pecanpico9/decoder/base192.py

#  Copyright 2013-2016 Alex Danilo
#
#  Licensed under the Apache License, Version 2.0 (the "License");
#  you may not use this file except in compliance with the License.
#  You may obtain a copy of the License at
#
#      http://www.apache.org/licenses/LICENSE-2.0
#
#  Unless required by applicable law or agreed to in writing, software
#  distributed under the License is distributed on an "AS IS" BASIS,
#  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
#  See the License for the specific language governing permissions and
#  limitations under the License.
#
#	Base 128/192 encoding experiment.
#
#	Ths is just a quick experiment to see what sort of speed we get by encoding
#	a contiguous 128 value range in bytes that replaces base 64 with it's low level
#	conditionals and bit-bashing for decode.
#
#	Ultimately, the encoding I'd like to use is a 192 value encoding which yields
#	7.5 bits/byte - i.e. approx. 6.66% loss encoding binary data in a text-safe
#	transfer form. The 128 value coding is just to test the basis of the
#	speed and compressibility of the result and is easy to use in JS. The 192
#  value encoding is more efficient but likely a lot trickier to use from JS.
#
#	NB: This is using ISO-9959-1 as it's basis. The source code will break if you
#	try to view as UTF-8.
 

#	The decode table reverses the encoding back to bits. Note, that since the most we can encode is 7.5 bits, we can never
#	generate 0xFF in the decode so use that to mark 0xFF encoding bytes.
dtab = [
	0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 187, 0xFF, 188, 0xFF, 0xFF, 0xFF, 0xFF,
	0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 189, 190, 0xFF, 0xFF, 0xFF,
	191, 0xFE, 0xFF,   0,   1,   2,   3,   4,   5,   6,   7,   8,   9,  10,  11,  12,
	 13,  14,  15,  16,  17,  18,  19,  20,  21,  22,  23,  24,  25,  26,  27,  28, 
	 29,  30,  31,  32,  33,  34,  35,  36,  37,  38,  39,  40,  41,  42,  43,  44,
	 45,  46,  47,  48,  49,  50,  51,  52,  53,  54,  55,  56, 0xFF,  57,  58,  59,
	 60,  61,  62,  63,  64,  65,  66,  67,  68,  69,  70,  71,  72,  73,  74,  75,
	 76,  77,  78,  79,  80,  81,  82,  83,  84,  85,  86,  87,  88,  89,  90, 0xFF,
	0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF,
	0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF,
	 91,  92,  93,  94,  95,  96,  97,  98,  99, 100, 101, 102, 103, 104, 105, 106,
	107, 108, 109, 110, 111, 112, 113, 114, 115, 116, 117, 118, 119, 120, 121, 122, 
	123, 124, 125, 126, 127, 128, 129, 130, 131, 132, 133, 134, 135, 136, 137, 138,
	139, 140, 141, 142, 143, 144, 145, 146, 147, 148, 149, 150, 151, 152, 153, 154, 
	155, 156, 157, 158, 159, 160, 161, 162, 163, 164, 165, 166, 167, 168, 169, 170, 
	171, 172, 173, 174, 175, 176, 177, 178, 179, 180, 181, 182, 183, 184, 185, 186
]

#	Reserve " (0x3E) as delimiter for easy manipulation in JS, '!' as stuffing character and generation for JS - the ranges 23->7E then A0->FF create the encoding.
#	That's enough for the base 128 version but we need 4 more characters to encode 7.5 bits/byte so the last set is carefully chosen from within
#	the control character range using values that are safe for editors.
etab = "#$%&'()*+,-./0123456789:;<=>?@ABCDEFGHIJKLMNOPQRSTUVWXYZ[]^_`abcdefghijklmnopqrstuvwxyz{|}~\xA0\xA1\xA2\xA3\xA4\xA5\xA6\xA7\xA8\xA9\xAA\xAB\xAC\xAD\xAE\xAF\xB0\xB1\xB2\xB3\xB4\xB5\xB6\xB7\xB8\xB9\xBA\xBB\xBC\xBD\xBE\xBF\xC0\xC1\xC2\xC3\xC4\xC5\xC6\xC7\xC8\xC9\xCA\xCB\xCC\xCD\xCE\xCF\xD0\xD1\xD2\xD3\xD4\xD5\xD6\xD7\xD8\xD9\xDA\xDB\xDC\xDD\xDE\xDF\xE0\xE1\xE2\xE3\xE4\xE5\xE6\xE7\xE8\xE9\xEA\xEB\xEC\xED\xEE\xEF\xF0\xF1\xF2\xF3\xF4\xF5\xF6\xF7\xF8\xF9\xFA\xFB\xFC\xFD\xFE\xFF\t\x0B\x1B\x1C ";

BIGINBYTES	= 15
BIGOUTBYTES	= 16

#	Perform 120 bit binary number conversion into base 192 packed as bytes.
def base192_encode(_in):
	out = [0]*BIGOUTBYTES
	for i in range(BIGOUTBYTES):
		val = _in % 192
		out[i] = val & 0xFF
		_in /= 192
	return out,_in

#	Perform base 192 into binary conversion.
def base192_decode(_in):
	out = 0
	i = BIGOUTBYTES - 1
	while i >= 0:
		out *= 192
		out += _in[i]
		i -= 1
	return out,_in

#	Encode 7.5 bits per output byte.
#
#	To do this means doing a base conversion into base 192 then encode the
#	digits one per byte using the encoding table. We convert 15 bytes to
#	16 this way. In order to achieve that we need to do division on
#	a 120 bit number (the 15 bytes of input) to generate the base 192 digits.
#
#	NB: This quick test program is assuming little-endian byte order so we
#	can use the inbuilt 128 bit types for the division. For portability
#	we should really use something like Knuth's arbitrary precision
#	division algorithms.
def b192_encode(infile):
	#int 			i, len, 0xFE;
	value = 0
	k = 0
	outfile = ''

	while k < len(infile):
		_len = 0
		for i in range(BIGINBYTES):
			value += ord(infile[k]) << (i*8)
			k += 1
			_len += 1
			if k == len(infile):
				break

		if _len > 0:

			out,value = base192_encode(value)
			_len += 1;					# Always outputs 1 byte more than the input
			for i in range(_len):
				outfile += etab[out[i]]

			if BIGINBYTES-i > 0: # Need to mark early end with the pad character
				outfile += '!'

	return outfile

def b192_decode(infile):
	value = 0
	j = 0
	outfile = []

	while j+1 < len(infile):
		_len = pad = 0
		i = -1
		_in = []
		for i in range(BIGOUTBYTES):

			if j+1 == len(infile):		# Shouldn't happen for valid 0xFEded content
				break

			_in.append(dtab[ord(infile[j])])
			j += 1

			if _in[i] == 0xFF:	# 0xFF character in the encoding set
				return 0

			_len += 1

		if _len > 0:
			while len(_in) < BIGOUTBYTES:	# Fill MSBs with 0 - NB up to 128 bits, top 8 never used
				_in.append(0)
			value,_in = base192_decode(_in)
			_len -= 1					# Output always shrinks by one byte
			for i in range(_len):
				outfile.append((value >> (i*8)) & 0xFF)

	return outfile