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function y = base64decode(x)
%BASE64DECODE Perform base64 decoding on a string.
%
% BASE64DECODE(STR) decodes the given base64 string STR.
%
% Any character not part of the 65-character base64 subset set is silently
% ignored. Characters occuring after a '=' padding character are never
% decoded.
%
% STR doesn't have to be a string. The only requirement is that it is a
% vector containing values in the range 0-255.
%
% If the length of the string to decode (after ignoring non-base64 chars) is
% not a multiple of 4, then a warning is generated.
%
% This function is used to decode strings from the Base64 encoding specified
% in RFC 2045 - MIME (Multipurpose Internet Mail Extensions). The Base64
% encoding is designed to represent arbitrary sequences of octets in a form
% that need not be humanly readable. A 65-character subset ([A-Za-z0-9+/=])
% of US-ASCII is used, enabling 6 bits to be represented per printable
% character.
%
% See also BASE64ENCODE.
% Author: Peter J. Acklam
% Time-stamp: 2004-09-20 08:20:50 +0200
% E-mail: pjacklam@online.no
% URL: http://home.online.no/~pjacklam
% check number of input arguments
error(nargchk(1, 1, nargin));
% remove non-base64 chars
x = x ( ( 'A' <= x & x <= 'Z' ) ...
| ( 'a' <= x & x <= 'z' ) ...
| ( '0' <= x & x <= '9' ) ...
| ( x == '+' ) | ( x == '=' ) | ( x == '/' ) );
if rem(length(x), 4)
warning('Length of base64 data not a multiple of 4; padding input.');
end
k = find(x == '=');
if ~isempty(k)
x = x(1:k(1)-1);
end
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
% Now perform the following mapping
%
% A-Z -> 0 - 25
% a-z -> 26 - 51
% 0-9 -> 52 - 61
% + -> 62
% / -> 63
y = repmat(uint8(0), size(x));
i = 'A' <= x & x <= 'Z'; y(i) = - 'A' + x(i);
i = 'a' <= x & x <= 'z'; y(i) = 26 - 'a' + x(i);
i = '0' <= x & x <= '9'; y(i) = 52 - '0' + x(i);
i = x == '+'; y(i) = 62 - '+' + x(i);
i = x == '/'; y(i) = 63 - '/' + x(i);
x = y;
nebytes = length(x); % number of encoded bytes
nchunks = ceil(nebytes/4); % number of chunks/groups
% add padding if necessary
if rem(nebytes, 4)
x(end+1 : 4*nchunks) = 0;
end
x = reshape(uint8(x), 4, nchunks);
y = repmat(uint8(0), 3, nchunks); % for the decoded data
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
% Rearrange every 4 bytes into 3 bytes
%
% 00aaaaaa 00bbbbbb 00cccccc 00dddddd
%
% to form
%
% aaaaaabb bbbbcccc ccdddddd
y(1,:) = bitshift(x(1,:), 2); % 6 highest bits of y(1,:)
y(1,:) = bitor(y(1,:), bitshift(x(2,:), -4)); % 2 lowest bits of y(1,:)
y(2,:) = bitshift(x(2,:), 4); % 4 highest bits of y(2,:)
y(2,:) = bitor(y(2,:), bitshift(x(3,:), -2)); % 4 lowest bits of y(2,:)
y(3,:) = bitshift(x(3,:), 6); % 2 highest bits of y(3,:)
y(3,:) = bitor(y(3,:), x(4,:)); % 6 lowest bits of y(3,:)
% remove padding
switch rem(nebytes, 4)
case 2
y = y(1:end-2);
case 3
y = y(1:end-1);
end
% reshape to a row vector and make it a character array
y = char(reshape(y, 1, numel(y)));
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