,n,encoding="b32"):
	"""Wraps generateRaw().

	:param secret: (str or bytes)
	:param k: number of shares necessary for secret recovery
	:param n: number of shares generated
	:param encoding: {hex, b32, b64} desired output encoding. Hexadecimal, Base32 or Base64.
	:return: [(str) share, ...]"""
	if isinstance(secret,str):
		secret=secret.encode("utf-8")
	shares=generateRaw(secret,k,n)
	return [encode(s,encoding) for s in shares]


def reconstruct(*shares,encoding="",raw=False):
	"""Wraps reconstructRaw.

	:param shares: ((str) share, ...)
	:param encoding: {hex, b32, b64, ""} encoding of share strings. If not provided or empty, the function tries to guess it.
	:param raw: (bool) whether to return bytes (True) or str (False)
	:return: (str or bytes) reconstructed secret. Too few shares returns garbage."""
	if not encoding:
		encoding=detectEncoding(shares)

	bs=reconstructRaw(*(decode(s,encoding) for s in shares))
	try:
		return bs if raw else bs.decode(encoding="utf-8")
	except UnicodeDecodeError:
		raise DecodingException('Failed to decode bytes to utf-8. Either you supplied invalid shares, or you missed the "raw" flag. Offending value: {0}'.format(bs))


def encode(share,encoding="b32"):
	if encoding=="hex": f=base64.b16encode
	elif encoding=="b32": f=base64.b32encode
	else: f=base64.b64encode
	(i,bs)=share
	return "{0}.{1}".format(i,f(bs).decode("utf-8"))


def decode(share,encoding="b32"):
	try:
		(i,_,shareStr)=share.partition(".")
		i=int(i)
		if not 1<=i<=255:
			raise MalformedShare("Malformed share: Failed 1<=k<=255, k={0}".format(i))
		if encoding=="hex": f=base64.b16decode
		elif encoding=="b32": f=base64.b32decode
		else: f=base64.b64decode
		shareBytes=f(shareStr)
		return (i,shareBytes)
	except (ValueError,binascii.Error):
		raise MalformedShare('Malformed share: share="{0}", encoding="{1}"'.format(share,encoding))


def detectEncoding(shares):
	classes=[
		(re.compile(r"\d+\.([0-9A-F]{2})+"), "hex"),
		(re.compile(r"\d+\.([A-Z2-7]{8})*([A-Z2-7]{8}|[A-Z2-7]{2}={6}|[A-Z2-7]{4}={4}|[A-Z2-7]{5}={3}|[A-Z2-7]{7}={1})"), "b32"),
		(re.compile(r"\d+\.([A-Za-z0-9+/]{4})*([A-Za-z0-9+/]{4}|[A-Za-z0-9+/]{2}={2}|[A-Za-z0-9+/]{3}={1})"), "b64")
	]
	for (regexp, res) in classes:
		if all(regexp.fullmatch(share) for share in shares):
			return res
	raise DetectionException("No expected encoding detected")


if __name__=="__main__":
	import cli
	cli.run()