Problem H
Base64 Decoding
Input: standard input
Output: standard
output
Time Limit: 1 second
Memory Limit: 32 MB
The following is an excerpt from section 6.8 (Base64
Content-Transfer-Encoding) of RFC 2045. It gives details of Base64 encoding that
is used for encoding of binary attchments in email.
The encoding process represents 24-bit groups of input bits
as output strings of 4 encoded characters. Proceeding from left to right, a
24-bit input group is formed by concatenating 3 8bit input groups. These 24
bits are then treated as 4 concatenated 6-bit groups, each of which is
translated into a single digit in the base64 alphabet. When encoding a bit
stream via the base64 encoding, the bit stream must be presumed to be ordered
with the most-significant-bit first. That is, the first bit in the stream will
be the high-order bit in the first 8bit byte, and the eighth bit will be the
low-order bit in the first 8bit byte, and so on.
Each 6-bit group is used as an index into an array of 64 printable characters.
The character referenced by the index is placed in the output string. These
characters are identified in table below.
Value Encoding Value Encoding Value Encoding Value Encoding
0 A 26 a 52 0 62 +
1 B 27 b 53 1 63 /
2 C 28 c 54 2
. . .. . .. . (pad) =
. . .. . .. .
24 Y 50 y 60 8
25 Z 51 z 61 9
The encoded output stream must be represented in lines of no
more than 76 characters each. All line breaks or other characters not found in
the table must be ignored by decoding software.
Special processing is performed if fewer than 24 bits are
available at the end of the data being encoded. A full encoding quantum is
always completed at the end of a body. When fewer than 24 input bits are
available in an input group, zero bits are added (on the right) to form an
integral number of 6-bit groups. Padding at the end of the data is performed
using the "=" character. Since all base64 input is an integral number
of octets, only the following cases can arise: (1) the final quantum of
encoding input is an integral multiple of 24 bits; here, the final unit of
encoded output will be an integral multiple of 4 characters with no
"=" padding, (2) the final quantum of encoding input is exactly 8
bits; here, the final unit of encoded output will be two characters followed by
two "=" padding characters, or (3) the final quantum of encoding
input is exactly 16 bits; here, the final unit of encoded output will be three
characters followed by one "=" padding character.
Because it is used only for padding at the end of the data,
the occurrence of any "=" characters may be taken as evidence that
the end of the data has been reached (without truncation in transit). No such
assurance is possible, however, when the number of octets transmitted was a
multiple of three and no "=" characters are present.
You are to write a Base64 decoder
for a web-based email client to be used in BUET server. However, in this
problem, to ease handing multiple datasets, you have to treat the character '#'
specially (i.e. NOT ignore it as RFC 2045 says above). See input specification
below.
Input
There are multiple datasets, each
terminated by the '#' mark. The last dataset, which should not be processed,
contains no character other than the terminating '#' mark. Each dataset
consists of a valid Base64 encoding of some (possibly binary) data.
Output
Output decoded data for each
dataset followed immediately by the '#' mark.
Sample Input
VGhpc0lzVGVzdA==
#
QSBUZXN0IElucHV0W3so
KX1d
##
Sample Output
ThisIsTest#A Test Input[{()}]#
Mustaq Ahmed