[codec] on the principle and implementation of Base64 coding

Reprinted from: http://www.cnblogs.com/hongru/archive/2012/01/14/2321397.html

"Base64"
-base64 are encoded in string lengths, in groups of 8bit characters per 3.
-Then for each group, first get the ASCII encoding for each character,
-then convert the ASCII encoding to 8bit binary to get a set of 3*8=24bit bytes
-then divide the 24bit into 4 6bit bytes and fill two high 0 in front of each 6bit byte to get 4 8bit bytes
-The 4 8bit bytes are then converted to 10, and the corresponding encoded characters are obtained against the BASE64 encoded table (table below).

(Note: 1.) The encoding character is required to be 8bit, so it must be within the ASCII encoding range, \U0000-\U00FF, Chinese is not.
2. If the encoded character length is not a multiple of 3, then use 0 instead of the corresponding output character =)

BASE64 Coding Table
Value	Char		Value	Char		Value	Char		Value	Char
0	A		16	Q		32	G		48	W
1	B		17	R		33	H		49	X
2	C		18	S		34	I		50	Y
3	D		19	T		35	J		51	Z
4	E		20	U		36	K		52	0
5	F		21st	V		37	L		53	1
6	G		22	W		38	M		54	2
7	H		23	X		39	N		55	3
8	I		24	Y		40	O		56	4
9	J		25	Z		41	P		57	5
10	K		26	A		42	Q		58	6
11	L		27	B		43	R		59	7
12	M		28	C		44	S		60	8
13	N		29	D		45	T		61	9
14	O		30	E		46	U		62	+
15	P		31	F		47	V		63	/

For example, the following 2 examples:
A) If the character length is divisible by 3: For example "Tom":

            T           o           m
ASCII:          109 111         
8bit bytes:   01010100    01101111    01101101
6bit bytes:     010101      000110      111101      101101
Decimal:          6          45
Corresponding code:   V           G           9           t  

So, Btoa (' Tom ') = vg9t

b) string lengths cannot be divisible by 3, such as "Lucy":

            L           u           c           y
ASCII:          117          121
8bit bytes:   01001100    01110101    01100011    01111001      00000000    00000000
6bit bytes:     010011      000111      010101      100011      011110  010000  000000  000000
Decimal:          7      (Exception) (Exception)      
Corresponding code:   T           H           V           J               e       Q       =       =

Since Lucy has only 4 letters, so by 3 groups, the second group has two vacancies, so it needs to be filled with a zero. Here it is necessary to note that, because it is necessary to make up the 0, so the conversion into a decimal time can not be used in accordance with the regular Base64 encoding table to correspond, so not a, can be understood as a special "anomaly", the code should correspond to "=".

With the above theory, it is easy for us to implement a Base64 code.

[codec] on the principle and implementation of Base64 coding