why is the base64 encode java code doing this -

-

so i'm trying understand base64 encoding better , came across implementation on wikipedia

private static string base64encode(byte[] in)       {         stringbuffer out = new stringbuffer((in.length * 4) / 3);         int b;         (int = 0; < in.length; += 3)  {             b = (in[i] & 0xfc) >> 2;             out.append(codes.charat(b));             b = (in[i] & 0x03) << 4;             if (i + 1 < in.length)      {                 b |= (in[i + 1] & 0xf0) >> 4;                 out.append(codes.charat(b));                 b = (in[i + 1] & 0x0f) << 2;                 if (i + 2 < in.length)  {                     b |= (in[i + 2] & 0xc0) >> 6;                     out.append(codes.charat(b));                     b = in[i + 2] & 0x3f;                     out.append(codes.charat(b));                 } else  {                     out.append(codes.charat(b));                     out.append('=');                 }             } else      {                 out.append(codes.charat(b));                 out.append("==");             }         }          return out.tostring();     }

and i'm following along , line: 

b = (in[i] & 0xfc) >> 2;

and don't it...why bitwise , 252 number shift right 2...wouldn't same if shifted byte without doing bitwise operation? example:

b = in[i] >> 2;

say in[i] letter e...represented 101 or in binary 01100101. if shift 2 right 011001 or 25. if bitwise & 

01100101 11111100 -------- 01100100

but shift going chop off last 2 anyway...so why bother doing it?

can clarify me please. thanks.

in in[i] >> 2, in[i] converted int first. if negative byte (with high bit set) converted negative int (with now-highest 24 bits set well).

in (in[i] & 0xfc) >> 2, in[i] converted int above, , & 0xfc makes sure bits reset 0.

you're partially right, in (in[i] & 0xff) >> 2 give same result. & 0xff common way convert byte non-negative int in range 0 255.

the way know sure why original developer used 0xfc, , not 0xff, ask them - speculate it's make more obvious bits being used.


Comments

Post a Comment

Popular posts from this blog

python - jinja2: TemplateSyntaxError: expected token ',', got 'string' -

-
i new jinja2 , having issue using python regular expression (re). in following code bold lines have error string in them. {% block content %} <div class="container"> {% l in lines %} {% if re.search(r"error", l) %} {# <<< throws error #} <b> {{ l }} </b> {% else %} {{ l }} <hr> {% endif %} {% endfor %} </div> {% endblock %} the re.search above throws following error: jinja2.exceptions.templatesyntaxerror templatesyntaxerror: expected token ',', got 'string' raw python code not supported in jinja2 template syntax. {% if re.search(r"error", l) %} replace line with {% if "error" in l %} can fix problem. if logical condition more complicated, should consider defining own custom filters (which can call python code) or complicated things in view layer. go check global namespace .
Read more

node.js - NodeJS remote terminal to Dropbear OpenWRT-Server -

-
i wandering whether there valid solution have remote terminal openwrt-box out of nodejs-app? connecting terminal works: ssh -i ~/.mykeys/id_rsa root@192.168.178.39 busybox v1.23.2 (2015-04-22 23:25:48 utc) built-in shell (ash) root@openwrt:~# the only interactive ssh solution nodejs doesn't interactive part described in readme.md following: var client = require('ssh2').client; var conn = new client(); conn.on('ready', function() { console.log('client :: ready'); conn.shell(function(err, stream) { if (err) throw err; stream.on('close', function() { console.log('stream :: close'); conn.end(); }).on('data', function(data) { console.log('stdout: ' + data); }).stderr.on('data', function(data) { console.log('stderr: ' + data); }); stream.end('ls -l\nexit\n'); }); }).connect({ host: '192.168.100.100', port: 22, username...
Read more

Qt4: how to send QString inside a struct via QSharedMemory -

-
i have struct struct control_data{ int column_number; qstring cell; }; i need send thread of qsharememory. read can't because qstring contains pointers inside. other ways? you have serialize struct byte array. can convert qstring const char* this: mystring.tostdstring().c_str(); but serializing qstring should work. the first step serialize struct qdatastream using qt, example here . then once struct can read , written can pass shared memory. a complete example of using qsharedmemory can found here . here relevant code: // first, test whether shared memory segment attached process. // if so, detach if (sharedmem.isattached()) { sharedmem.detach(); } ... qbuffer buffer; buffer.open( qbuffer::readwrite ); qdatastream out( &buffer ); out << youstruct; int size = buffer.size(); // size of int + size of qstring in bytes if ( !sharedmem.create( size ) ) { return; } // write shared memory sharedmem.lock(); char *to = (char*...
Read more