utils.py: Add helper method to encode Service Table

This method helps in encoding of Service Tables in EF.SST, EF.UST, EF.EST, EF.IST.
Takes original hex string of EF.SST, EF.UST, EF.EST, EF.IST, Service number,
Service to be enabled or disable flag as input and outputs the modified Service Table.

Change-Id: I0c97317d5a17aa0df720659d021b5cbf7d30edcc

pySim/utils.py

@@ -460,6 +460,49 @@ def TLV_parser(bytelist):
 			bytelist = bytelist[ L+2 : ]
 	return ret
 
+def enc_st(st, service, state=1):
+	"""
+	Encodes the EF S/U/IST/EST and returns the updated Service Table
+
+	Parameters:
+		st - Current value of SIM/USIM/ISIM Service Table
+		service - Service Number to encode as activated/de-activated
+		state - 1 mean activate, 0 means de-activate
+
+	Returns:
+		s - Modified value of SIM/USIM/ISIM Service Table
+
+	Default values:
+		- state: 1 - Sets the particular Service bit to 1
+	"""
+	st_bytes = [st[i:i+2] for i in range(0, len(st), 2) ]
+
+	s = ""
+	# Check whether the requested service is present in each byte
+	for i in range(0, len(st_bytes)):
+		# Byte i contains info about Services num (8i+1) to num (8i+8)
+		if service in range((8*i) + 1, (8*i) + 9):
+			byte = int(st_bytes[i], 16)
+			# Services in each byte are in order MSB to LSB
+			# MSB - Service (8i+8)
+			# LSB - Service (8i+1)
+			mod_byte = 0x00
+			# Copy bit by bit contents of byte to mod_byte with modified bit
+			# for requested service
+			for j in range(1, 9):
+				mod_byte = mod_byte >> 1
+				if service == (8*i) + j:
+					mod_byte = state == 1 and mod_byte|0x80 or mod_byte&0x7f
+				else:
+					mod_byte = byte&0x01 == 0x01 and mod_byte|0x80 or mod_byte&0x7f
+				byte = byte >> 1
+
+			s += ('%02x' % (mod_byte))
+		else:
+			s += st_bytes[i]
+
+	return s
+
 def dec_epdgid(hexstr):
 	"""
 	Decode ePDG Id to get EF.ePDGId or EF.ePDGIdEm.